diff --git a/src/liballoc/benches/string.rs b/src/liballoc/benches/string.rs
index 599c8b1682851..5c95160ba2d14 100644
--- a/src/liballoc/benches/string.rs
+++ b/src/liballoc/benches/string.rs
@@ -1,5 +1,5 @@
 use std::iter::repeat;
-use test::Bencher;
+use test::{black_box, Bencher};
 
 #[bench]
 fn bench_with_capacity(b: &mut Bencher) {
diff --git a/src/libarena/lib.rs b/src/libarena/lib.rs
index beb0bac17d2ea..2a3d92edc4956 100644
--- a/src/libarena/lib.rs
+++ b/src/libarena/lib.rs
@@ -21,7 +21,6 @@
 extern crate alloc;
 
 use rustc_data_structures::cold_path;
-use rustc_data_structures::sync::MTLock;
 use smallvec::SmallVec;
 
 use std::cell::{Cell, RefCell};
@@ -116,11 +115,6 @@ impl<T> Default for TypedArena<T> {
 }
 
 impl<T> TypedArena<T> {
-    pub fn in_arena(&self, ptr: *const T) -> bool {
-        let ptr = ptr as *const T as *mut T;
-
-        self.chunks.borrow().iter().any(|chunk| chunk.start() <= ptr && ptr < chunk.end())
-    }
     /// Allocates an object in the `TypedArena`, returning a reference to it.
     #[inline]
     pub fn alloc(&self, object: T) -> &mut T {
@@ -334,12 +328,6 @@ impl Default for DroplessArena {
 }
 
 impl DroplessArena {
-    pub fn in_arena<T: ?Sized>(&self, ptr: *const T) -> bool {
-        let ptr = ptr as *const u8 as *mut u8;
-
-        self.chunks.borrow().iter().any(|chunk| chunk.start() <= ptr && ptr < chunk.end())
-    }
-
     #[inline]
     fn align(&self, align: usize) {
         let final_address = ((self.ptr.get() as usize) + align - 1) & !(align - 1);
@@ -500,66 +488,5 @@ impl DroplessArena {
     }
 }
 
-#[derive(Default)]
-// FIXME(@Zoxc): this type is entirely unused in rustc
-pub struct SyncTypedArena<T> {
-    lock: MTLock<TypedArena<T>>,
-}
-
-impl<T> SyncTypedArena<T> {
-    #[inline(always)]
-    pub fn alloc(&self, object: T) -> &mut T {
-        // Extend the lifetime of the result since it's limited to the lock guard
-        unsafe { &mut *(self.lock.lock().alloc(object) as *mut T) }
-    }
-
-    #[inline(always)]
-    pub fn alloc_slice(&self, slice: &[T]) -> &mut [T]
-    where
-        T: Copy,
-    {
-        // Extend the lifetime of the result since it's limited to the lock guard
-        unsafe { &mut *(self.lock.lock().alloc_slice(slice) as *mut [T]) }
-    }
-
-    #[inline(always)]
-    pub fn clear(&mut self) {
-        self.lock.get_mut().clear();
-    }
-}
-
-#[derive(Default)]
-pub struct SyncDroplessArena {
-    lock: MTLock<DroplessArena>,
-}
-
-impl SyncDroplessArena {
-    #[inline(always)]
-    pub fn in_arena<T: ?Sized>(&self, ptr: *const T) -> bool {
-        self.lock.lock().in_arena(ptr)
-    }
-
-    #[inline(always)]
-    pub fn alloc_raw(&self, bytes: usize, align: usize) -> &mut [u8] {
-        // Extend the lifetime of the result since it's limited to the lock guard
-        unsafe { &mut *(self.lock.lock().alloc_raw(bytes, align) as *mut [u8]) }
-    }
-
-    #[inline(always)]
-    pub fn alloc<T>(&self, object: T) -> &mut T {
-        // Extend the lifetime of the result since it's limited to the lock guard
-        unsafe { &mut *(self.lock.lock().alloc(object) as *mut T) }
-    }
-
-    #[inline(always)]
-    pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
-    where
-        T: Copy,
-    {
-        // Extend the lifetime of the result since it's limited to the lock guard
-        unsafe { &mut *(self.lock.lock().alloc_slice(slice) as *mut [T]) }
-    }
-}
-
 #[cfg(test)]
 mod tests;
diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index 4fcd0d9737d56..e7eecf7540ad7 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -187,8 +187,6 @@
 //! ```
 //!
 
-// ignore-tidy-undocumented-unsafe
-
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::cmp::Ordering;
@@ -368,6 +366,10 @@ impl<T> Cell<T> {
         if ptr::eq(self, other) {
             return;
         }
+        // SAFETY: This can be risky if called from separate threads, but `Cell`
+        // is `!Sync` so this won't happen. This also won't invalidate any
+        // pointers since `Cell` makes sure nothing else will be pointing into
+        // either of these `Cell`s.
         unsafe {
             ptr::swap(self.value.get(), other.value.get());
         }
@@ -387,6 +389,8 @@ impl<T> Cell<T> {
     /// ```
     #[stable(feature = "move_cell", since = "1.17.0")]
     pub fn replace(&self, val: T) -> T {
+        // SAFETY: This can cause data races if called from a separate thread,
+        // but `Cell` is `!Sync` so this won't happen.
         mem::replace(unsafe { &mut *self.value.get() }, val)
     }
 
@@ -423,6 +427,8 @@ impl<T: Copy> Cell<T> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get(&self) -> T {
+        // SAFETY: This can cause data races if called from a separate thread,
+        // but `Cell` is `!Sync` so this won't happen.
         unsafe { *self.value.get() }
     }
 
@@ -491,6 +497,9 @@ impl<T: ?Sized> Cell<T> {
     #[inline]
     #[stable(feature = "cell_get_mut", since = "1.11.0")]
     pub fn get_mut(&mut self) -> &mut T {
+        // SAFETY: This can cause data races if called from a separate thread,
+        // but `Cell` is `!Sync` so this won't happen, and `&mut` guarantees
+        // unique access.
         unsafe { &mut *self.value.get() }
     }
 
@@ -510,6 +519,7 @@ impl<T: ?Sized> Cell<T> {
     #[inline]
     #[stable(feature = "as_cell", since = "1.37.0")]
     pub fn from_mut(t: &mut T) -> &Cell<T> {
+        // SAFETY: `&mut` ensures unique access.
         unsafe { &*(t as *mut T as *const Cell<T>) }
     }
 }
@@ -553,6 +563,7 @@ impl<T> Cell<[T]> {
     /// ```
     #[stable(feature = "as_cell", since = "1.37.0")]
     pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
+        // SAFETY: `Cell<T>` has the same memory layout as `T`.
         unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
     }
 }
@@ -816,6 +827,8 @@ impl<T: ?Sized> RefCell<T> {
     #[inline]
     pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
         match BorrowRef::new(&self.borrow) {
+            // SAFETY: `BorrowRef` ensures that there is only immutable access
+            // to the value while borrowed.
             Some(b) => Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b }),
             None => Err(BorrowError { _private: () }),
         }
@@ -891,6 +904,7 @@ impl<T: ?Sized> RefCell<T> {
     #[inline]
     pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
         match BorrowRefMut::new(&self.borrow) {
+            // SAFETY: `BorrowRef` guarantees unique access.
             Some(b) => Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b }),
             None => Err(BorrowMutError { _private: () }),
         }
@@ -940,6 +954,7 @@ impl<T: ?Sized> RefCell<T> {
     #[inline]
     #[stable(feature = "cell_get_mut", since = "1.11.0")]
     pub fn get_mut(&mut self) -> &mut T {
+        // SAFETY: `&mut` guarantees unique access.
         unsafe { &mut *self.value.get() }
     }
 
diff --git a/src/libcore/str/lossy.rs b/src/libcore/str/lossy.rs
index 8a1fb9de54667..88b2bc551b7d1 100644
--- a/src/libcore/str/lossy.rs
+++ b/src/libcore/str/lossy.rs
@@ -3,8 +3,6 @@ use crate::fmt::{self, Write};
 use crate::mem;
 use crate::str as core_str;
 
