Auto merge of #45853 - nikomatsakis:chalk-simplify-hr-lub-glb, r=arielb1

Simplify higher-ranked LUB/GLB

This is a better version of https://github.com/rust-lang/rust/pull/44211. It still makes higher-ranked LUB/GLB into a hard equality test, however, it does try to identify that something changed and issue a notice to the user. I wroteup https://github.com/rust-lang/rust/issues/45852 as a tracking issue for this change.

Currently, this moves straight to a hard-error, on the basis that the crater run in #44211 saw no impact. It might be good to retest -- or perhaps to try for a warning period. Trying to do the latter in a precise way would be somewhat painful, but an imprecise way might suffice -- that is, we could issue warning *whenever* a LUB/GLB operation succeeds that will later fail, even if it doesn't ultimately impact the type check. I could experiment with this.

~~I am *mildly* wary about landing this independently of other code that moves to a universe-based system. In particular, I was nervous that this change would make coherence accepts new pairs of impls that will later be errors. I have the code for the universe-based approach available, I hope to open an PR and run some tests on its impact very shortly.~~ @arielb1 points out that I was being silly.

r? @arielb1

src/test/run-pass/lub-glb-with-unbound-infer-var.rs

@@ -0,0 +1,24 @@
+// 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.
+
+// Test for a specific corner case: when we compute the LUB of two fn
+// types and their parameters have unbound variables. In that case, we
+// wind up relating those two variables. This was causing an ICE in an
+// in-progress PR.
+
+fn main() {
+    let a_f: fn(_) = |_| ();
+    let b_f: fn(_) = |_| ();
+    let c_f = match 22 {
+        0 => a_f,
+        _ => b_f,
+    };
+    c_f(4);
+}

src/test/ui/lub-glb/old-lub-glb-hr.rs

@@ -0,0 +1,36 @@
+// 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.
+
+// Test that we give a note when the old LUB/GLB algorithm would have
+// succeeded but the new code (which is stricter) gives an error.
+
+fn foo(
+    x: fn(&u8, &u8),
+    y: for<'a> fn(&'a u8, &'a u8),
+) {
+    let z = match 22 {
+        0 => x,
+        _ => y,
+    };
+}
+
+fn bar(
+    x: fn(&u8, &u8),
+    y: for<'a> fn(&'a u8, &'a u8),
+) {
+    let z = match 22 {
+        // No error with an explicit cast:
+        0 => x as for<'a> fn(&'a u8, &'a u8),
+        _ => y,
+    };
+}
+
+fn main() {
+}

src/test/ui/lub-glb/old-lub-glb-hr.stderr

@@ -0,0 +1,22 @@
+error[E0308]: match arms have incompatible types
+  --> $DIR/old-lub-glb-hr.rs:18:13
+   |
+18 |       let z = match 22 {
+   |  _____________^
+19 | |         0 => x,
+20 | |         _ => y,
+21 | |     };
+   | |_____^ expected bound lifetime parameter, found concrete lifetime
+   |
+   = note: expected type `for<'r, 's> fn(&'r u8, &'s u8)`
+              found type `for<'a> fn(&'a u8, &'a u8)`
+   = note: this was previously accepted by the compiler but has been phased out
+   = note: for more information, see https://github.com/rust-lang/rust/issues/45852
+note: match arm with an incompatible type
+  --> $DIR/old-lub-glb-hr.rs:20:14
+   |
+20 |         _ => y,
+   |              ^
+
+error: aborting due to previous error
+

src/test/ui/lub-glb/old-lub-glb-object.rs

@@ -0,0 +1,38 @@
+// 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.
+
+// Test that we give a note when the old LUB/GLB algorithm would have
+// succeeded but the new code (which is stricter) gives an error.
+
+trait Foo<T, U> { }
+
+fn foo(
+    x: &for<'a, 'b> Foo<&'a u8, &'b u8>,
+    y: &for<'a> Foo<&'a u8, &'a u8>,
+) {
+    let z = match 22 {
+        0 => x,
+        _ => y,
+    };
+}
+
+fn bar(
+    x: &for<'a, 'b> Foo<&'a u8, &'b u8>,
+    y: &for<'a> Foo<&'a u8, &'a u8>,
+) {
+    // Accepted with explicit case:
+    let z = match 22 {
+        0 => x as &for<'a> Foo<&'a u8, &'a u8>,
+        _ => y,
+    };
+}
+
+fn main() {
+}

src/test/ui/lub-glb/old-lub-glb-object.stderr

@@ -0,0 +1,22 @@
+error[E0308]: match arms have incompatible types
+  --> $DIR/old-lub-glb-object.rs:20:13
+   |
+20 |       let z = match 22 {
+   |  _____________^
+21 | |         0 => x,
+22 | |         _ => y,
+23 | |     };
+   | |_____^ expected bound lifetime parameter 'a, found concrete lifetime
+   |
+   = note: expected type `&for<'a, 'b> Foo<&'a u8, &'b u8>`
+              found type `&for<'a> Foo<&'a u8, &'a u8>`
+   = note: this was previously accepted by the compiler but has been phased out
+   = note: for more information, see https://github.com/rust-lang/rust/issues/45852
+note: match arm with an incompatible type
+  --> $DIR/old-lub-glb-object.rs:22:14
+   |
+22 |         _ => y,
+   |              ^
+
+error: aborting due to previous error
+