Auto merge of #16061 - Veykril:vec-completion, r=Veykril

fix: Fix fragment parser replacing matches with dummies on incomplete parses

Notably, this caused some completions in the `vec!` macro to no longer work. Fixes rust-lang/rust-analyzer#15016

crates/hir-def/src/macro_expansion_tests/mbe/regression.rs

@@ -13,37 +13,97 @@ fn test_vec() {
     check(
         r#"
 macro_rules! vec {
-   ($($item:expr),*) => {{
-           let mut v = Vec::new();
-           $( v.push($item); )*
-           v
-    }};
+    () => (
+        $crate::__rust_force_expr!($crate::vec::Vec::new())
+    );
+    ($elem:expr; $n:expr) => (
+        $crate::__rust_force_expr!($crate::vec::from_elem($elem, $n))
+    );
+    ($($x:expr),+ $(,)?) => (
+        $crate::__rust_force_expr!(<[_]>::into_vec(
+            // This rustc_box is not required, but it produces a dramatic improvement in compile
+            // time when constructing arrays with many elements.
+            #[rustc_box]
+            $crate::boxed::Box::new([$($x),+])
+        ))
+    );
+}
+
+macro_rules! __rust_force_expr {
+    ($e:expr) => {
+        $e
+    };
 }
+
 fn main() {
     vec!();
     vec![1u32,2];
+    vec![a.];
 }
 "#,
         expect![[r#"
 macro_rules! vec {
-   ($($item:expr),*) => {{
-           let mut v = Vec::new();
-           $( v.push($item); )*
-           v
-    }};
+    () => (
+        $crate::__rust_force_expr!($crate::vec::Vec::new())
+    );
+    ($elem:expr; $n:expr) => (
+        $crate::__rust_force_expr!($crate::vec::from_elem($elem, $n))
+    );
+    ($($x:expr),+ $(,)?) => (
+        $crate::__rust_force_expr!(<[_]>::into_vec(
+            // This rustc_box is not required, but it produces a dramatic improvement in compile
+            // time when constructing arrays with many elements.
+            #[rustc_box]
+            $crate::boxed::Box::new([$($x),+])
+        ))
+    );
+}
+
+macro_rules! __rust_force_expr {
+    ($e:expr) => {
+        $e
+    };
 }
+
 fn main() {
-     {
-        let mut v = Vec::new();
-        v
+    $crate::__rust_force_expr!($crate:: vec:: Vec:: new());
+    $crate::__rust_force_expr!(<[_]>:: into_vec(#[rustc_box]$crate:: boxed:: Box:: new([1u32, 2])));
+    /* error: expected Expr */$crate::__rust_force_expr!($crate:: vec:: from_elem((a.), $n));
+}
+"#]],
+    );
+    // FIXME we should ahev testing infra for multi level expansion tests
+    check(
+        r#"
+macro_rules! __rust_force_expr {
+    ($e:expr) => {
+        $e
     };
-     {
-        let mut v = Vec::new();
-        v.push(1u32);
-        v.push(2);
-        v
+}
+
+fn main() {
+    __rust_force_expr!(crate:: vec:: Vec:: new());
+    __rust_force_expr!(<[_]>:: into_vec(#[rustc_box] crate:: boxed:: Box:: new([1u32, 2])));
+    __rust_force_expr/*+errors*/!(crate:: vec:: from_elem((a.), $n));
+}
+"#,
+        expect![[r#"
+macro_rules! __rust_force_expr {
+    ($e:expr) => {
+        $e
     };
 }
+
+fn main() {
+    (crate ::vec::Vec::new());
+    (<[_]>::into_vec(#[rustc_box] crate ::boxed::Box::new([1u32, 2])));
+    /* error: expected Expr *//* parse error: expected field name or number */
+/* parse error: expected expression */
+/* parse error: expected R_PAREN */
+/* parse error: expected COMMA */
+/* parse error: expected expression, item or let statement */
+(crate ::vec::from_elem((a.), $n));
+}
 "#]],
     );
 }

crates/mbe/src/tt_iter.rs

@@ -131,7 +131,6 @@ impl<'a, S: Span> TtIter<'a, S> {
         let buffer = tt::buffer::TokenBuffer::from_tokens(self.inner.as_slice());
         let parser_input = to_parser_input(&buffer);
         let tree_traversal = entry_point.parse(&parser_input);
-
         let mut cursor = buffer.begin();
         let mut error = false;
         for step in tree_traversal.iter() {
@@ -163,12 +162,10 @@ impl<'a, S: Span> TtIter<'a, S> {
         let mut curr = buffer.begin();
         let mut res = vec![];
 
-        if cursor.is_root() {
-            while curr != cursor {
-                let Some(token) = curr.token_tree() else { break };
-                res.push(token.cloned());
-                curr = curr.bump();
-            }
+        while curr != cursor {
+            let Some(token) = curr.token_tree() else { break };
+            res.push(token.cloned());
+            curr = curr.bump();
         }
 
         self.inner = self.inner.as_slice()[res.len()..].iter();