Make dedicated recovery for missing braces on closure with return

compiler/rustc_parse/src/parser/expr.rs

@@ -2329,7 +2329,8 @@ impl<'a> Parser<'a> {
         let capture_clause = self.parse_capture_clause()?;
         let (fn_decl, fn_arg_span) = self.parse_fn_block_decl()?;
         let decl_hi = self.prev_token.span;
-        let mut body = match fn_decl.output {
+        let mut body = match &fn_decl.output {
+            // No return type.
             FnRetTy::Default(_) => {
                 let restrictions =
                     self.restrictions - Restrictions::STMT_EXPR - Restrictions::ALLOW_LET;
@@ -2341,11 +2342,8 @@ impl<'a> Parser<'a> {
                     Err(err) => self.recover_closure_body(err, before, prev, token, lo, decl_hi)?,
                 }
             }
-            _ => {
+            // Explicit return type (`->`) needs block `-> T { }`.
-                // If an explicit return type is given, require a block to appear (RFC 968).
+            FnRetTy::Ty(ty) => self.parse_closure_block_body(ty.span)?,
-                let body_lo = self.token.span;
-                self.parse_expr_block(None, body_lo, BlockCheckMode::Default)?
-            }
         };
 
         match coroutine_kind {
@@ -2397,6 +2395,49 @@ impl<'a> Parser<'a> {
         Ok(closure)
     }
 
+    /// If an explicit return type is given, require a block to appear (RFC 968).
+    fn parse_closure_block_body(&mut self, ret_span: Span) -> PResult<'a, P<Expr>> {
+        if self.may_recover()
+            && self.token.can_begin_expr()
+            && !matches!(self.token.kind, TokenKind::OpenDelim(Delimiter::Brace))
+            && !self.token.is_whole_block()
+        {
+            let snapshot = self.create_snapshot_for_diagnostic();
+            let restrictions =
+                self.restrictions - Restrictions::STMT_EXPR - Restrictions::ALLOW_LET;
+            let tok = self.token.clone();
+            match self.parse_expr_res(restrictions, AttrWrapper::empty()) {
+                Ok((expr, _)) => {
+                    let descr = super::token_descr(&tok);
+                    let mut diag = self
+                        .dcx()
+                        .struct_span_err(tok.span, format!("expected `{{`, found {descr}"));
+                    diag.span_label(
+                        ret_span,
+                        "explicit return type requires closure body to be enclosed in braces",
+                    );
+                    diag.multipart_suggestion_verbose(
+                        "wrap the expression in curly braces",
+                        vec![
+                            (expr.span.shrink_to_lo(), "{ ".to_string()),
+                            (expr.span.shrink_to_hi(), " }".to_string()),
+                        ],
+                        Applicability::MachineApplicable,
+                    );
+                    diag.emit();
+                    return Ok(expr);
+                }
+                Err(diag) => {
+                    diag.cancel();
+                    self.restore_snapshot(snapshot);
+                }
+            }
+        }
+
+        let body_lo = self.token.span;
+        self.parse_expr_block(None, body_lo, BlockCheckMode::Default)
+    }
+
     /// Parses an optional `move` or `use` prefix to a closure-like construct.
     fn parse_capture_clause(&mut self) -> PResult<'a, CaptureBy> {
         if self.eat_keyword(exp!(Move)) {

tests/ui/parser/closure-return-syntax.stderr

@@ -2,9 +2,11 @@ error: expected `{`, found `22`
   --> $DIR/closure-return-syntax.rs:5:23
    |
 LL |     let x = || -> i32 22;
-   |                       ^^ expected `{`
+   |                   --- ^^
+   |                   |
+   |                   explicit return type requires closure body to be enclosed in braces
    |
-help: you might have meant to write this as part of a block
+help: wrap the expression in curly braces
    |
 LL |     let x = || -> i32 { 22 };
    |                       +    +
@@ -13,9 +15,11 @@ error: expected `{`, found `(`
   --> $DIR/closure-return-syntax.rs:12:34
    |
 LL |         let x = || -> (i32, i32) (1, 2);
-   |                                  ^ expected `{`
+   |                       ---------- ^
+   |                       |
+   |                       explicit return type requires closure body to be enclosed in braces
    |
-help: you might have meant to write this as part of a block
+help: wrap the expression in curly braces
    |
 LL |         let x = || -> (i32, i32) { (1, 2) };
    |                                  +        +
@@ -24,9 +28,11 @@ error: expected `{`, found `[`
   --> $DIR/closure-return-syntax.rs:17:32
    |
 LL |         let c = || -> [i32; 2] [1, 2];
-   |                                ^ expected `{`
+   |                       -------- ^
+   |                       |
+   |                       explicit return type requires closure body to be enclosed in braces
    |
-help: you might have meant to write this as part of a block
+help: wrap the expression in curly braces
    |
 LL |         let c = || -> [i32; 2] { [1, 2] };
    |                                +        +