-// ignore-tidy-undocumented-unsafe
-
 /// Lossy UTF-8 string.
 #[unstable(feature = "str_internals", issue = "none")]
 pub struct Utf8Lossy {
@@ -17,6 +15,7 @@ impl Utf8Lossy {
     }
 
     pub fn from_bytes(bytes: &[u8]) -> &Utf8Lossy {
+        // SAFETY: Both use the same memory layout, and UTF-8 correctness isn't required.
         unsafe { mem::transmute(bytes) }
     }
 
@@ -60,6 +59,8 @@ impl<'a> Iterator for Utf8LossyChunksIter<'a> {
         while i < self.source.len() {
             let i_ = i;
 
+            // SAFETY: `i` starts at `0`, is less than `self.source.len()`, and
+            // only increases, so `0 <= i < self.source.len()`.
             let byte = unsafe { *self.source.get_unchecked(i) };
             i += 1;
 
@@ -69,6 +70,7 @@ impl<'a> Iterator for Utf8LossyChunksIter<'a> {
 
                 macro_rules! error {
                     () => {{
+                        // SAFETY: We have checked up to `i` that source is valid UTF-8.
                         unsafe {
                             let r = Utf8LossyChunk {
                                 valid: core_str::from_utf8_unchecked(&self.source[0..i_]),
@@ -130,6 +132,7 @@ impl<'a> Iterator for Utf8LossyChunksIter<'a> {
         }
 
         let r = Utf8LossyChunk {
+            // SAFETY: We have checked that the entire source is valid UTF-8.
             valid: unsafe { core_str::from_utf8_unchecked(self.source) },
             broken: &[],
         };
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index ab771b1164bad..5a7cddd4041d5 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -1,5 +1,4 @@
 // ignore-tidy-filelength
-// ignore-tidy-undocumented-unsafe
 
 //! String manipulation.
 //!
@@ -341,6 +340,7 @@ impl Utf8Error {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error> {
     run_utf8_validation(v)?;
+    // SAFETY: Just ran validation.
     Ok(unsafe { from_utf8_unchecked(v) })
 }
 
@@ -379,6 +379,7 @@ pub fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error> {
 #[stable(feature = "str_mut_extras", since = "1.20.0")]
 pub fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
     run_utf8_validation(v)?;
+    // SAFETY: Just ran validation.
     Ok(unsafe { from_utf8_unchecked_mut(v) })
 }
 
@@ -581,7 +582,7 @@ impl<'a> Iterator for Chars<'a> {
     #[inline]
     fn next(&mut self) -> Option<char> {
         next_code_point(&mut self.iter).map(|ch| {
-            // str invariant says `ch` is a valid Unicode Scalar Value
+            // SAFETY: `str` invariant says `ch` is a valid Unicode Scalar Value.
             unsafe { char::from_u32_unchecked(ch) }
         })
     }
@@ -628,7 +629,7 @@ impl<'a> DoubleEndedIterator for Chars<'a> {
     #[inline]
     fn next_back(&mut self) -> Option<char> {
         next_code_point_reverse(&mut self.iter).map(|ch| {
-            // str invariant says `ch` is a valid Unicode Scalar Value
+            // SAFETY: `str` invariant says `ch` is a valid Unicode Scalar Value.
             unsafe { char::from_u32_unchecked(ch) }
         })
     }
@@ -658,6 +659,7 @@ impl<'a> Chars<'a> {
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
     #[inline]
     pub fn as_str(&self) -> &'a str {
+        // SAFETY: `Chars` is only made from a str, which guarantees the iter is valid UTF-8.
         unsafe { from_utf8_unchecked(self.iter.as_slice()) }
     }
 }
@@ -1102,6 +1104,7 @@ impl<'a, P: Pattern<'a>> SplitInternal<'a, P> {
     fn get_end(&mut self) -> Option<&'a str> {
         if !self.finished && (self.allow_trailing_empty || self.end - self.start > 0) {
             self.finished = true;
+            // SAFETY: `self.start` and `self.end` always lie on unicode boundaries.
             unsafe {
                 let string = self.matcher.haystack().get_unchecked(self.start..self.end);
                 Some(string)
@@ -1119,6 +1122,7 @@ impl<'a, P: Pattern<'a>> SplitInternal<'a, P> {
 
         let haystack = self.matcher.haystack();
         match self.matcher.next_match() {
+            // SAFETY: `Searcher` guarantees that `a` and `b` lie on unicode boundaries.
             Some((a, b)) => unsafe {
                 let elt = haystack.get_unchecked(self.start..a);
                 self.start = b;
@@ -1151,11 +1155,13 @@ impl<'a, P: Pattern<'a>> SplitInternal<'a, P> {
 
         let haystack = self.matcher.haystack();
         match self.matcher.next_match_back() {
+            // SAFETY: `Searcher` guarantees that `a` and `b` lie on unicode boundaries.
             Some((a, b)) => unsafe {
                 let elt = haystack.get_unchecked(b..self.end);
                 self.end = a;
                 Some(elt)
             },
+            // SAFETY: `self.start` and `self.end` always lie on unicode boundaries.
             None => unsafe {
                 self.finished = true;
                 Some(haystack.get_unchecked(self.start..self.end))
@@ -1297,6 +1303,7 @@ impl<'a, P: Pattern<'a>> MatchIndicesInternal<'a, P> {
     fn next(&mut self) -> Option<(usize, &'a str)> {
         self.0
             .next_match()
+            // SAFETY: `Searcher` guarantees that `start` and `end` lie on unicode boundaries.
             .map(|(start, end)| unsafe { (start, self.0.haystack().get_unchecked(start..end)) })
     }
 
