Rollup merge of #67820 - ecstatic-morse:const-trait, r=oli-obk

Parse the syntax described in RFC 2632

This adds support for both `impl const Trait for Ty` and `?const Trait` bound syntax from rust-lang/rfcs#2632 to the parser. For now, both modifiers end up in a newly-added `constness` field on `ast::TraitRef`, although this may change once the implementation is fleshed out.

I was planning on using `delay_span_bug` when this syntax is encountered during lowering, but I can't write `should-ice` UI tests. I emit a normal error instead, which causes duplicates when the feature gate is not enabled (see the `.stderr` files for the feature gate tests). Not sure what the desired approach is; Maybe just do nothing when the syntax is encountered with the feature gate is enabled?

@oli-obk I went with `const_trait_impl` and `const_trait_bound_opt_out` for the names of these features. Are these to your liking?

cc #67792 #67794

r? @Centril

src/test/ui/parser/bounds-type.rs

@@ -8,6 +8,11 @@ struct S<
     T: ?for<'a> Trait, // OK
     T: Tr +, // OK
     T: ?'a, //~ ERROR `?` may only modify trait bounds, not lifetime bounds
+
+    T: ?const Tr, // OK
+    T: ?const ?Tr, // OK
+    T: ?const Tr + 'a, // OK
+    T: ?const 'a, //~ ERROR `?const` may only modify trait bounds, not lifetime bounds
 >;
 
 fn main() {}

src/test/ui/parser/bounds-type.stderr

@@ -4,5 +4,11 @@ error: `?` may only modify trait bounds, not lifetime bounds
 LL |     T: ?'a,
    |        ^
 
-error: aborting due to previous error
+error: `?const` may only modify trait bounds, not lifetime bounds
+  --> $DIR/bounds-type.rs:15:8
+   |
+LL |     T: ?const 'a,
+   |        ^^^^^^
+
+error: aborting due to 2 previous errors
 

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/feature-gate.gated.stderr

@@ -0,0 +1,8 @@
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/feature-gate.rs:11:29
+   |
+LL | const fn get_assoc_const<S: ?const T>() -> i32 { <S as T>::CONST }
+   |                             ^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/feature-gate.rs

@@ -0,0 +1,15 @@
+// revisions: stock gated
+// gate-test-const_trait_bound_opt_out
+
+#![cfg_attr(gated, feature(const_trait_bound_opt_out))]
+#![allow(incomplete_features)]
+
+trait T {
+    const CONST: i32;
+}
+
+const fn get_assoc_const<S: ?const T>() -> i32 { <S as T>::CONST }
+//[stock]~^ ERROR `?const` on trait bounds is experimental
+//[stock,gated]~^^ ERROR `?const` on trait bounds is not yet implemented
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/feature-gate.stock.stderr

@@ -0,0 +1,18 @@
+error[E0658]: `?const` on trait bounds is experimental
+  --> $DIR/feature-gate.rs:11:29
+   |
+LL | const fn get_assoc_const<S: ?const T>() -> i32 { <S as T>::CONST }
+   |                             ^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/67794
+   = help: add `#![feature(const_trait_bound_opt_out)]` to the crate attributes to enable
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/feature-gate.rs:11:29
+   |
+LL | const fn get_assoc_const<S: ?const T>() -> i32 { <S as T>::CONST }
+   |                             ^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/in-impl-trait.rs

@@ -0,0 +1,25 @@
+#![feature(const_trait_bound_opt_out)]
+#![feature(associated_type_bounds)]
+#![allow(incomplete_features)]
+
+trait T {}
+struct S;
+impl T for S {}
+
+fn rpit() -> impl ?const T { S }
+//~^ ERROR `?const` is not permitted in `impl Trait`
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn apit(_: impl ?const T) {}
+//~^ ERROR `?const` is not permitted in `impl Trait`
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn rpit_assoc_bound() -> impl IntoIterator<Item: ?const T> { Some(S) }
+//~^ ERROR `?const` is not permitted in `impl Trait`
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn apit_assoc_bound(_: impl IntoIterator<Item: ?const T>) {}
+//~^ ERROR `?const` is not permitted in `impl Trait`
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/in-impl-trait.stderr

