Catch an edge case

compiler/rustc_parse/src/lexer/tokentrees.rs

@@ -131,7 +131,11 @@ impl<'a> TokenTreesReader<'a> {
                     diff_errs.push(diff_err);
                 } else if parser.token.is_keyword(kw::If) {
                     in_cond = true;
-                } else if parser.token == token::CloseDelim(Delimiter::Brace) {
+                } else if matches!(
+                    parser.token.kind,
+                    token::CloseDelim(Delimiter::Brace) | token::FatArrow
+                ) {
+                    // end of the `if`/`while` body, or the end of a `match` guard
                     in_cond = false;
                 } else if in_cond && parser.token == token::OpenDelim(Delimiter::Brace) {
                     // Store the `&&` and `let` to use their spans later when creating the diagnostic

tests/ui/parser/brace-in-let-chain.rs

@@ -9,6 +9,15 @@ fn main() {
     }
 }
 
+fn qux() {
+    let foo = false;
+    match foo {
+        _ if foo => {
+            && let () = ()
+        _ => {}
+    }
+}
+
 fn foo() {
     {
     && let () = ()

tests/ui/parser/brace-in-let-chain.stderr

@@ -1,9 +1,12 @@
 error: this file contains an unclosed delimiter
-  --> $DIR/brace-in-let-chain.rs:28:54
+  --> $DIR/brace-in-let-chain.rs:37:54
    |
 LL | fn main() {
    |           - unclosed delimiter
 ...
+LL | fn qux() {
+   |          - unclosed delimiter
+...
 LL | fn foo() {
    |          - unclosed delimiter
 ...