Emit clearer diagnostics for parens around for loop heads

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1334,30 +1334,25 @@ impl<'a> Parser<'a> {
     pub(super) fn recover_parens_around_for_head(
         &mut self,
         pat: P<Pat>,
-        expr: &Expr,
         begin_paren: Option<Span>,
     ) -> P<Pat> {
         match (&self.token.kind, begin_paren) {
             (token::CloseDelim(token::Paren), Some(begin_par_sp)) => {
                 self.bump();
 
-                let pat_str = self
-                    // Remove the `(` from the span of the pattern:
-                    .span_to_snippet(pat.span.trim_start(begin_par_sp).unwrap())
-                    .unwrap_or_else(|_| pprust::pat_to_string(&pat));
-
-                self.struct_span_err(self.prev_token.span, "unexpected closing `)`")
-                    .span_label(begin_par_sp, "opening `(`")
-                    .span_suggestion(
-                        begin_par_sp.to(self.prev_token.span),
-                        "remove parenthesis in `for` loop",
-                        format!("{} in {}", pat_str, pprust::expr_to_string(&expr)),
-                        // With e.g. `for (x) in y)` this would replace `(x) in y)`
-                        // with `x) in y)` which is syntactically invalid.
-                        // However, this is prevented before we get here.
-                        Applicability::MachineApplicable,
-                    )
-                    .emit();
+                self.struct_span_err(
+                    MultiSpan::from_spans(vec![begin_par_sp, self.prev_token.span]),
+                    "unexpected parenthesis surrounding `for` loop head",
+                )
+                .multipart_suggestion(
+                    "remove parenthesis in `for` loop",
+                    vec![(begin_par_sp, String::new()), (self.prev_token.span, String::new())],
+                    // With e.g. `for (x) in y)` this would replace `(x) in y)`
+                    // with `x) in y)` which is syntactically invalid.
+                    // However, this is prevented before we get here.
+                    Applicability::MachineApplicable,
+                )
+                .emit();
 
                 // Unwrap `(pat)` into `pat` to avoid the `unused_parens` lint.
                 pat.and_then(|pat| match pat.kind {

compiler/rustc_parse/src/parser/expr.rs

@@ -2042,7 +2042,7 @@ impl<'a> Parser<'a> {
         self.check_for_for_in_in_typo(self.prev_token.span);
         let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
 
-        let pat = self.recover_parens_around_for_head(pat, &expr, begin_paren);
+        let pat = self.recover_parens_around_for_head(pat, begin_paren);
 
         let (iattrs, loop_block) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);

src/test/ui/parser/recover-for-loop-parens-around-head.rs

@@ -9,7 +9,7 @@ fn main() {
 
     for ( elem in vec ) {
         //~^ ERROR expected one of `)`, `,`, `@`, or `|`, found keyword `in`
-        //~| ERROR unexpected closing `)`
+        //~| ERROR unexpected parenthesis surrounding `for` loop head
         const RECOVERY_WITNESS: () = 0; //~ ERROR mismatched types
     }
 }

src/test/ui/parser/recover-for-loop-parens-around-head.stderr

@@ -4,14 +4,17 @@ error: expected one of `)`, `,`, `@`, or `|`, found keyword `in`
 LL |     for ( elem in vec ) {
    |                ^^ expected one of `)`, `,`, `@`, or `|`
 
-error: unexpected closing `)`
-  --> $DIR/recover-for-loop-parens-around-head.rs:10:23
+error: unexpected parenthesis surrounding `for` loop head
+  --> $DIR/recover-for-loop-parens-around-head.rs:10:9
    |
 LL |     for ( elem in vec ) {
-   |         --------------^
-   |         |
-   |         opening `(`
-   |         help: remove parenthesis in `for` loop: `elem in vec`
+   |         ^             ^
+   |
+help: remove parenthesis in `for` loop
+   |
+LL -     for ( elem in vec ) {
+LL +     for  elem in vec  {
+   | 
 
 error[E0308]: mismatched types
   --> $DIR/recover-for-loop-parens-around-head.rs:13:38