Auto merge of #45316 - goffrie:exitable-breakable-block, r=nikomatsakis

Mark block exits as reachable if the block can break.

This only happens when desugaring `catch` expressions for now, but regular blocks (in HIR) can be broken from - respect that when doing reachability analysis.

Fixes #45124.

src/librustc/hir/lowering.rs

@@ -2547,7 +2547,7 @@ impl<'a> LoweringContext<'a> {
                 };
 
                 // Err(err) => #[allow(unreachable_code)]
-                //             return Carrier::from_error(From::from(err)),
+                //             return Try::from_error(From::from(err)),
                 let err_arm = {
                     let err_ident = self.str_to_ident("err");
                     let err_local = self.pat_ident(e.span, err_ident);

src/librustc_typeck/check/mod.rs

@@ -4291,6 +4291,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             CoerceMany::with_coercion_sites(coerce_to_ty, tail_expr)
         };
 
+        let prev_diverges = self.diverges.get();
         let ctxt = BreakableCtxt {
             coerce: Some(coerce),
             may_break: false,
@@ -4340,6 +4341,12 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             }
         });
 
+        if ctxt.may_break {
+            // If we can break from the block, then the block's exit is always reachable
+            // (... as long as the entry is reachable) - regardless of the tail of the block.
+            self.diverges.set(prev_diverges);
+        }
+
         let mut ty = ctxt.coerce.unwrap().complete(self);
 
         if self.has_errors.get() || ty.references_error() {

src/test/run-pass/issue-45124.rs

@@ -0,0 +1,24 @@
+// Copyright 2017 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.
+
+#![feature(catch_expr)]
+
+fn main() {
+    let mut a = 0;
+    let () = {
+        let _: Result<(), ()> = do catch {
+            let _ = Err(())?;
+            return
+        };
+        a += 1;
+    };
+    a += 2;
+    assert_eq!(a, 3);
+}