@@ -0,0 +1,50 @@
+error: `?const` is not permitted in `impl Trait`
+  --> $DIR/in-impl-trait.rs:9:19
+   |
+LL | fn rpit() -> impl ?const T { S }
+   |                   ^^^^^^^^
+
+error: `?const` is not permitted in `impl Trait`
+  --> $DIR/in-impl-trait.rs:13:17
+   |
+LL | fn apit(_: impl ?const T) {}
+   |                 ^^^^^^^^
+
+error: `?const` is not permitted in `impl Trait`
+  --> $DIR/in-impl-trait.rs:17:50
+   |
+LL | fn rpit_assoc_bound() -> impl IntoIterator<Item: ?const T> { Some(S) }
+   |                                                  ^^^^^^^^
+
+error: `?const` is not permitted in `impl Trait`
+  --> $DIR/in-impl-trait.rs:21:48
+   |
+LL | fn apit_assoc_bound(_: impl IntoIterator<Item: ?const T>) {}
+   |                                                ^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/in-impl-trait.rs:9:19
+   |
+LL | fn rpit() -> impl ?const T { S }
+   |                   ^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/in-impl-trait.rs:13:17
+   |
+LL | fn apit(_: impl ?const T) {}
+   |                 ^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/in-impl-trait.rs:17:50
+   |
+LL | fn rpit_assoc_bound() -> impl IntoIterator<Item: ?const T> { Some(S) }
+   |                                                  ^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/in-impl-trait.rs:21:48
+   |
+LL | fn apit_assoc_bound(_: impl IntoIterator<Item: ?const T>) {}
+   |                                                ^^^^^^^^
+
+error: aborting due to 8 previous errors
+

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/in-trait-bounds.rs

@@ -0,0 +1,9 @@
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+trait Super {}
+trait T: ?const Super {}
+//~^ ERROR `?const` is not permitted in supertraits
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/in-trait-bounds.stderr

@@ -0,0 +1,14 @@
+error: `?const` is not permitted in supertraits
+  --> $DIR/in-trait-bounds.rs:5:10
+   |
+LL | trait T: ?const Super {}
+   |          ^^^^^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/in-trait-bounds.rs:5:10
+   |
+LL | trait T: ?const Super {}
+   |          ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/in-trait-object.rs

@@ -0,0 +1,22 @@
+#![feature(const_trait_bound_opt_out)]
+#![allow(bare_trait_objects)]
+#![allow(incomplete_features)]
+
+struct S;
+trait T {}
+impl T for S {}
+
+// An inherent impl for the trait object `?const T`.
+impl ?const T {}
+//~^ ERROR `?const` is not permitted in trait objects
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn trait_object() -> &'static dyn ?const T { &S }
+//~^ ERROR `?const` is not permitted in trait objects
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn trait_object_in_apit(_: impl IntoIterator<Item = Box<dyn ?const T>>) {}
+//~^ ERROR `?const` is not permitted in trait objects
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/in-trait-object.stderr

@@ -0,0 +1,38 @@
+error: `?const` is not permitted in trait objects
+  --> $DIR/in-trait-object.rs:10:6
+   |
+LL | impl ?const T {}
+   |      ^^^^^^^^
+
+error: `?const` is not permitted in trait objects
+  --> $DIR/in-trait-object.rs:14:35
+   |
+LL | fn trait_object() -> &'static dyn ?const T { &S }
+   |                                   ^^^^^^^^
+
+error: `?const` is not permitted in trait objects
+  --> $DIR/in-trait-object.rs:18:61
+   |
+LL | fn trait_object_in_apit(_: impl IntoIterator<Item = Box<dyn ?const T>>) {}
+   |                                                             ^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/in-trait-object.rs:10:6
+   |
+LL | impl ?const T {}
+   |      ^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/in-trait-object.rs:14:35
+   |
+LL | fn trait_object() -> &'static dyn ?const T { &S }
+   |                                   ^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/in-trait-object.rs:18:61
+   |
+LL | fn trait_object_in_apit(_: impl IntoIterator<Item = Box<dyn ?const T>>) {}
+   |                                                             ^^^^^^^^
+
+error: aborting due to 6 previous errors
+

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/opt-out-twice.rs

@@ -0,0 +1,8 @@
+// compile-flags: -Z parse-only
+
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+struct S<T: ?const ?const Tr>;
+//~^ ERROR expected identifier, found keyword `const`
+//~| ERROR expected one of `(`, `+`, `,`, `::`, `<`, `=`, or `>`

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/opt-out-twice.stderr

@@ -0,0 +1,14 @@
+error: expected identifier, found keyword `const`
+  --> $DIR/opt-out-twice.rs:6:21
+   |
+LL | struct S<T: ?const ?const Tr>;
+   |                     ^^^^^ expected identifier, found keyword
+
+error: expected one of `(`, `+`, `,`, `::`, `<`, `=`, or `>`, found `Tr`
+  --> $DIR/opt-out-twice.rs:6:27
+   |
+LL | struct S<T: ?const ?const Tr>;
+   |                           ^^ expected one of 7 possible tokens
+
+error: aborting due to 2 previous errors
+