@@ -1307,6 +1314,7 @@ impl<'a, P: Pattern<'a>> MatchIndicesInternal<'a, P> {
     {
         self.0
             .next_match_back()
+            // SAFETY: `Searcher` guarantees that `start` and `end` lie on unicode boundaries.
             .map(|(start, end)| unsafe { (start, self.0.haystack().get_unchecked(start..end)) })
     }
 }
@@ -1348,6 +1356,7 @@ where
 impl<'a, P: Pattern<'a>> MatchesInternal<'a, P> {
     #[inline]
     fn next(&mut self) -> Option<&'a str> {
+        // SAFETY: `Searcher` guarantees that `start` and `end` lie on unicode boundaries.
         self.0.next_match().map(|(a, b)| unsafe {
             // Indices are known to be on utf8 boundaries
             self.0.haystack().get_unchecked(a..b)
@@ -1359,6 +1368,7 @@ impl<'a, P: Pattern<'a>> MatchesInternal<'a, P> {
     where
         P::Searcher: ReverseSearcher<'a>,
     {
+        // SAFETY: `Searcher` guarantees that `start` and `end` lie on unicode boundaries.
         self.0.next_match_back().map(|(a, b)| unsafe {
             // Indices are known to be on utf8 boundaries
             self.0.haystack().get_unchecked(a..b)
@@ -1579,6 +1589,10 @@ fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
             if align != usize::max_value() && align.wrapping_sub(index) % usize_bytes == 0 {
                 let ptr = v.as_ptr();
                 while index < blocks_end {
+                    // SAFETY: since `align - index` and `ascii_block_size` are
+                    // multiples of `usize_bytes`, `block = ptr.add(index)` is
+                    // always aligned with a `usize` so it's safe to dereference
+                    // both `block` and `block.offset(1)`.
                     unsafe {
                         let block = ptr.add(index) as *const usize;
                         // break if there is a nonascii byte
@@ -1804,6 +1818,7 @@ mod traits {
                 && slice.is_char_boundary(self.start)
                 && slice.is_char_boundary(self.end)
             {
+                // SAFETY: just checked that `start` and `end` are on a char boundary.
                 Some(unsafe { self.get_unchecked(slice) })
             } else {
                 None
@@ -1815,6 +1830,7 @@ mod traits {
                 && slice.is_char_boundary(self.start)
                 && slice.is_char_boundary(self.end)
             {
+                // SAFETY: just checked that `start` and `end` are on a char boundary.
                 Some(unsafe { self.get_unchecked_mut(slice) })
             } else {
                 None
@@ -1845,6 +1861,7 @@ mod traits {
                 && slice.is_char_boundary(self.start)
                 && slice.is_char_boundary(self.end)
             {
+                // SAFETY: just checked that `start` and `end` are on a char boundary.
                 unsafe { self.get_unchecked_mut(slice) }
             } else {
                 super::slice_error_fail(slice, self.start, self.end)
@@ -1873,6 +1890,7 @@ mod traits {
         #[inline]
         fn get(self, slice: &str) -> Option<&Self::Output> {
             if slice.is_char_boundary(self.end) {
+                // SAFETY: just checked that `end` is on a char boundary.
                 Some(unsafe { self.get_unchecked(slice) })
             } else {
                 None
@@ -1881,6 +1899,7 @@ mod traits {
         #[inline]
         fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
             if slice.is_char_boundary(self.end) {
+                // SAFETY: just checked that `end` is on a char boundary.
                 Some(unsafe { self.get_unchecked_mut(slice) })
             } else {
                 None
@@ -1903,8 +1922,8 @@ mod traits {
         }
         #[inline]
         fn index_mut(self, slice: &mut str) -> &mut Self::Output {
-            // is_char_boundary checks that the index is in [0, .len()]
             if slice.is_char_boundary(self.end) {
+                // SAFETY: just checked that `end` is on a char boundary.
                 unsafe { self.get_unchecked_mut(slice) }
             } else {
                 super::slice_error_fail(slice, 0, self.end)
@@ -1934,6 +1953,7 @@ mod traits {
         #[inline]
         fn get(self, slice: &str) -> Option<&Self::Output> {
             if slice.is_char_boundary(self.start) {
+                // SAFETY: just checked that `start` is on a char boundary.
                 Some(unsafe { self.get_unchecked(slice) })
             } else {
                 None
@@ -1942,6 +1962,7 @@ mod traits {
         #[inline]
         fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
             if slice.is_char_boundary(self.start) {
+                // SAFETY: just checked that `start` is on a char boundary.
                 Some(unsafe { self.get_unchecked_mut(slice) })
             } else {
                 None
@@ -1966,8 +1987,8 @@ mod traits {
         }
         #[inline]
         fn index_mut(self, slice: &mut str) -> &mut Self::Output {
-            // is_char_boundary checks that the index is in [0, .len()]
             if slice.is_char_boundary(self.start) {
+                // SAFETY: just checked that `start` is on a char boundary.
                 unsafe { self.get_unchecked_mut(slice) }
             } else {
                 super::slice_error_fail(slice, self.start, slice.len())
@@ -2238,7 +2259,6 @@ impl str {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "str_as_bytes", since = "1.32.0")]
     #[inline(always)]
-    // SAFETY: const sound because we transmute two types with the same layout
     #[allow(unused_attributes)]
     #[allow_internal_unstable(const_fn_union)]
     pub const fn as_bytes(&self) -> &[u8] {
@@ -2247,6 +2267,7 @@ impl str {
             str: &'a str,
             slice: &'a [u8],
         }
+        // SAFETY: const sound because we transmute two types with the same layout
         unsafe { Slices { str: self }.slice }
     }
 
@@ -2573,6 +2594,7 @@ impl str {
     pub fn split_at(&self, mid: usize) -> (&str, &str) {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {
+            // SAFETY: just checked that `mid` is on a char boundary.
             unsafe { (self.get_unchecked(0..mid), self.get_unchecked(mid..self.len())) }
         } else {
             slice_error_fail(self, 0, mid)
@@ -2617,6 +2639,7 @@ impl str {
         if self.is_char_boundary(mid) {
             let len = self.len();
             let ptr = self.as_mut_ptr();
+            // SAFETY: just checked that `mid` is on a char boundary.
             unsafe {
                 (
                     from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr, mid)),
@@ -3805,10 +3828,8 @@ impl str {
         if let Some((_, b)) = matcher.next_reject_back() {
             j = b;
         }
-        unsafe {
-            // Searcher is known to return valid indices
-            self.get_unchecked(i..j)
-        }
+        // SAFETY: `Searcher` is known to return valid indices.
+        unsafe { self.get_unchecked(i..j) }
     }
 
     /// Returns a string slice with all prefixes that match a pattern
@@ -3844,10 +3865,8 @@ impl str {
         if let Some((a, _)) = matcher.next_reject() {
             i = a;
         }
-        unsafe {
-            // Searcher is known to return valid indices
-            self.get_unchecked(i..self.len())
-        }
+        // SAFETY: `Searcher` is known to return valid indices.
+        unsafe { self.get_unchecked(i..self.len()) }
     }
 
     /// Returns a string slice with the prefix removed.
@@ -3878,10 +3897,8 @@ impl str {
                 "The first search step from Searcher \
                 must include the first character"
             );
-            unsafe {
-                // Searcher is known to return valid indices.
-                Some(self.get_unchecked(len..))
-            }
+            // SAFETY: `Searcher` is known to return valid indices.
+            unsafe { Some(self.get_unchecked(len..)) }
         } else {
             None
         }
@@ -3919,10 +3936,8 @@ impl str {
                 "The first search step from ReverseSearcher \
                 must include the last character"
             );
-            unsafe {
-                // Searcher is known to return valid indices.
-                Some(self.get_unchecked(..start))
-            }
+            // SAFETY: `Searcher` is known to return valid indices.
+            unsafe { Some(self.get_unchecked(..start)) }
         } else {
             None
         }
@@ -3970,10 +3985,8 @@ impl str {
         if let Some((_, b)) = matcher.next_reject_back() {
             j = b;
         }
-        unsafe {
-            // Searcher is known to return valid indices
-            self.get_unchecked(0..j)
-        }
+        // SAFETY: `Searcher` is known to return valid indices.
+        unsafe { self.get_unchecked(0..j) }
     }
 
