Parse loop labels missing a leading '

When encountering the following typo:

```rust
a: loop { break 'a; }
```

provide an appropriate suggestion.

compiler/rustc_parse/src/parser/expr.rs

@@ -585,7 +585,7 @@ fn parse_assoc_op_cast(
         lhs_span: Span,
         expr_kind: fn(P<Expr>, P<Ty>) -> ExprKind,
     ) -> PResult<'a, P<Expr>> {
-        let mk_expr = |this: &mut Self, rhs: P<Ty>| {
+        let mk_expr = |this: &mut Self, lhs: P<Expr>, rhs: P<Ty>| {
             this.mk_expr(
                 this.mk_expr_sp(&lhs, lhs_span, rhs.span),
                 expr_kind(lhs, rhs),
@@ -597,13 +597,49 @@ fn parse_assoc_op_cast(
         // LessThan comparison after this cast.
         let parser_snapshot_before_type = self.clone();
         let cast_expr = match self.parse_ty_no_plus() {
-            Ok(rhs) => mk_expr(self, rhs),
+            Ok(rhs) => mk_expr(self, lhs, rhs),
             Err(mut type_err) => {
                 // Rewind to before attempting to parse the type with generics, to recover
                 // from situations like `x as usize < y` in which we first tried to parse
                 // `usize < y` as a type with generic arguments.
                 let parser_snapshot_after_type = mem::replace(self, parser_snapshot_before_type);
 
+                // Check for typo of `'a: loop { break 'a }` with a missing `'`.
+                match (&lhs.kind, &self.token.kind) {
+                    (
+                        // `foo: `
+                        ExprKind::Path(None, ast::Path { segments, .. }),
+                        TokenKind::Ident(kw::For | kw::Loop | kw::While, false),
+                    ) if segments.len() == 1 => {
+                        let snapshot = self.clone();
+                        let label = Label {
+                            ident: Ident::from_str_and_span(
+                                &format!("'{}", segments[0].ident),
+                                segments[0].ident.span,
+                            ),
+                        };
+                        match self.parse_labeled_expr(label, AttrVec::new(), false) {
+                            Ok(expr) => {
+                                type_err.cancel();
+                                self.struct_span_err(label.ident.span, "malformed loop label")
+                                    .span_suggestion(
+                                        label.ident.span,
+                                        "use the correct loop label format",
+                                        label.ident.to_string(),
+                                        Applicability::MachineApplicable,
+                                    )
+                                    .emit();
+                                return Ok(expr);
+                            }
+                            Err(mut err) => {
+                                err.cancel();
+                                *self = snapshot;
+                            }
+                        }
+                    }
+                    _ => {}
+                }
+
                 match self.parse_path(PathStyle::Expr) {
                     Ok(path) => {
                         let (op_noun, op_verb) = match self.token.kind {
@@ -630,7 +666,8 @@ fn parse_assoc_op_cast(
                             op_noun,
                         );
                         let span_after_type = parser_snapshot_after_type.token.span;
-                        let expr = mk_expr(self, self.mk_ty(path.span, TyKind::Path(None, path)));
+                        let expr =
+                            mk_expr(self, lhs, self.mk_ty(path.span, TyKind::Path(None, path)));
 
                         let expr_str = self
                             .span_to_snippet(expr.span)
@@ -1067,7 +1104,7 @@ fn parse_bottom_expr(&mut self) -> PResult<'a, P<Expr>> {
         } else if self.eat_keyword(kw::While) {
             self.parse_while_expr(None, self.prev_token.span, attrs)
         } else if let Some(label) = self.eat_label() {
-            self.parse_labeled_expr(label, attrs)
+            self.parse_labeled_expr(label, attrs, true)
         } else if self.eat_keyword(kw::Loop) {
             self.parse_loop_expr(None, self.prev_token.span, attrs)
         } else if self.eat_keyword(kw::Continue) {
@@ -1228,7 +1265,12 @@ fn parse_path_start_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
     }
 
     /// Parse `'label: $expr`. The label is already parsed.
-    fn parse_labeled_expr(&mut self, label: Label, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+    fn parse_labeled_expr(
+        &mut self,
+        label: Label,
+        attrs: AttrVec,
+        consume_colon: bool,
+    ) -> PResult<'a, P<Expr>> {
         let lo = label.ident.span;
         let label = Some(label);
         let ate_colon = self.eat(&token::Colon);
@@ -1247,7 +1289,7 @@ fn parse_labeled_expr(&mut self, label: Label, attrs: AttrVec) -> PResult<'a, P<
             self.parse_expr()
         }?;
 
-        if !ate_colon {
+        if !ate_colon && consume_colon {
             self.error_labeled_expr_must_be_followed_by_colon(lo, expr.span);
         }
 

src/test/ui/label/label_misspelled_2.rs

@@ -7,11 +7,10 @@ fn main() {
     'b: for _ in 0..1 {
         break b; //~ ERROR cannot find value `b` in this scope
     }
-    c: for _ in 0..1 { //~ ERROR expected identifier, found keyword `for`
+    c: for _ in 0..1 { //~ ERROR malformed loop label
-        //~^ ERROR expected `<`, found reserved identifier `_`
         break 'c;
     }
-    d: for _ in 0..1 {
+    d: for _ in 0..1 { //~ ERROR malformed loop label
-        break ;
+        break d; //~ ERROR cannot find value `d` in this scope
     }
 }

src/test/ui/label/label_misspelled_2.stderr

@@ -1,19 +1,14 @@
-error: expected identifier, found keyword `for`
+error: malformed loop label
-  --> $DIR/label_misspelled_2.rs:10:8
+  --> $DIR/label_misspelled_2.rs:10:5
    |
 LL |     c: for _ in 0..1 {
-   |        ^^^ expected identifier, found keyword
+   |     ^ help: use the correct loop label format: `'c`
 
-error: expected `<`, found reserved identifier `_`
+error: malformed loop label
-  --> $DIR/label_misspelled_2.rs:10:12
+  --> $DIR/label_misspelled_2.rs:13:5
    |
-LL |     c: for _ in 0..1 {
+LL |     d: for _ in 0..1 {
-   |      -     ^ expected `<`
+   |     ^ help: use the correct loop label format: `'d`
-   |      |
-   |      tried to parse a type due to this type ascription
-   |
-   = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
 
 error[E0425]: cannot find value `b` in this scope
   --> $DIR/label_misspelled_2.rs:8:15
@@ -26,6 +21,17 @@ LL |         break b;
    |               not found in this scope
    |               help: use the similarly named label: `'b`
 
-error: aborting due to 3 previous errors
+error[E0425]: cannot find value `d` in this scope
+  --> $DIR/label_misspelled_2.rs:14:15
+   |
+LL |     d: for _ in 0..1 {
+   |     - a label with a similar name exists
+LL |         break d;
+   |               ^
+   |               |
+   |               not found in this scope
+   |               help: use the similarly named label: `'d`
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0425`.