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/syntax.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z parse-only
+// check-pass
+
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+struct S<
+    T: ?const ?for<'a> Tr<'a> + 'static + ?const std::ops::Add,
+    T: ?const ?for<'a: 'b> m::Trait<'a>,
+>;

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/with-maybe-sized.rs

@@ -0,0 +1,8 @@
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+struct S<T: ?const ?Sized>(std::marker::PhantomData<T>);
+//~^ ERROR `?const` and `?` are mutually exclusive
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/with-maybe-sized.stderr

@@ -0,0 +1,14 @@
+error: `?const` and `?` are mutually exclusive
+  --> $DIR/with-maybe-sized.rs:4:13
+   |
+LL | struct S<T: ?const ?Sized>(std::marker::PhantomData<T>);
+   |             ^^^^^^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/with-maybe-sized.rs:4:13
+   |
+LL | struct S<T: ?const ?Sized>(std::marker::PhantomData<T>);
+   |             ^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/without-question-mark.rs

@@ -0,0 +1,7 @@
+// compile-flags: -Z parse-only
+
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+struct S<T: const Tr>;
+//~^ ERROR expected one of `!`, `(`, `,`, `=`, `>`, `?`, `for`, lifetime, or path

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/without-question-mark.stderr

@@ -0,0 +1,8 @@
+error: expected one of `!`, `(`, `,`, `=`, `>`, `?`, `for`, lifetime, or path, found keyword `const`
+  --> $DIR/without-question-mark.rs:6:13
+   |
+LL | struct S<T: const Tr>;
+   |             ^^^^^ expected one of 9 possible tokens
+
+error: aborting due to previous error
+

src/test/ui/rfc-2632-const-trait-impl/feature-gate.gated.stderr

@@ -0,0 +1,8 @@
+error: const trait impls are not yet implemented
+  --> $DIR/feature-gate.rs:9:1
+   |
+LL | impl const T for S {}
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/rfc-2632-const-trait-impl/feature-gate.rs

@@ -0,0 +1,13 @@
+// revisions: stock gated
+// gate-test-const_trait_impl
+
+#![cfg_attr(gated, feature(const_trait_impl))]
+#![allow(incomplete_features)]
+
+struct S;
+trait T {}
+impl const T for S {}
+//[stock]~^ ERROR const trait impls are experimental
+//[stock,gated]~^^ ERROR const trait impls are not yet implemented
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/feature-gate.stock.stderr

@@ -0,0 +1,18 @@
+error[E0658]: const trait impls are experimental
+  --> $DIR/feature-gate.rs:9:6
+   |
+LL | impl const T for S {}
+   |      ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/67792
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
+
+error: const trait impls are not yet implemented
+  --> $DIR/feature-gate.rs:9:1
+   |
+LL | impl const T for S {}
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2632-const-trait-impl/impl-opt-out-trait.rs

@@ -0,0 +1,11 @@
+#![feature(const_trait_bound_opt_out)]
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+struct S;
+trait T {}
+
+impl ?const T for S {}
+//~^ ERROR expected a trait, found type
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/impl-opt-out-trait.stderr

@@ -0,0 +1,8 @@
+error: expected a trait, found type
+  --> $DIR/impl-opt-out-trait.rs:8:6
+   |
+LL | impl ?const T for S {}
+   |      ^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/rfc-2632-const-trait-impl/inherent-impl.rs

@@ -0,0 +1,14 @@
+// compile-flags: -Z parse-only
+
+#![feature(const_trait_impl)]
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+#![allow(bare_trait_objects)]
+
+struct S;
+trait T {}
+
+impl const T {}
+//~^ ERROR `const` cannot modify an inherent impl
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/inherent-impl.stderr

@@ -0,0 +1,10 @@
+error: `const` cannot modify an inherent impl
+  --> $DIR/inherent-impl.rs:11:6
+   |
+LL | impl const T {}
+   |      ^^^^^
+   |
+   = help: only a trait impl can be `const`
+
+error: aborting due to previous error
+

src/test/ui/rfc-2632-const-trait-impl/syntax.rs

@@ -0,0 +1,9 @@
+// compile-flags: -Z parse-only
+// check-pass
+
+#![feature(const_trait_bound_opt_out)]
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+// For now, this parses since an error does not occur until AST lowering.
+impl ?const T {}