     /// Returns a string slice with all prefixes that match a pattern
@@ -4166,6 +4179,7 @@ impl str {
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     pub fn make_ascii_uppercase(&mut self) {
+        // SAFETY: safe because we transmute two types with the same layout.
         let me = unsafe { self.as_bytes_mut() };
         me.make_ascii_uppercase()
     }
@@ -4191,6 +4205,7 @@ impl str {
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     pub fn make_ascii_lowercase(&mut self) {
+        // SAFETY: safe because we transmute two types with the same layout.
         let me = unsafe { self.as_bytes_mut() };
         me.make_ascii_lowercase()
     }
@@ -4357,6 +4372,7 @@ impl Default for &str {
 impl Default for &mut str {
     /// Creates an empty mutable str
     fn default() -> Self {
+        // SAFETY: The empty string is valid UTF-8.
         unsafe { from_utf8_unchecked_mut(&mut []) }
     }
 }
@@ -4412,6 +4428,7 @@ impl_fn_for_zst! {
 
     #[derive(Clone)]
     struct UnsafeBytesToStr impl<'a> Fn = |bytes: &'a [u8]| -> &'a str {
+        // SAFETY: not safe
         unsafe { from_utf8_unchecked(bytes) }
     };
 }
diff --git a/src/libcore/str/pattern.rs b/src/libcore/str/pattern.rs
index 46d9499394a38..ef64d8b0fdf88 100644
--- a/src/libcore/str/pattern.rs
+++ b/src/libcore/str/pattern.rs
@@ -3,8 +3,6 @@
 //! For more details, see the traits [`Pattern`], [`Searcher`],
 //! [`ReverseSearcher`], and [`DoubleEndedSearcher`].
 
-// ignore-tidy-undocumented-unsafe
-
 #![unstable(
     feature = "pattern",
     reason = "API not fully fleshed out and ready to be stabilized",
@@ -271,6 +269,14 @@ unsafe impl<'a> Searcher<'a> for CharSearcher<'a> {
     #[inline]
     fn next(&mut self) -> SearchStep {
         let old_finger = self.finger;
+        // SAFETY: 1-4 guarantee safety of `get_unchecked`
+        // 1. `self.finger` and `self.finger_back` are kept on unicode boundaries
+        //    (this is invariant)
+        // 2. `self.finger >= 0` since it starts at 0 and only increases
+        // 3. `self.finger < self.finger_back` because otherwise the char `iter`
+        //    would return `SearchStep::Done`
+        // 4. `self.finger` comes before the end of the haystack because `self.finger_back`
+        //    starts at the end and only decreases
         let slice = unsafe { self.haystack.get_unchecked(old_finger..self.finger_back) };
         let mut iter = slice.chars();
         let old_len = iter.iter.len();
@@ -293,6 +299,7 @@ unsafe impl<'a> Searcher<'a> for CharSearcher<'a> {
             // get the haystack after the last character found
             let bytes = self.haystack.as_bytes().get(self.finger..self.finger_back)?;
             // the last byte of the utf8 encoded needle
+            // SAFETY: we have an invariant that `utf8_size < 5`
             let last_byte = unsafe { *self.utf8_encoded.get_unchecked(self.utf8_size - 1) };
             if let Some(index) = memchr::memchr(last_byte, bytes) {
                 // The new finger is the index of the byte we found,
@@ -336,6 +343,7 @@ unsafe impl<'a> ReverseSearcher<'a> for CharSearcher<'a> {
     #[inline]
     fn next_back(&mut self) -> SearchStep {
         let old_finger = self.finger_back;
+        // SAFETY: see the comment for next() above
         let slice = unsafe { self.haystack.get_unchecked(self.finger..old_finger) };
         let mut iter = slice.chars();
         let old_len = iter.iter.len();
@@ -363,6 +371,7 @@ unsafe impl<'a> ReverseSearcher<'a> for CharSearcher<'a> {
                 return None;
             };
             // the last byte of the utf8 encoded needle
+            // SAFETY: we have an invariant that `utf8_size < 5`
             let last_byte = unsafe { *self.utf8_encoded.get_unchecked(self.utf8_size - 1) };
             if let Some(index) = memchr::memrchr(last_byte, bytes) {
                 // we searched a slice that was offset by self.finger,
diff --git a/src/libcore/sync/atomic.rs b/src/libcore/sync/atomic.rs
index fae95ca5cdb36..9d449bb991507 100644
--- a/src/libcore/sync/atomic.rs
+++ b/src/libcore/sync/atomic.rs
@@ -112,8 +112,6 @@
 //! println!("live threads: {}", old_thread_count + 1);
 //! ```
 
-// ignore-tidy-undocumented-unsafe
-
 #![stable(feature = "rust1", since = "1.0.0")]
 #![cfg_attr(not(target_has_atomic_load_store = "8"), allow(dead_code))]
 #![cfg_attr(not(target_has_atomic_load_store = "8"), allow(unused_imports))]
@@ -350,6 +348,7 @@ impl AtomicBool {
     #[inline]
     #[stable(feature = "atomic_access", since = "1.15.0")]
     pub fn get_mut(&mut self) -> &mut bool {
+        // SAFETY: the mutable reference guarantees unique ownership.
         unsafe { &mut *(self.v.get() as *mut bool) }
     }
 
@@ -400,6 +399,8 @@ impl AtomicBool {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn load(&self, order: Ordering) -> bool {
+        // SAFETY: any data races are prevented by atomic intrinsics and the raw
+        // pointer passed in is valid because we got it from a reference.
         unsafe { atomic_load(self.v.get(), order) != 0 }
     }
 
@@ -432,6 +433,8 @@ impl AtomicBool {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn store(&self, val: bool, order: Ordering) {
+        // SAFETY: any data races are prevented by atomic intrinsics and the raw
+        // pointer passed in is valid because we got it from a reference.
         unsafe {
             atomic_store(self.v.get(), val as u8, order);
         }
@@ -463,6 +466,7 @@ impl AtomicBool {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[cfg(target_has_atomic = "8")]
     pub fn swap(&self, val: bool, order: Ordering) -> bool {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe { atomic_swap(self.v.get(), val as u8, order) != 0 }
     }
 
@@ -558,6 +562,7 @@ impl AtomicBool {
         success: Ordering,
         failure: Ordering,
     ) -> Result<bool, bool> {
+        // SAFETY: data races are prevented by atomic intrinsics.
         match unsafe {
             atomic_compare_exchange(self.v.get(), current as u8, new as u8, success, failure)
         } {
@@ -615,6 +620,7 @@ impl AtomicBool {
         success: Ordering,
         failure: Ordering,
     ) -> Result<bool, bool> {
+        // SAFETY: data races are prevented by atomic intrinsics.
         match unsafe {
             atomic_compare_exchange_weak(self.v.get(), current as u8, new as u8, success, failure)
         } {
@@ -661,6 +667,7 @@ impl AtomicBool {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[cfg(target_has_atomic = "8")]
     pub fn fetch_and(&self, val: bool, order: Ordering) -> bool {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe { atomic_and(self.v.get(), val as u8, order) != 0 }
     }
 
