ensure RET assignments do not get propagated on unwinding

tests/fail/function_calls/return_pointer_aliasing2.rs

@@ -1,4 +1,6 @@
-//@compile-flags: -Zmiri-tree-borrows
+// This does need an aliasing model.
+//@revisions: stack tree
+//@[tree]compile-flags: -Zmiri-tree-borrows
 #![feature(raw_ref_op)]
 #![feature(core_intrinsics)]
 #![feature(custom_mir)]
@@ -25,6 +27,7 @@ pub fn main() {
 fn myfun(ptr: *mut i32) -> i32 {
     // This overwrites the return place, which shouldn't be possible through another pointer.
     unsafe { ptr.write(0) };
-    //~^ ERROR: /write access .* forbidden/
+    //~[stack]^ ERROR: tag does not exist in the borrow stack
+    //~[tree]| ERROR: /write access .* forbidden/
     13
 }

tests/fail/function_calls/return_pointer_aliasing2.stack.stderr

@@ -0,0 +1,40 @@
+error: Undefined Behavior: attempting a write access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+  --> $DIR/return_pointer_aliasing2.rs:LL:CC
+   |
+LL |     unsafe { ptr.write(0) };
+   |              ^^^^^^^^^^^^
+   |              |
+   |              attempting a write access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |              this error occurs as part of an access at ALLOC[0x0..0x4]
+   |
+   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
+   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
+help: <TAG> was created by a SharedReadWrite retag at offsets [0x0..0x4]
+  --> $DIR/return_pointer_aliasing2.rs:LL:CC
+   |
+LL | /     mir! {
+LL | |         {
+LL | |             let _x = 0;
+LL | |             let ptr = &raw mut _x;
+...  |
+LL | |         }
+LL | |     }
+   | |_____^
+help: <TAG> was later invalidated at offsets [0x0..0x4] by a Unique in-place function argument/return passing protection
+  --> $DIR/return_pointer_aliasing2.rs:LL:CC
+   |
+LL |     unsafe { ptr.write(0) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: BACKTRACE (of the first span):
+   = note: inside `myfun` at $DIR/return_pointer_aliasing2.rs:LL:CC
+note: inside `main`
+  --> $DIR/return_pointer_aliasing2.rs:LL:CC
+   |
+LL |             Call(_x = myfun(ptr), after_call)
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `::core::intrinsics::mir::__internal_remove_let` which comes from the expansion of the macro `mir` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

tests/fail/function_calls/return_pointer_on_unwind.rs

@@ -0,0 +1,55 @@
+// Doesn't need an aliasing model.
+//@compile-flags: -Zmiri-disable-stacked-borrows
+#![feature(raw_ref_op)]
+#![feature(core_intrinsics)]
+#![feature(custom_mir)]
+
+use std::intrinsics::mir::*;
+use std::panic;
+
+#[repr(C)]
+struct S(i32, [u8; 128]);
+
+#[custom_mir(dialect = "runtime", phase = "optimized")]
+fn docall(out: &mut S) {
+    mir! {
+        {
+            Call(*out = callee(), after_call)
+        }
+
+        after_call = {
+            Return()
+        }
+    }
+}
+
+fn startpanic() -> () {
+    panic!()
+}
+
+#[custom_mir(dialect = "runtime", phase = "optimized")]
+fn callee() -> S {
+    mir! {
+        type RET = S;
+        let _unit: ();
+        {
+            // We test whether changes done to RET before unwinding
+            // become visible to the outside. In codegen we can see them
+            // but Miri should detect this as UB!
+            RET.0 = 42;
+            Call(_unit = startpanic(), after_call)
+        }
+
+        after_call = {
+            Return()
+        }
+    }
+}
+
+fn main() {
+    let mut x = S(0, [0; 128]);
+    panic::catch_unwind(panic::AssertUnwindSafe(|| docall(&mut x))).unwrap_err();
+    // The return place got de-initialized before the call and assigning to RET
+    // does not propagate if we do not reach the `Return`.
+    dbg!(x.0); //~ERROR: uninitialized
+}

tests/fail/function_calls/return_pointer_on_unwind.stderr

@@ -0,0 +1,19 @@
+thread 'main' panicked at $DIR/return_pointer_on_unwind.rs:LL:CC:
+explicit panic
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
+  --> $DIR/return_pointer_on_unwind.rs:LL:CC
+   |
+LL |     dbg!(x.0);
+   |     ^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at RUSTLIB/std/src/macros.rs:LL:CC
+   = note: this error originates in the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

tests/pass/calls.rs

@@ -34,10 +34,26 @@ fn const_fn_call() -> i64 {
     x
 }
 
+fn call_return_into_passed_reference() {
+    pub fn func<T>(v: &mut T, f: fn(&T) -> T) {
+        // MIR building will introduce a temporary, so this becomes
+        // `let temp = f(v); *v = temp;`.
+        // If this got optimized to `*v = f(v)` on the MIR level we'd have UB
+        // since the return place may not be observed while the function runs!
+        *v = f(v);
+    }
+
+    let mut x = 0;
+    func(&mut x, |v| v + 1);
+    assert_eq!(x, 1);
+}
+
 fn main() {
     assert_eq!(call(), 2);
     assert_eq!(factorial_recursive(), 3628800);
     assert_eq!(call_generic(), (42, true));
     assert_eq!(cross_crate_fn_call(), 1);
     assert_eq!(const_fn_call(), 11);
+
+    call_return_into_passed_reference();
 }