Auto merge of #45294 - petrochenkov:prioplus, r=nikomatsakis

syntax: Lower priority of `+` in `impl Trait`/`dyn Trait`

Now you have to write `Fn() -> (impl A + B)` instead of `Fn() -> impl A + B`, this is consistent with priority of `+` in trait objects (`Fn() -> A + B` means `(Fn() -> A) + B`).

To make this viable I changed the syntax to also permit `+` in return types in function declarations
```
fn f() -> dyn A + B { ... } // OK, don't have to write `-> (dyn A + B)`

// This is acceptable, because `dyn A + B` here is an isolated type and
// not part of a larger type with various operator priorities in play
// like `dyn A + B` in `Fn() -> dyn A + B` despite syntax similarities.
```
but you still have to use `-> (dyn A + B)` in function types and function-like trait object types (see this PR's tests for examples).

This can be a breaking change for code using `impl Trait` on nightly. The thing that is most likely to break is `&impl A + B`, it needs to be rewritten as `&(impl A + B)`.

cc https://github.com/rust-lang/rust/issues/34511 https://github.com/rust-lang/rust/issues/44662 https://github.com/rust-lang/rfcs/pull/438

src/test/parse-fail/closure-return-syntax.rs

@@ -13,5 +13,5 @@
 
 fn main() {
     let x = || -> i32 22;
-    //~^ ERROR expected one of `!`, `(`, `::`, `<`, or `{`, found `22`
+    //~^ ERROR expected one of `!`, `(`, `+`, `::`, `<`, or `{`, found `22`
 }

src/test/parse-fail/issue-24780.rs

@@ -15,6 +15,6 @@
 // compile-flags: -Z parse-only
 
 fn foo() -> Vec<usize>> {
-    //~^ ERROR expected one of `!`, `::`, `where`, or `{`, found `>`
+    //~^ ERROR expected one of `!`, `+`, `::`, `where`, or `{`, found `>`
     Vec::new()
 }

src/test/ui/impl-trait/impl-trait-plus-priority.rs

@@ -0,0 +1,59 @@
+// 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.
+
+// compile-flags: -Z parse-only -Z continue-parse-after-error
+
+fn f() -> impl A + {} // OK
+fn f() -> impl A + B {} // OK
+fn f() -> dyn A + B {} // OK
+fn f() -> A + B {} // OK
+
+impl S {
+    fn f(self) -> impl A + { // OK
+        let _ = |a, b| -> impl A + {}; // OK
+    }
+    fn f(self) -> impl A + B { // OK
+        let _ = |a, b| -> impl A + B {}; // OK
+    }
+    fn f(self) -> dyn A + B { // OK
+        let _ = |a, b| -> dyn A + B {}; // OK
+    }
+    fn f(self) -> A + B { // OK
+        let _ = |a, b| -> A + B {}; // OK
+    }
+}
+
+type A = fn() -> impl A +;
+//~^ ERROR ambiguous `+` in a type
+type A = fn() -> impl A + B;
+//~^ ERROR ambiguous `+` in a type
+type A = fn() -> dyn A + B;
+//~^ ERROR ambiguous `+` in a type
+type A = fn() -> A + B;
+//~^ ERROR expected a path on the left-hand side of `+`, not `fn() -> A`
+
+type A = Fn() -> impl A +;
+//~^ ERROR ambiguous `+` in a type
+type A = Fn() -> impl A + B;
+//~^ ERROR ambiguous `+` in a type
+type A = Fn() -> dyn A + B;
+//~^ ERROR ambiguous `+` in a type
+type A = Fn() -> A + B; // OK, interpreted as `(Fn() -> A) + B` for compatibility
+
+type A = &impl A +;
+//~^ ERROR ambiguous `+` in a type
+type A = &impl A + B;
+//~^ ERROR ambiguous `+` in a type
+type A = &dyn A + B;
+//~^ ERROR ambiguous `+` in a type
+type A = &A + B;
+//~^ ERROR expected a path on the left-hand side of `+`, not `&A`
+
+fn main() {}

src/test/ui/impl-trait/impl-trait-plus-priority.stderr

@@ -0,0 +1,68 @@
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:33:18
+   |
+33 | type A = fn() -> impl A +;
+   |                  ^^^^^^^^ help: use parentheses to disambiguate: `(impl A)`
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:35:18
+   |
+35 | type A = fn() -> impl A + B;
+   |                  ^^^^^^^^^^ help: use parentheses to disambiguate: `(impl A + B)`
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:37:18
+   |
+37 | type A = fn() -> dyn A + B;
+   |                  ^^^^^^^^^ help: use parentheses to disambiguate: `(dyn A + B)`
+
+error[E0178]: expected a path on the left-hand side of `+`, not `fn() -> A`
+  --> $DIR/impl-trait-plus-priority.rs:39:10
+   |
+39 | type A = fn() -> A + B;
+   |          ^^^^^^^^^^^^^ perhaps you forgot parentheses?
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:42:18
+   |
+42 | type A = Fn() -> impl A +;
+   |                  ^^^^^^^^ help: use parentheses to disambiguate: `(impl A)`
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:44:18
+   |
+44 | type A = Fn() -> impl A + B;
+   |                  ^^^^^^^^^^ help: use parentheses to disambiguate: `(impl A + B)`
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:46:18
+   |
+46 | type A = Fn() -> dyn A + B;
+   |                  ^^^^^^^^^ help: use parentheses to disambiguate: `(dyn A + B)`
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:50:11
+   |
+50 | type A = &impl A +;
+   |           ^^^^^^^^ help: use parentheses to disambiguate: `(impl A)`
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:52:11
+   |
+52 | type A = &impl A + B;
+   |           ^^^^^^^^^^ help: use parentheses to disambiguate: `(impl A + B)`
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-trait-plus-priority.rs:54:11
+   |
+54 | type A = &dyn A + B;
+   |           ^^^^^^^^^ help: use parentheses to disambiguate: `(dyn A + B)`
+
+error[E0178]: expected a path on the left-hand side of `+`, not `&A`
+  --> $DIR/impl-trait-plus-priority.rs:56:10
+   |
+56 | type A = &A + B;
+   |          ^^^^^^ help: try adding parentheses: `&(A + B)`
+
+error: aborting due to 11 previous errors
+