@@ -756,6 +763,7 @@ impl AtomicBool {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[cfg(target_has_atomic = "8")]
     pub fn fetch_or(&self, val: bool, order: Ordering) -> bool {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe { atomic_or(self.v.get(), val as u8, order) != 0 }
     }
 
@@ -797,6 +805,7 @@ impl AtomicBool {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[cfg(target_has_atomic = "8")]
     pub fn fetch_xor(&self, val: bool, order: Ordering) -> bool {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe { atomic_xor(self.v.get(), val as u8, order) != 0 }
     }
 
@@ -872,6 +881,7 @@ impl<T> AtomicPtr<T> {
     #[inline]
     #[stable(feature = "atomic_access", since = "1.15.0")]
     pub fn get_mut(&mut self) -> &mut *mut T {
+        // SAFETY: the mutable reference guarantees unique ownership.
         unsafe { &mut *self.p.get() }
     }
 
@@ -923,6 +933,7 @@ impl<T> AtomicPtr<T> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn load(&self, order: Ordering) -> *mut T {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe { atomic_load(self.p.get() as *mut usize, order) as *mut T }
     }
 
@@ -957,6 +968,7 @@ impl<T> AtomicPtr<T> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn store(&self, ptr: *mut T, order: Ordering) {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe {
             atomic_store(self.p.get() as *mut usize, ptr as usize, order);
         }
@@ -990,6 +1002,7 @@ impl<T> AtomicPtr<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[cfg(target_has_atomic = "ptr")]
     pub fn swap(&self, ptr: *mut T, order: Ordering) -> *mut T {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe { atomic_swap(self.p.get() as *mut usize, ptr as usize, order) as *mut T }
     }
 
@@ -1074,6 +1087,7 @@ impl<T> AtomicPtr<T> {
         success: Ordering,
         failure: Ordering,
     ) -> Result<*mut T, *mut T> {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe {
             let res = atomic_compare_exchange(
                 self.p.get() as *mut usize,
@@ -1137,6 +1151,7 @@ impl<T> AtomicPtr<T> {
         success: Ordering,
         failure: Ordering,
     ) -> Result<*mut T, *mut T> {
+        // SAFETY: data races are prevented by atomic intrinsics.
         unsafe {
             let res = atomic_compare_exchange_weak(
                 self.p.get() as *mut usize,
@@ -1290,6 +1305,7 @@ assert_eq!(some_var.load(Ordering::SeqCst), 5);
                 #[inline]
                 #[$stable_access]
                 pub fn get_mut(&mut self) -> &mut $int_type {
+                    // SAFETY: the mutable reference guarantees unique ownership.
                     unsafe { &mut *self.v.get() }
                 }
             }
@@ -1344,6 +1360,7 @@ assert_eq!(some_var.load(Ordering::Relaxed), 5);
                 #[inline]
                 #[$stable]
                 pub fn load(&self, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_load(self.v.get(), order) }
                 }
             }
@@ -1378,6 +1395,7 @@ assert_eq!(some_var.load(Ordering::Relaxed), 10);
                 #[inline]
                 #[$stable]
                 pub fn store(&self, val: $int_type, order: Ordering) {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_store(self.v.get(), val, order); }
                 }
             }
@@ -1408,6 +1426,7 @@ assert_eq!(some_var.swap(10, Ordering::Relaxed), 5);
                 #[$stable]
                 #[$cfg_cas]
                 pub fn swap(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_swap(self.v.get(), val, order) }
                 }
             }
@@ -1510,6 +1529,7 @@ assert_eq!(some_var.load(Ordering::Relaxed), 10);
                                         new: $int_type,
                                         success: Ordering,
                                         failure: Ordering) -> Result<$int_type, $int_type> {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_compare_exchange(self.v.get(), current, new, success, failure) }
                 }
             }
@@ -1562,6 +1582,7 @@ loop {
                                              new: $int_type,
                                              success: Ordering,
                                              failure: Ordering) -> Result<$int_type, $int_type> {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe {
                         atomic_compare_exchange_weak(self.v.get(), current, new, success, failure)
                     }
@@ -1596,6 +1617,7 @@ assert_eq!(foo.load(Ordering::SeqCst), 10);
                 #[$stable]
                 #[$cfg_cas]
                 pub fn fetch_add(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_add(self.v.get(), val, order) }
                 }
             }
@@ -1628,6 +1650,7 @@ assert_eq!(foo.load(Ordering::SeqCst), 10);
                 #[$stable]
                 #[$cfg_cas]
                 pub fn fetch_sub(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_sub(self.v.get(), val, order) }
                 }
             }
@@ -1663,6 +1686,7 @@ assert_eq!(foo.load(Ordering::SeqCst), 0b100001);
                 #[$stable]
                 #[$cfg_cas]
                 pub fn fetch_and(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_and(self.v.get(), val, order) }
                 }
             }
@@ -1699,6 +1723,7 @@ assert_eq!(foo.load(Ordering::SeqCst), !(0x13 & 0x31));
                 #[$stable_nand]
                 #[$cfg_cas]
                 pub fn fetch_nand(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_nand(self.v.get(), val, order) }
                 }
             }
@@ -1734,6 +1759,7 @@ assert_eq!(foo.load(Ordering::SeqCst), 0b111111);
                 #[$stable]
                 #[$cfg_cas]
                 pub fn fetch_or(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_or(self.v.get(), val, order) }
                 }
             }
@@ -1769,6 +1795,7 @@ assert_eq!(foo.load(Ordering::SeqCst), 0b011110);
                 #[$stable]
                 #[$cfg_cas]
                 pub fn fetch_xor(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { atomic_xor(self.v.get(), val, order) }
                 }
             }
@@ -1880,6 +1907,7 @@ assert!(max_foo == 42);
                        issue = "48655")]
                 #[$cfg_cas]
                 pub fn fetch_max(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { $max_fn(self.v.get(), val, order) }
                 }
             }
@@ -1932,6 +1960,7 @@ assert_eq!(min_foo, 12);
                        issue = "48655")]
                 #[$cfg_cas]
                 pub fn fetch_min(&self, val: $int_type, order: Ordering) -> $int_type {
+                    // SAFETY: data races are prevented by atomic intrinsics.
                     unsafe { $min_fn(self.v.get(), val, order) }
                 }
             }
@@ -1960,7 +1989,9 @@ extern {
 }
 
