Enhance error message when misspelled label to value in break expression

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -542,6 +542,26 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                 err.span_label(base_span, fallback_label);
             }
         }
+        if let Some(err_code) = &err.code {
+            if err_code == &rustc_errors::error_code!(E0425) {
+                for label_rib in &self.label_ribs {
+                    for (label_ident, _) in &label_rib.bindings {
+                        if format!("'{}", ident) == label_ident.to_string() {
+                            let msg = "a label with a similar name exists";
+                            // FIXME: consider only emitting this suggestion if a label would be valid here
+                            // which is pretty much only the case for `break` expressions.
+                            err.span_suggestion(
+                                span,
+                                &msg,
+                                label_ident.name.to_string(),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
+                }
+            }
+        }
+
         (err, candidates)
     }
 

src/test/ui/label/label_misspelled.rs

@@ -0,0 +1,18 @@
+fn main() {
+    'LOOP: loop {
+        LOOP;
+        //~^ ERROR cannot find value `LOOP` in this scope
+    };
+    'while_loop: while true { //~ WARN denote infinite loops with
+        while_loop;
+        //~^ ERROR cannot find value `while_loop` in this scope
+    };
+    'while_let: while let Some(_) = Some(()) {
+        while_let;
+        //~^ ERROR cannot find value `while_let` in this scope
+    }
+    'for_loop: for _ in 0..3 {
+        for_loop;
+        //~^ ERROR cannot find value `for_loop` in this scope
+    };
+}

src/test/ui/label/label_misspelled.stderr

@@ -0,0 +1,47 @@
+error[E0425]: cannot find value `LOOP` in this scope
+  --> $DIR/label_misspelled.rs:3:9
+   |
+LL |         LOOP;
+   |         ^^^^
+   |         |
+   |         not found in this scope
+   |         help: a label with a similar name exists: `'LOOP`
+
+error[E0425]: cannot find value `while_loop` in this scope
+  --> $DIR/label_misspelled.rs:7:9
+   |
+LL |         while_loop;
+   |         ^^^^^^^^^^
+   |         |
+   |         not found in this scope
+   |         help: a label with a similar name exists: `'while_loop`
+
+error[E0425]: cannot find value `while_let` in this scope
+  --> $DIR/label_misspelled.rs:11:9
+   |
+LL |         while_let;
+   |         ^^^^^^^^^
+   |         |
+   |         not found in this scope
+   |         help: a label with a similar name exists: `'while_let`
+
+error[E0425]: cannot find value `for_loop` in this scope
+  --> $DIR/label_misspelled.rs:15:9
+   |
+LL |         for_loop;
+   |         ^^^^^^^^
+   |         |
+   |         not found in this scope
+   |         help: a label with a similar name exists: `'for_loop`
+
+warning: denote infinite loops with `loop { ... }`
+  --> $DIR/label_misspelled.rs:6:5
+   |
+LL |     'while_loop: while true {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ help: use `loop`
+   |
+   = note: `#[warn(while_true)]` on by default
+
+error: aborting due to 4 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0425`.

src/test/ui/loops/loop-break-value.rs

@@ -90,4 +90,10 @@ fn main() {
         break; //~ ERROR mismatched types
         break 4;
     };
+
+    'LOOP: for _ in 0 .. 9 {
+        break LOOP;
+        //~^ ERROR cannot find value `LOOP` in this scope
+        //~| ERROR `break` with value from a `for` loop
+    }
 }

src/test/ui/loops/loop-break-value.stderr

@@ -1,3 +1,12 @@
+error[E0425]: cannot find value `LOOP` in this scope
+  --> $DIR/loop-break-value.rs:95:15
+   |
+LL |         break LOOP;
+   |               ^^^^
+   |               |
+   |               not found in this scope
+   |               help: a label with a similar name exists: `'LOOP`
+
 warning: denote infinite loops with `loop { ... }`
   --> $DIR/loop-break-value.rs:26:5
    |
@@ -94,6 +103,17 @@ help: instead, use `break` on its own without a value inside this `for` loop
 LL |             break;
    |             ^^^^^
 
+error[E0571]: `break` with value from a `for` loop
+  --> $DIR/loop-break-value.rs:95:9
+   |
+LL |         break LOOP;
+   |         ^^^^^^^^^^ can only break with a value inside `loop` or breakable block
+   |
+help: instead, use `break` on its own without a value inside this `for` loop
+   |
+LL |         break;
+   |         ^^^^^
+
 error[E0308]: mismatched types
   --> $DIR/loop-break-value.rs:4:31
    |
@@ -151,7 +171,7 @@ LL |         break;
    |         expected integer, found `()`
    |         help: give it a value of the expected type: `break value`
 
-error: aborting due to 16 previous errors; 1 warning emitted
+error: aborting due to 18 previous errors; 1 warning emitted
 
-Some errors have detailed explanations: E0308, E0571.
+Some errors have detailed explanations: E0308, E0425, E0571.
 For more information about an error, try `rustc --explain E0308`.