Rollup merge of #39009 - canndrew:default-unit-warnings, r=nikomatsakis

Add warning for () to ! switch

With feature(never_type) enabled diverging type variables will default to `!` instead of `()`. This can cause breakages where a trait is resolved on such a type.

This PR emits a future-compatibility warning when it sees this happen.

1 files changed, 51 insertions, 0 deletions

diff --git a/src/test/compile-fail/defaulted-unit-warning.rs b/src/test/compile-fail/defaulted-unit-warning.rs
new file mode 100644
index 00000000000..5213a189714
--- /dev/null
+++ b/src/test/compile-fail/defaulted-unit-warning.rs
@@ -0,0 +1,51 @@
+// Copyright 2016 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.
+
+#![allow(dead_code)]
+#![allow(unreachable_code)]
+#![deny(resolve_trait_on_defaulted_unit)]
+
+trait Deserialize: Sized {
+    fn deserialize() -> Result<Self, String>;
+}
+
+impl Deserialize for () {
+    fn deserialize() -> Result<(), String> {
+        Ok(())
+    }
+}
+
+fn doit() -> Result<(), String> {
+    let _ = match Deserialize::deserialize() {
+        //~^ ERROR code relies on type
+        //~| WARNING previously accepted
+        Ok(x) => x,
+        Err(e) => return Err(e),
+    };
+    Ok(())
+}
+
+trait ImplementedForUnitButNotNever {}
+
+impl ImplementedForUnitButNotNever for () {}
+
+fn foo<T: ImplementedForUnitButNotNever>(_t: T) {}
+
+fn smeg() {
+    let _x = return;
+    foo(_x);
+    //~^ ERROR code relies on type
+    //~| WARNING previously accepted
+}
+
+fn main() {
+    let _ = doit();
+}
+