Rollup merge of #119894 - fmease:tilde-const-assoc-ty-bounds, r=compiler-errors

Allow `~const` on associated type bounds again

This follows from [this Zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/419616-t-compiler.2Fproject-const-traits/topic/projections.20on.20.28~.29const.20Trait.20.26.20.28~.29const.20assoc.20ty.20bounds).

Basically in my opinion, it makes sense to allow `~const` on associated type bounds again since they're quite useful even though we haven't implemented the proposed syntax `<Ty as ~const Trait>::Proj`/`<Ty as const Trait>::Proj` yet; that can happen as a follow-up.

This already allows more code to compile since `T::Assoc` where `T` is a type parameter and where the predicate `<T as ~const Trait>` is in the environment gets elaborated to (pseudo) `<T as ~const Trait>::Assoc`.

```rs
#[const_trait]
trait Trait {
    type Assoc: ~const Trait;
    fn func() -> i32;
}

const fn function<T: ~const Trait>() -> i32 {
    T::Assoc::func()
}
```

`~const` associated type bounds also work together with `const` bounds:

```rs
struct Type<const N: i32>;

fn procedure<T: const Trait>() -> Type<{ T::Assoc::func() }> { // `Trait` comes from above
    Type
}
```

NB: This PR also starts allowing `~const` bounds in the generics and the where-clause of trait associated types since it's trivial to support them. However, I don't know if those bounds are actually useful. Maybe we should continue to reject them?
For reference, it wouldn't make any sense to allow `~const Trait` in GACs (generic associated constants, `generic_const_items`) because they'd be absolutely useless (contrary to `const Trait`).

~~[``@]rustbot`` ping project-const-traits~~
r? project-const-traits

Author: matthiaskrgr

compiler/rustc_ast_passes/messages.ftl

@@ -232,6 +232,9 @@ ast_passes_tilde_const_disallowed = `~const` is not allowed here
     .trait = this trait is not a `#[const_trait]`, so it cannot have `~const` trait bounds
     .trait_impl = this impl is not `const`, so it cannot have `~const` trait bounds
     .impl = inherent impls cannot have `~const` trait bounds
+    .trait_assoc_ty = associated types in non-`#[const_trait]` traits cannot have `~const` trait bounds
+    .trait_impl_assoc_ty = associated types in non-const impls cannot have `~const` trait bounds
+    .inherent_assoc_ty = inherent associated types cannot have `~const` trait bounds
     .object = trait objects cannot have `~const` trait bounds
     .item = this item cannot have `~const` trait bounds
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -37,12 +37,17 @@ enum SelfSemantic {
 }
 
 /// What is the context that prevents using `~const`?
+// FIXME(effects): Consider getting rid of this in favor of `errors::TildeConstReason`, they're
+// almost identical. This gets rid of an abstraction layer which might be considered bad.
 enum DisallowTildeConstContext<'a> {
     TraitObject,
     Fn(FnKind<'a>),
     Trait(Span),
     TraitImpl(Span),
     Impl(Span),
+    TraitAssocTy(Span),
+    TraitImplAssocTy(Span),
+    InherentAssocTy(Span),
     Item,
 }
 
@@ -316,6 +321,7 @@ impl<'a> AstValidator<'a> {
                 constness: Const::No,
                 polarity: ImplPolarity::Positive,
                 trait_ref,
+                ..
             } = parent
         {
             Some(trait_ref.path.span.shrink_to_lo())
@@ -1286,6 +1292,15 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                             // suggestion for moving such bounds to the assoc const fns if available.
                             errors::TildeConstReason::Impl { span }
                         }
+                        &DisallowTildeConstContext::TraitAssocTy(span) => {
+                            errors::TildeConstReason::TraitAssocTy { span }
+                        }
+                        &DisallowTildeConstContext::TraitImplAssocTy(span) => {
+                            errors::TildeConstReason::TraitImplAssocTy { span }
+                        }
+                        &DisallowTildeConstContext::InherentAssocTy(span) => {
+                            errors::TildeConstReason::InherentAssocTy { span }
+                        }
                         DisallowTildeConstContext::TraitObject => {
                             errors::TildeConstReason::TraitObject
                         }
@@ -1483,13 +1498,12 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             self.check_item_named(item.ident, "const");
         }
 
+        let parent_is_const =
+            self.outer_trait_or_trait_impl.as_ref().and_then(TraitOrTraitImpl::constness).is_some();
+
         match &item.kind {
             AssocItemKind::Fn(box Fn { sig, generics, body, .. })
-                if self
-                    .outer_trait_or_trait_impl
-                    .as_ref()
-                    .and_then(TraitOrTraitImpl::constness)
-                    .is_some()
+                if parent_is_const
                     || ctxt == AssocCtxt::Trait
                     || matches!(sig.header.constness, Const::Yes(_)) =>
             {
@@ -1505,6 +1519,20 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 );
                 self.visit_fn(kind, item.span, item.id);
             }
+            AssocItemKind::Type(_) => {
+                let disallowed = (!parent_is_const).then(|| match self.outer_trait_or_trait_impl {
+                    Some(TraitOrTraitImpl::Trait { .. }) => {
+                        DisallowTildeConstContext::TraitAssocTy(item.span)
+                    }
+                    Some(TraitOrTraitImpl::TraitImpl { .. }) => {
+                        DisallowTildeConstContext::TraitImplAssocTy(item.span)
+                    }
+                    None => DisallowTildeConstContext::InherentAssocTy(item.span),
+                });
+                self.with_tilde_const(disallowed, |this| {
+                    this.with_in_trait_impl(None, |this| visit::walk_assoc_item(this, item, ctxt))
+                })
+            }
             _ => self.with_in_trait_impl(None, |this| visit::walk_assoc_item(this, item, ctxt)),
         }
     }

