Auto merge of #125915 - camelid:const-arg-refactor, r=BoxyUwU

Represent type-level consts with new-and-improved `hir::ConstArg`

### Summary

This is a step toward `min_generic_const_exprs`. We now represent all const
generic arguments using an enum that differentiates between const *paths*
(temporarily just bare const params) and arbitrary anon consts that may perform
computations. This will enable us to cleanly implement the `min_generic_const_args`
plan of allowing the use of generics in paths used as const args, while
disallowing their use in arbitrary anon consts. Here is a summary of the salient
aspects of this change:

- Add `current_def_id_parent` to `LoweringContext`

  This is needed to track anon const parents properly once we implement
  `ConstArgKind::Path` (which requires moving anon const def-creation
  outside of `DefCollector`).

- Create `hir::ConstArgKind` enum with `Path` and `Anon` variants. Use it in the
  existing `hir::ConstArg` struct, replacing the previous `hir::AnonConst` field.

- Use `ConstArg` for all instances of const args. Specifically, use it instead
  of `AnonConst` for assoc item constraints, array lengths, and const param
  defaults.

- Some `ast::AnonConst`s now have their `DefId`s created in
  rustc_ast_lowering rather than `DefCollector`. This is because in some
  cases they will end up becoming a `ConstArgKind::Path` instead, which
  has no `DefId`. We have to solve this in a hacky way where we guess
  whether the `AnonConst` could end up as a path const since we can't
  know for sure until after name resolution (`N` could refer to a free
  const or a nullary struct). If it has no chance as being a const
  param, then we create a `DefId` in `DefCollector` -- otherwise we
  decide during ast_lowering. This will have to be updated once all path
  consts use `ConstArgKind::Path`.

- We explicitly use `ConstArgHasType` for array lengths, rather than
  implicitly relying on anon const type feeding -- this is due to the
  addition of `ConstArgKind::Path`.

- Some tests have their outputs changed, but the changes are for the
  most part minor (including removing duplicate or almost-duplicate
  errors). One test now ICEs, but it is for an incomplete, unstable
  feature and is now tracked at https://github.com/rust-lang/rust/issues/127009.

### Followup items post-merge

- Use `ConstArgKind::Path` for all const paths, not just const params.
- Fix (no github dont close this issue) #127009
- If a path in generic args doesn't resolve as a type, try to resolve as a const
  instead (do this in rustc_resolve). Then remove the special-casing from
  `rustc_ast_lowering`, so that all params will automatically be lowered as
  `ConstArgKind::Path`.
- (?) Consider making `const_evaluatable_unchecked` a hard error, or at least
  trying it in crater

r? `@BoxyUwU`

1 files changed, 20 insertions, 6 deletions

diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index 9bb3245ae05..c202ee41e31 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -347,6 +347,9 @@ pub trait Visitor<'v>: Sized {
     fn visit_inline_const(&mut self, c: &'v ConstBlock) -> Self::Result {
         walk_inline_const(self, c)
     }
+    fn visit_const_arg(&mut self, c: &'v ConstArg<'v>) -> Self::Result {
+        walk_const_arg(self, c)
+    }
     fn visit_expr(&mut self, ex: &'v Expr<'v>) -> Self::Result {
         walk_expr(self, ex)
     }
@@ -364,7 +367,7 @@ pub trait Visitor<'v>: Sized {
     fn visit_generic_param(&mut self, p: &'v GenericParam<'v>) -> Self::Result {
         walk_generic_param(self, p)
     }
-    fn visit_const_param_default(&mut self, _param: HirId, ct: &'v AnonConst) -> Self::Result {
+    fn visit_const_param_default(&mut self, _param: HirId, ct: &'v ConstArg<'v>) -> Self::Result {
         walk_const_param_default(self, ct)
     }
     fn visit_generics(&mut self, g: &'v Generics<'v>) -> Self::Result {
@@ -708,7 +711,7 @@ pub fn walk_array_len<'v, V: Visitor<'v>>(visitor: &mut V, len: &'v ArrayLen<'v>
     match len {
         // FIXME: Use `visit_infer` here.
         ArrayLen::Infer(InferArg { hir_id, span: _ }) => visitor.visit_id(*hir_id),
-        ArrayLen::Body(c) => visitor.visit_anon_const(c),
+        ArrayLen::Body(c) => visitor.visit_const_arg(c),
     }
 }
 
@@ -725,6 +728,17 @@ pub fn walk_inline_const<'v, V: Visitor<'v>>(
     visitor.visit_nested_body(constant.body)
 }
 
+pub fn walk_const_arg<'v, V: Visitor<'v>>(
+    visitor: &mut V,
+    const_arg: &'v ConstArg<'v>,
+) -> V::Result {
+    try_visit!(visitor.visit_id(const_arg.hir_id));
+    match &const_arg.kind {
+        ConstArgKind::Path(qpath) => visitor.visit_qpath(qpath, const_arg.hir_id, qpath.span()),
+        ConstArgKind::Anon(anon) => visitor.visit_anon_const(*anon),
+    }
+}
+
 pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>) -> V::Result {
     try_visit!(visitor.visit_id(expression.hir_id));
     match expression.kind {
@@ -928,9 +942,9 @@ pub fn walk_generic_param<'v, V: Visitor<'v>>(
 
 pub fn walk_const_param_default<'v, V: Visitor<'v>>(
     visitor: &mut V,
-    ct: &'v AnonConst,
+    ct: &'v ConstArg<'v>,
 ) -> V::Result {
-    visitor.visit_anon_const(ct)
+    visitor.visit_const_arg(ct)
 }
 
 pub fn walk_generics<'v, V: Visitor<'v>>(visitor: &mut V, generics: &'v Generics<'v>) -> V::Result {
@@ -1216,7 +1230,7 @@ pub fn walk_generic_arg<'v, V: Visitor<'v>>(
     match generic_arg {
         GenericArg::Lifetime(lt) => visitor.visit_lifetime(lt),
         GenericArg::Type(ty) => visitor.visit_ty(ty),
-        GenericArg::Const(ct) => visitor.visit_anon_const(&ct.value),
+        GenericArg::Const(ct) => visitor.visit_const_arg(ct),
         GenericArg::Infer(inf) => visitor.visit_infer(inf),
     }
 }
@@ -1278,7 +1292,7 @@ pub fn walk_assoc_item_constraint<'v, V: Visitor<'v>>(
     match constraint.kind {
         AssocItemConstraintKind::Equality { ref term } => match term {
             Term::Ty(ref ty) => try_visit!(visitor.visit_ty(ty)),
-            Term::Const(ref c) => try_visit!(visitor.visit_anon_const(c)),
+            Term::Const(ref c) => try_visit!(visitor.visit_const_arg(c)),
         },
         AssocItemConstraintKind::Bound { bounds } => {
             walk_list!(visitor, visit_param_bound, bounds)