 let mut atomic = ", stringify!($atomic_type), "::new(1);
-unsafe {
+",
+// SAFETY: Safe as long as `my_atomic_op` is atomic.
+"unsafe {
     my_atomic_op(atomic.as_mut_ptr());
 }
 # }
@@ -2526,6 +2557,7 @@ pub fn fence(order: Ordering) {
     // https://github.com/WebAssembly/tool-conventions/issues/59. We should
     // follow that discussion and implement a solution when one comes about!
     #[cfg(not(target_arch = "wasm32"))]
+    // SAFETY: using an atomic fence is safe.
     unsafe {
         match order {
             Acquire => intrinsics::atomic_fence_acq(),
@@ -2613,6 +2645,7 @@ pub fn fence(order: Ordering) {
 #[inline]
 #[stable(feature = "compiler_fences", since = "1.21.0")]
 pub fn compiler_fence(order: Ordering) {
+    // SAFETY: using an atomic fence is safe.
     unsafe {
         match order {
             Acquire => intrinsics::atomic_singlethreadfence_acq(),
diff --git a/src/librustc/arena.rs b/src/librustc/arena.rs
index cb3fdff53a3b0..15e92d8d84219 100644
--- a/src/librustc/arena.rs
+++ b/src/librustc/arena.rs
@@ -123,6 +123,9 @@ macro_rules! arena_types {
             [few] inferred_outlives_crate: rustc::ty::CratePredicatesMap<'tcx>,
             [] upvars: rustc_data_structures::fx::FxIndexMap<rustc_hir::HirId, rustc_hir::Upvar>,
 
+            // Interned types
+            [] tys: rustc::ty::TyS<$tcx>,
+
             // HIR types
             [few] hir_forest: rustc::hir::map::Forest<$tcx>,
             [] arm: rustc_hir::Arm<$tcx>,
@@ -176,7 +179,7 @@ macro_rules! declare_arena {
     ([], [$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
         #[derive(Default)]
         pub struct Arena<$tcx> {
-            dropless: DroplessArena,
+            pub dropless: DroplessArena,
             drop: DropArena,
             $($name: arena_for_type!($a[$ty]),)*
         }
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index ef776c88a8f7c..a51f0f7f24c36 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -51,10 +51,9 @@ use rustc_session::config::CrateType;
 use rustc_session::config::{BorrowckMode, OutputFilenames};
 use rustc_session::Session;
 
-use arena::SyncDroplessArena;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::profiling::SelfProfilerRef;
-use rustc_data_structures::sharded::ShardedHashMap;
+use rustc_data_structures::sharded::{IntoPointer, ShardedHashMap};
 use rustc_data_structures::stable_hasher::{
     hash_stable_hashmap, HashStable, StableHasher, StableVec,
 };
@@ -83,21 +82,11 @@ use syntax::ast;
 use syntax::attr;
 use syntax::expand::allocator::AllocatorKind;
 
-pub struct AllArenas {
-    pub interner: SyncDroplessArena,
-}
-
-impl AllArenas {
-    pub fn new() -> Self {
-        AllArenas { interner: SyncDroplessArena::default() }
-    }
-}
-
 type InternedSet<'tcx, T> = ShardedHashMap<Interned<'tcx, T>, ()>;
 
 pub struct CtxtInterners<'tcx> {
     /// The arena that types, regions, etc. are allocated from.
-    arena: &'tcx SyncDroplessArena,
+    arena: &'tcx WorkerLocal<Arena<'tcx>>,
 
     /// Specifically use a speedy hash algorithm for these hash sets, since
     /// they're accessed quite often.
@@ -117,7 +106,7 @@ pub struct CtxtInterners<'tcx> {
 }
 
 impl<'tcx> CtxtInterners<'tcx> {
-    fn new(arena: &'tcx SyncDroplessArena) -> CtxtInterners<'tcx> {
+    fn new(arena: &'tcx WorkerLocal<Arena<'tcx>>) -> CtxtInterners<'tcx> {
         CtxtInterners {
             arena,
             type_: Default::default(),
@@ -1125,7 +1114,6 @@ impl<'tcx> TyCtxt<'tcx> {
         lint_store: Lrc<dyn Any + sync::Send + sync::Sync>,
         local_providers: ty::query::Providers<'tcx>,
         extern_providers: ty::query::Providers<'tcx>,
-        arenas: &'tcx AllArenas,
         arena: &'tcx WorkerLocal<Arena<'tcx>>,
         resolutions: ty::ResolverOutputs,
         hir: hir_map::Map<'tcx>,
@@ -1136,7 +1124,7 @@ impl<'tcx> TyCtxt<'tcx> {
         let data_layout = TargetDataLayout::parse(&s.target.target).unwrap_or_else(|err| {
             s.fatal(&err);
         });
-        let interners = CtxtInterners::new(&arenas.interner);
+        let interners = CtxtInterners::new(arena);
         let common_types = CommonTypes::new(&interners);
         let common_lifetimes = CommonLifetimes::new(&interners);
         let common_consts = CommonConsts::new(&interners, &common_types);
@@ -1567,11 +1555,11 @@ pub trait Lift<'tcx>: fmt::Debug {
 }
 
 macro_rules! nop_lift {
-    ($ty:ty => $lifted:ty) => {
+    ($set:ident; $ty:ty => $lifted:ty) => {
         impl<'a, 'tcx> Lift<'tcx> for $ty {
             type Lifted = $lifted;
             fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
-                if tcx.interners.arena.in_arena(*self as *const _) {
+                if tcx.interners.$set.contains_pointer_to(&Interned(*self)) {
                     Some(unsafe { mem::transmute(*self) })
                 } else {
                     None
@@ -1582,14 +1570,14 @@ macro_rules! nop_lift {
 }
 
 macro_rules! nop_list_lift {
-    ($ty:ty => $lifted:ty) => {
+    ($set:ident; $ty:ty => $lifted:ty) => {
         impl<'a, 'tcx> Lift<'tcx> for &'a List<$ty> {
             type Lifted = &'tcx List<$lifted>;
             fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
                 if self.is_empty() {
                     return Some(List::empty());
                 }
-                if tcx.interners.arena.in_arena(*self as *const _) {
+                if tcx.interners.$set.contains_pointer_to(&Interned(*self)) {
                     Some(unsafe { mem::transmute(*self) })
                 } else {
                     None
@@ -1599,21 +1587,21 @@ macro_rules! nop_list_lift {
     };
 }
 
-nop_lift! {Ty<'a> => Ty<'tcx>}
-nop_lift! {Region<'a> => Region<'tcx>}
-nop_lift! {Goal<'a> => Goal<'tcx>}
-nop_lift! {&'a Const<'a> => &'tcx Const<'tcx>}
+nop_lift! {type_; Ty<'a> => Ty<'tcx>}
+nop_lift! {region; Region<'a> => Region<'tcx>}
+nop_lift! {goal; Goal<'a> => Goal<'tcx>}
+nop_lift! {const_; &'a Const<'a> => &'tcx Const<'tcx>}
 