compiler/rustc_ast_passes/src/errors.rs

@@ -565,6 +565,8 @@ pub struct ConstBoundTraitObject {
     pub span: Span,
 }
 
+// FIXME(effects): Consider making the note/reason the message of the diagnostic.
+// FIXME(effects): Provide structured suggestions (e.g., add `const` / `#[const_trait]` here).
 #[derive(Diagnostic)]
 #[diag(ast_passes_tilde_const_disallowed)]
 pub struct TildeConstDisallowed {
@@ -598,6 +600,21 @@ pub enum TildeConstReason {
         #[primary_span]
         span: Span,
     },
+    #[note(ast_passes_trait_assoc_ty)]
+    TraitAssocTy {
+        #[primary_span]
+        span: Span,
+    },
+    #[note(ast_passes_trait_impl_assoc_ty)]
+    TraitImplAssocTy {
+        #[primary_span]
+        span: Span,
+    },
+    #[note(ast_passes_inherent_assoc_ty)]
+    InherentAssocTy {
+        #[primary_span]
+        span: Span,
+    },
     #[note(ast_passes_object)]
     TraitObject,
     #[note(ast_passes_item)]

tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-0.qualified.stderr

@@ -0,0 +1,14 @@
+error[E0277]: the trait bound `T: Trait` is not satisfied
+  --> $DIR/assoc-type-const-bound-usage-0.rs:21:6
+   |
+LL |     <T as /* FIXME: ~const */ Trait>::Assoc::func()
+   |      ^ the trait `Trait` is not implemented for `T`
+   |
+help: consider further restricting this bound
+   |
+LL | const fn qualified<T: ~const Trait + Trait>() -> i32 {
+   |                                    +++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-0.rs

@@ -0,0 +1,24 @@
+// FIXME(effects): Collapse the revisions into one once we support `<Ty as ~const Trait>::Proj`.
+// revisions: unqualified qualified
+//[unqualified] check-pass
+//[qualified] known-bug: unknown
+
+#![feature(const_trait_impl, effects)]
+
+#[const_trait]
+trait Trait {
+    type Assoc: ~const Trait;
+    fn func() -> i32;
+}
+
+#[cfg(unqualified)]
+const fn unqualified<T: ~const Trait>() -> i32 {
+    T::Assoc::func()
+}
+
+#[cfg(qualified)]
+const fn qualified<T: ~const Trait>() -> i32 {
+    <T as /* FIXME: ~const */ Trait>::Assoc::func()
+}
+
+fn main() {}

tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-1.qualified.stderr

@@ -0,0 +1,14 @@
+error[E0277]: the trait bound `T: Trait` is not satisfied
+  --> $DIR/assoc-type-const-bound-usage-1.rs:23:43
+   |
+LL | fn qualified<T: const Trait>() -> Type<{ <T as /* FIXME: const */ Trait>::Assoc::func() }> {
+   |                                           ^ the trait `Trait` is not implemented for `T`
+   |
+help: consider further restricting this bound
+   |
+LL | fn qualified<T: const Trait + Trait>() -> Type<{ <T as /* FIXME: const */ Trait>::Assoc::func() }> {
+   |                             +++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-1.rs

@@ -0,0 +1,27 @@
+// FIXME(effects): Collapse the revisions into one once we support `<Ty as const Trait>::Proj`.
+// revisions: unqualified qualified
+//[unqualified] check-pass
+//[qualified] known-bug: unknown
+
+#![feature(const_trait_impl, effects, generic_const_exprs)]
+#![allow(incomplete_features)]
+
+#[const_trait]
+trait Trait {
+    type Assoc: ~const Trait;
+    fn func() -> i32;
+}
+
+struct Type<const N: i32>;
+
+#[cfg(unqualified)]
+fn unqualified<T: const Trait>() -> Type<{ T::Assoc::func() }> {
+    Type
+}
+
+#[cfg(qualified)]
+fn qualified<T: const Trait>() -> Type<{ <T as /* FIXME: const */ Trait>::Assoc::func() }> {
+    Type
+}
+
+fn main() {}

tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage.rs

@@ -1,29 +1,43 @@
-// known-bug: #110395
+// FIXME(effects): Replace `Add` with `std::ops::Add` once the latter a `#[const_trait]` again.
+#![feature(const_trait_impl, effects)]
 
-#![feature(const_trait_impl)]
+#[const_trait]
+trait Add<Rhs = Self> {
+    type Output;
+
+    fn add(self, other: Rhs) -> Self::Output;
+}
+
+impl const Add for i32 {
+    type Output = Self;
+
+    fn add(self, other: Self) -> Self::Output {
+        self + other
+    }
+}
 
 struct NonConstAdd(i32);
 
-impl std::ops::Add for NonConstAdd {
+impl Add for NonConstAdd {
     type Output = Self;
 
     fn add(self, rhs: Self) -> Self {
-        NonConstAdd(self.0 + rhs.0)
+        NonConstAdd(self.0.add(rhs.0))
     }
 }
 
 #[const_trait]
 trait Foo {
-    type Bar: ~const std::ops::Add;
+    type Bar: ~const Add;
 }
 
 impl const Foo for NonConstAdd {
-    type Bar = NonConstAdd;
+    type Bar = NonConstAdd; //~ ERROR the trait bound `NonConstAdd: ~const Add` is not satisfied
 }
 
 #[const_trait]
 trait Baz {
-    type Qux: std::ops::Add;
+    type Qux: Add;
 }
 
 impl const Baz for NonConstAdd {