Fix control flow check for breaking with diverging values

compiler/rustc_typeck/src/check/expr.rs

@@ -627,7 +627,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 assert!(expr_opt.is_none() || self.tcx.sess.has_errors());
             }
 
-            ctxt.may_break = true;
+            // If we encountered a `break`, then (no surprise) it may be possible to break from the
+            // loop... unless the value being returned from the loop diverges itself, e.g.
+            // `break return 5` or `break loop {}`.
+            ctxt.may_break |= !e_ty.is_never();
 
             // the type of a `break` is always `!`, since it diverges
             tcx.types.never

src/test/ui/break-diverging-value.rs

@@ -0,0 +1,13 @@
+fn loop_break_return() -> i32 {
+    let loop_value = loop { break return 0 }; // ok
+}
+
+fn loop_break_loop() -> i32 {
+    let loop_value = loop { break loop {} }; // ok
+}
+
+fn loop_break_break() -> i32 { //~ ERROR mismatched types
+    let loop_value = loop { break break };
+}
+
+fn main() {}

src/test/ui/break-diverging-value.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/break-diverging-value.rs:9:26
+   |
+LL | fn loop_break_break() -> i32 {
+   |    ----------------      ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.