-nop_list_lift! {Goal<'a> => Goal<'tcx>}
-nop_list_lift! {Clause<'a> => Clause<'tcx>}
-nop_list_lift! {Ty<'a> => Ty<'tcx>}
-nop_list_lift! {ExistentialPredicate<'a> => ExistentialPredicate<'tcx>}
-nop_list_lift! {Predicate<'a> => Predicate<'tcx>}
-nop_list_lift! {CanonicalVarInfo => CanonicalVarInfo}
-nop_list_lift! {ProjectionKind => ProjectionKind}
+nop_list_lift! {goal_list; Goal<'a> => Goal<'tcx>}
+nop_list_lift! {clauses; Clause<'a> => Clause<'tcx>}
+nop_list_lift! {type_list; Ty<'a> => Ty<'tcx>}
+nop_list_lift! {existential_predicates; ExistentialPredicate<'a> => ExistentialPredicate<'tcx>}
+nop_list_lift! {predicates; Predicate<'a> => Predicate<'tcx>}
+nop_list_lift! {canonical_var_infos; CanonicalVarInfo => CanonicalVarInfo}
+nop_list_lift! {projs; ProjectionKind => ProjectionKind}
 
 // This is the impl for `&'a InternalSubsts<'a>`.
-nop_list_lift! {GenericArg<'a> => GenericArg<'tcx>}
+nop_list_lift! {substs; GenericArg<'a> => GenericArg<'tcx>}
 
 pub mod tls {
     use super::{ptr_eq, GlobalCtxt, TyCtxt};
@@ -1937,6 +1925,11 @@ impl<'tcx, T: 'tcx + ?Sized> Clone for Interned<'tcx, T> {
 }
 impl<'tcx, T: 'tcx + ?Sized> Copy for Interned<'tcx, T> {}
 
+impl<'tcx, T: 'tcx + ?Sized> IntoPointer for Interned<'tcx, T> {
+    fn into_pointer(&self) -> *const () {
+        self.0 as *const _ as *const ()
+    }
+}
 // N.B., an `Interned<Ty>` compares and hashes as a `TyKind`.
 impl<'tcx> PartialEq for Interned<'tcx, TyS<'tcx>> {
     fn eq(&self, other: &Interned<'tcx, TyS<'tcx>>) -> bool {
@@ -2089,7 +2082,7 @@ macro_rules! slice_interners {
         $(impl<'tcx> TyCtxt<'tcx> {
             pub fn $method(self, v: &[$ty]) -> &'tcx List<$ty> {
                 self.interners.$field.intern_ref(v, || {
-                    Interned(List::from_arena(&self.interners.arena, v))
+                    Interned(List::from_arena(&*self.arena, v))
                 }).0
             }
         })+
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index e6acb6b74dc63..282136fa3546b 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -6,6 +6,7 @@ pub use self::BorrowKind::*;
 pub use self::IntVarValue::*;
 pub use self::Variance::*;
 
+use crate::arena::Arena;
 use crate::hir::exports::ExportMap;
 use crate::hir::map as hir_map;
 
@@ -25,7 +26,6 @@ use crate::ty::layout::VariantIdx;
 use crate::ty::subst::{InternalSubsts, Subst, SubstsRef};
 use crate::ty::util::{Discr, IntTypeExt};
 use crate::ty::walk::TypeWalker;
-use arena::SyncDroplessArena;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::fx::FxIndexMap;
@@ -74,7 +74,7 @@ pub use crate::ty::diagnostics::*;
 pub use self::binding::BindingMode;
 pub use self::binding::BindingMode::*;
 
-pub use self::context::{keep_local, tls, AllArenas, FreeRegionInfo, TyCtxt};
+pub use self::context::{keep_local, tls, FreeRegionInfo, TyCtxt};
 pub use self::context::{
     CanonicalUserType, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, ResolvedOpaqueTy,
     UserType, UserTypeAnnotationIndex,
@@ -604,7 +604,7 @@ unsafe impl<T: Sync> Sync for List<T> {}
 
 impl<T: Copy> List<T> {
     #[inline]
-    fn from_arena<'tcx>(arena: &'tcx SyncDroplessArena, slice: &[T]) -> &'tcx List<T> {
+    fn from_arena<'tcx>(arena: &'tcx Arena<'tcx>, slice: &[T]) -> &'tcx List<T> {
         assert!(!mem::needs_drop::<T>());
         assert!(mem::size_of::<T>() != 0);
         assert!(slice.len() != 0);
@@ -617,7 +617,9 @@ impl<T: Copy> List<T> {
 
         let size = offset + slice.len() * mem::size_of::<T>();
 
-        let mem = arena.alloc_raw(size, cmp::max(mem::align_of::<T>(), mem::align_of::<usize>()));
+        let mem = arena
+            .dropless
+            .alloc_raw(size, cmp::max(mem::align_of::<T>(), mem::align_of::<usize>()));
         unsafe {
             let result = &mut *(mem.as_mut_ptr() as *mut List<T>);
             // Write the length
diff --git a/src/librustc_data_structures/profiling.rs b/src/librustc_data_structures/profiling.rs
index 004db0a79a880..44cef727f034b 100644
--- a/src/librustc_data_structures/profiling.rs
+++ b/src/librustc_data_structures/profiling.rs
@@ -136,9 +136,11 @@ bitflags::bitflags! {
     }
 }
 
+// keep this in sync with the `-Z self-profile-events` help message in librustc_session/options.rs
 const EVENT_FILTERS_BY_NAME: &[(&str, EventFilter)] = &[
     ("none", EventFilter::NONE),
     ("all", EventFilter::ALL),
+    ("default", EventFilter::DEFAULT),
     ("generic-activity", EventFilter::GENERIC_ACTIVITIES),
     ("query-provider", EventFilter::QUERY_PROVIDERS),
     ("query-cache-hit", EventFilter::QUERY_CACHE_HITS),
diff --git a/src/librustc_data_structures/sharded.rs b/src/librustc_data_structures/sharded.rs
index 8b85d97a1d4fe..ee3f88ff1675f 100644
--- a/src/librustc_data_structures/sharded.rs
+++ b/src/librustc_data_structures/sharded.rs
@@ -137,6 +137,20 @@ impl<K: Eq + Hash + Copy> ShardedHashMap<K, ()> {
     }
 }
 
