Auto merge of #119099 - fmease:always-const-trait-bounds, r=fee1-dead

 Introduce `const Trait` (always-const trait bounds)

Feature `const_trait_impl` currently lacks a way to express “always const” trait bounds. This makes it impossible to define generic items like fns or structs which contain types that depend on const method calls (\*). While the final design and esp. the syntax of effects / keyword generics isn't set in stone, some version of “always const” trait bounds will very likely form a part of it. Further, their implementation is trivial thanks to the `effects` backbone.

Not sure if this needs t-lang sign-off though.

(\*):

```rs
#![feature(const_trait_impl, effects, generic_const_exprs)]

fn compute<T: const Trait>() -> Type<{ T::generate() }> { /*…*/ }

struct Store<T: const Trait>
where
    Type<{ T::generate() }>:,
{
    field: Type<{ T::generate() }>,
}
```

Lastly, “always const” trait bounds are a perfect fit for `generic_const_items`.

```rs
#![feature(const_trait_impl, effects, generic_const_items)]

const DEFAULT<T: const Default>: T = T::default();
```

Previously, we (oli, fee1-dead and I) wanted to reinterpret `~const Trait` as `const Trait` in generic const items which would've been quite surprising and not very generalizable.
Supersedes #117530.

---

cc `@oli-obk`

As discussed
r? fee1-dead (or compiler)

1 files changed, 1 insertions, 17 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 53cce9e2883..e276b34ca37 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -2556,19 +2556,12 @@ pub(crate) struct AssocLifetime {
 }
 
 #[derive(Diagnostic)]
-#[diag(parse_tilde_const_lifetime)]
-pub(crate) struct TildeConstLifetime {
-    #[primary_span]
-    pub span: Span,
-}
-
-#[derive(Diagnostic)]
 #[diag(parse_modifier_lifetime)]
 pub(crate) struct ModifierLifetime {
     #[primary_span]
     #[suggestion(style = "tool-only", applicability = "maybe-incorrect", code = "")]
     pub span: Span,
-    pub sigil: &'static str,
+    pub modifier: &'static str,
 }
 
 #[derive(Diagnostic)]
@@ -2582,15 +2575,6 @@ pub(crate) struct ParenthesizedLifetime {
 }
 
 #[derive(Diagnostic)]
-#[diag(parse_const_bounds_missing_tilde)]
-pub(crate) struct ConstMissingTilde {
-    #[primary_span]
-    pub span: Span,
-    #[suggestion(code = "~", applicability = "machine-applicable")]
-    pub start: Span,
-}
-
-#[derive(Diagnostic)]
 #[diag(parse_underscore_literal_suffix)]
 pub(crate) struct UnderscoreLiteralSuffix {
     #[primary_span]