Fail typecheck if we encounter a bogus break

Lone breaks outside of loops create errors in the
loop check pass but as they are not fatal,
compilation continues.

MIR building code assumes all HIR break statements
to point to valid locations and fires ICEs if this
assumption is violated. In normal compilation,
this causes no issues, as code apparently prevents
MIR from being built if errors are present.

However, before that, typecheck runs and with it
MIR const eval. Here we operate differently
from normal compilation: it doesn't check for any
errors except for type checker ones and then
directly builds the MIR.

This constellation causes an ICE-on-error if
bogus break statements are being put into array
length expressions.

This commit fixes this ICE by letting typecheck
fail if bogus break statements are encountered.
This way, MIR const eval fails cleanly with a
type check error.

Fixes rust-lang#50576
Fixes rust-lang#50581

Author: est31

src/librustc_typeck/check/mod.rs

@@ -3764,6 +3764,9 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                   }
 
                   ctxt.may_break = true;
+
+                  // the type of a `break` is always `!`, since it diverges
+                  tcx.types.never
               } else {
                   // Otherwise, we failed to find the enclosing loop;
                   // this can only happen if the `break` was not
@@ -3784,10 +3787,10 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                           }
                       }
                   }
+                  // There was an error, make typecheck fail
+                  tcx.types.err
               }
 
-              // the type of a `break` is always `!`, since it diverges
-              tcx.types.never
           }
           hir::ExprAgain(_) => { tcx.types.never }
           hir::ExprRet(ref expr_opt) => {

src/test/compile-fail/issue-43162.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 fn foo() -> bool {
+    //~^ ERROR E0308
     break true; //~ ERROR E0268
 }
 

src/test/ui/issue-50576.rs

@@ -0,0 +1,16 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    |bool: [u8; break 'L]| 0;
+    //~^ ERROR [E0426]
+    //~| ERROR [E0268]
+    Vec::<[u8; break]>::new(); //~ ERROR [E0268]
+}

src/test/ui/issue-50576.stderr

@@ -0,0 +1,22 @@
+error[E0426]: use of undeclared label `'L`
+  --> $DIR/issue-50576.rs:12:23
+   |
+LL |     |bool: [u8; break 'L]| 0;
+   |                       ^^ undeclared label `'L`
+
+error[E0268]: `break` outside of loop
+  --> $DIR/issue-50576.rs:12:17
+   |
+LL |     |bool: [u8; break 'L]| 0;
+   |                 ^^^^^^^^ cannot break outside of a loop
+
+error[E0268]: `break` outside of loop
+  --> $DIR/issue-50576.rs:15:16
+   |
+LL |     Vec::<[u8; break]>::new(); //~ ERROR [E0268]
+   |                ^^^^^ cannot break outside of a loop
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0268, E0426.
+For more information about an error, try `rustc --explain E0268`.

src/test/ui/issue-50581.rs

@@ -0,0 +1,13 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    |_: [u8; break]| (); //~ ERROR [E0268]
+}

src/test/ui/issue-50581.stderr

@@ -0,0 +1,9 @@
+error[E0268]: `break` outside of loop
+  --> $DIR/issue-50581.rs:12:14
+   |
+LL |     |_: [u8; break]| (); //~ ERROR [E0268]
+   |              ^^^^^ cannot break outside of a loop
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0268`.