+pub trait IntoPointer {
+    /// Returns a pointer which outlives `self`.
+    fn into_pointer(&self) -> *const ();
+}
+
+impl<K: Eq + Hash + Copy + IntoPointer> ShardedHashMap<K, ()> {
+    pub fn contains_pointer_to<T: Hash + IntoPointer>(&self, value: &T) -> bool {
+        let hash = make_hash(&value);
+        let shard = self.get_shard_by_hash(hash).lock();
+        let value = value.into_pointer();
+        shard.raw_entry().from_hash(hash, |entry| entry.into_pointer() == value).is_some()
+    }
+}
+
 #[inline]
 fn make_hash<K: Hash + ?Sized>(val: &K) -> u64 {
     let mut state = FxHasher::default();
diff --git a/src/librustc_interface/passes.rs b/src/librustc_interface/passes.rs
index c4444fbaa2fc7..d62c7539d5f21 100644
--- a/src/librustc_interface/passes.rs
+++ b/src/librustc_interface/passes.rs
@@ -15,7 +15,7 @@ use rustc::session::search_paths::PathKind;
 use rustc::session::Session;
 use rustc::traits;
 use rustc::ty::steal::Steal;
-use rustc::ty::{self, AllArenas, GlobalCtxt, ResolverOutputs, TyCtxt};
+use rustc::ty::{self, GlobalCtxt, ResolverOutputs, TyCtxt};
 use rustc::util::common::ErrorReported;
 use rustc_builtin_macros;
 use rustc_codegen_ssa::back::link::emit_metadata;
@@ -715,7 +715,6 @@ pub fn create_global_ctxt<'tcx>(
     outputs: OutputFilenames,
     crate_name: &str,
     global_ctxt: &'tcx Once<GlobalCtxt<'tcx>>,
-    all_arenas: &'tcx AllArenas,
     arena: &'tcx WorkerLocal<Arena<'tcx>>,
 ) -> QueryContext<'tcx> {
     let sess = &compiler.session();
@@ -746,7 +745,6 @@ pub fn create_global_ctxt<'tcx>(
                 lint_store,
                 local_providers,
                 extern_providers,
-                &all_arenas,
                 arena,
                 resolver_outputs,
                 hir_map,
diff --git a/src/librustc_interface/queries.rs b/src/librustc_interface/queries.rs
index bd9717d3f3d02..0af9b17a2995b 100644
--- a/src/librustc_interface/queries.rs
+++ b/src/librustc_interface/queries.rs
@@ -7,7 +7,7 @@ use rustc::hir::map;
 use rustc::session::config::{OutputFilenames, OutputType};
 use rustc::session::Session;
 use rustc::ty::steal::Steal;
-use rustc::ty::{AllArenas, GlobalCtxt, ResolverOutputs};
+use rustc::ty::{GlobalCtxt, ResolverOutputs};
 use rustc::util::common::ErrorReported;
 use rustc_codegen_utils::codegen_backend::CodegenBackend;
 use rustc_data_structures::sync::{Lrc, Once, WorkerLocal};
@@ -66,7 +66,6 @@ pub struct Queries<'tcx> {
     compiler: &'tcx Compiler,
     gcx: Once<GlobalCtxt<'tcx>>,
 
-    all_arenas: AllArenas,
     arena: WorkerLocal<Arena<'tcx>>,
 
     dep_graph_future: Query<Option<DepGraphFuture>>,
@@ -86,7 +85,6 @@ impl<'tcx> Queries<'tcx> {
         Queries {
             compiler,
             gcx: Once::new(),
-            all_arenas: AllArenas::new(),
             arena: WorkerLocal::new(|_| Arena::default()),
             dep_graph_future: Default::default(),
             parse: Default::default(),
@@ -265,7 +263,6 @@ impl<'tcx> Queries<'tcx> {
                 outputs,
                 &crate_name,
                 &self.gcx,
-                &self.all_arenas,
                 &self.arena,
             ))
         })
diff --git a/src/librustc_session/options.rs b/src/librustc_session/options.rs
index 4b5736adc17c3..2a0ed27b63b08 100644
--- a/src/librustc_session/options.rs
+++ b/src/librustc_session/options.rs
@@ -923,8 +923,12 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     self_profile: SwitchWithOptPath = (SwitchWithOptPath::Disabled,
         parse_switch_with_opt_path, [UNTRACKED],
         "run the self profiler and output the raw event data"),
+    // keep this in sync with the event filter names in librustc_data_structures/profiling.rs
     self_profile_events: Option<Vec<String>> = (None, parse_opt_comma_list, [UNTRACKED],
-        "specifies which kinds of events get recorded by the self profiler"),
+        "specifies which kinds of events get recorded by the self profiler;
+        for example: `-Z self-profile-events=default,query-keys`
+        all options: none, all, default, generic-activity, query-provider, query-cache-hit
+                     query-blocked, incr-cache-load, query-keys"),
     emit_stack_sizes: bool = (false, parse_bool, [UNTRACKED],
         "emits a section containing stack size metadata"),
     plt: Option<bool> = (None, parse_opt_bool, [TRACKED],
diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
index a3be264ddc128..173bb29e964d1 100644
--- a/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@ -1320,10 +1320,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 // those that do.
                 self.one_bound_for_assoc_type(
                     || traits::supertraits(tcx, trait_ref),
-                    &trait_ref.print_only_trait_path().to_string(),
+                    || trait_ref.print_only_trait_path().to_string(),
                     binding.item_name,
                     path_span,
-                    match binding.kind {
+                    || match binding.kind {
                         ConvertedBindingKind::Equality(ty) => Some(ty.to_string()),
                         _ => None,
                     },
@@ -1880,10 +1880,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     predicates.iter().filter_map(|(p, _)| p.to_opt_poly_trait_ref()),
                 )
             },
-            &param_name.as_str(),
+            || param_name.to_string(),
             assoc_name,
             span,
-            None,
+            || None,
         )
     }
 
@@ -1892,10 +1892,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     fn one_bound_for_assoc_type<I>(
         &self,
         all_candidates: impl Fn() -> I,
-        ty_param_name: &str,
+        ty_param_name: impl Fn() -> String,
         assoc_name: ast::Ident,
         span: Span,
-        is_equality: Option<String>,
+        is_equality: impl Fn() -> Option<String>,
     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorReported>
     where
         I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
@@ -1908,7 +1908,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             None => {
                 self.complain_about_assoc_type_not_found(
                     all_candidates,
-                    ty_param_name,
+                    &ty_param_name(),
                     assoc_name,
                     span,
                 );
@@ -1921,6 +1921,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         if let Some(bound2) = matching_candidates.next() {
             debug!("one_bound_for_assoc_type: bound2 = {:?}", bound2);
 
+            let is_equality = is_equality();
             let bounds = iter::once(bound).chain(iter::once(bound2)).chain(matching_candidates);
             let mut err = if is_equality.is_some() {
                 // More specific Error Index entry.
@@ -1930,7 +1931,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     E0222,
                     "ambiguous associated type `{}` in bounds of `{}`",
                     assoc_name,
-                    ty_param_name
+                    ty_param_name()
                 )
             } else {
                 struct_span_err!(
@@ -1939,7 +1940,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     E0221,
                     "ambiguous associated type `{}` in bounds of `{}`",
                     assoc_name,
-                    ty_param_name
+                    ty_param_name()
                 )
             };
             err.span_label(span, format!("ambiguous associated type `{}`", assoc_name));
@@ -1977,7 +1978,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                             "use fully qualified syntax to disambiguate",
                             format!(
                                 "<{} as {}>::{}",
-                                ty_param_name,
+                                ty_param_name(),
                                 bound.print_only_trait_path(),
                                 assoc_name,
                             ),
@@ -1987,7 +1988,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 } else {
                     err.note(&format!(
                         "associated type `{}` could derive from `{}`",
-                        ty_param_name,
+                        ty_param_name(),
                         bound.print_only_trait_path(),
                     ));
                 }
@@ -1996,7 +1997,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 err.help(&format!(
                     "consider introducing a new type parameter `T` and adding `where` constraints:\
                      \n    where\n        T: {},\n{}",
-                    ty_param_name,
+                    ty_param_name(),
                     where_bounds.join(",\n"),
                 ));
             }
@@ -2110,10 +2111,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
                 self.one_bound_for_assoc_type(
                     || traits::supertraits(tcx, ty::Binder::bind(trait_ref)),
-                    "Self",
+                    || "Self".to_string(),
                     assoc_ident,
                     span,
-                    None,
+                    || None,
                 )?
             }
             (&ty::Param(_), Res::SelfTy(Some(param_did), None))