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`

2 files changed, 74 insertions, 18 deletions

diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index 3bd7b300758..bf773f2d487 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -228,13 +228,53 @@ impl<'hir> PathSegment<'hir> {
     }
 }
 
+/// A constant that enters the type system, used for arguments to const generics (e.g. array lengths).
+///
+/// These are distinct from [`AnonConst`] as anon consts in the type system are not allowed
+/// to use any generic parameters, therefore we must represent `N` differently. Additionally
+/// future designs for supporting generic parameters in const arguments will likely not use
+/// an anon const based design.
+///
+/// So, `ConstArg` (specifically, [`ConstArgKind`]) distinguishes between const args
+/// that are [just paths](ConstArgKind::Path) (currently just bare const params)
+/// versus const args that are literals or have arbitrary computations (e.g., `{ 1 + 3 }`).
 #[derive(Clone, Copy, Debug, HashStable_Generic)]
 pub struct ConstArg<'hir> {
-    pub value: &'hir AnonConst,
+    #[stable_hasher(ignore)]
+    pub hir_id: HirId,
+    pub kind: ConstArgKind<'hir>,
     /// Indicates whether this comes from a `~const` desugaring.
     pub is_desugared_from_effects: bool,
 }
 
+impl<'hir> ConstArg<'hir> {
+    pub fn anon_const_hir_id(&self) -> Option<HirId> {
+        match self.kind {
+            ConstArgKind::Anon(ac) => Some(ac.hir_id),
+            _ => None,
+        }
+    }
+
+    pub fn span(&self) -> Span {
+        match self.kind {
+            ConstArgKind::Path(path) => path.span(),
+            ConstArgKind::Anon(anon) => anon.span,
+        }
+    }
+}
+
+/// See [`ConstArg`].
+#[derive(Clone, Copy, Debug, HashStable_Generic)]
+pub enum ConstArgKind<'hir> {
+    /// **Note:** Currently this is only used for bare const params
+    /// (`N` where `fn foo<const N: usize>(...)`),
+    /// not paths to any const (`N` where `const N: usize = ...`).
+    ///
+    /// However, in the future, we'll be using it for all of those.
+    Path(QPath<'hir>),
+    Anon(&'hir AnonConst),
+}
+
 #[derive(Clone, Copy, Debug, HashStable_Generic)]
 pub struct InferArg {
     pub hir_id: HirId,
@@ -251,7 +291,7 @@ impl InferArg {
 pub enum GenericArg<'hir> {
     Lifetime(&'hir Lifetime),
     Type(&'hir Ty<'hir>),
-    Const(ConstArg<'hir>),
+    Const(&'hir ConstArg<'hir>),
     Infer(InferArg),
 }
 
@@ -260,7 +300,7 @@ impl GenericArg<'_> {
         match self {
             GenericArg::Lifetime(l) => l.ident.span,
             GenericArg::Type(t) => t.span,
-            GenericArg::Const(c) => c.value.span,
+            GenericArg::Const(c) => c.span(),
             GenericArg::Infer(i) => i.span,
         }
     }
@@ -269,7 +309,7 @@ impl GenericArg<'_> {
         match self {
             GenericArg::Lifetime(l) => l.hir_id,
             GenericArg::Type(t) => t.hir_id,
-            GenericArg::Const(c) => c.value.hir_id,
+            GenericArg::Const(c) => c.hir_id,
             GenericArg::Infer(i) => i.hir_id,
         }
     }
@@ -524,7 +564,7 @@ pub enum GenericParamKind<'hir> {
     Const {
         ty: &'hir Ty<'hir>,
         /// Optional default value for the const generic param
-        default: Option<&'hir AnonConst>,
+        default: Option<&'hir ConstArg<'hir>>,
         is_host_effect: bool,
         synthetic: bool,
     },
@@ -1598,13 +1638,13 @@ pub type Lit = Spanned<LitKind>;
 #[derive(Copy, Clone, Debug, HashStable_Generic)]
 pub enum ArrayLen<'hir> {
     Infer(InferArg),
-    Body(&'hir AnonConst),
+    Body(&'hir ConstArg<'hir>),
 }
 
 impl ArrayLen<'_> {
     pub fn hir_id(&self) -> HirId {
         match self {
-            ArrayLen::Infer(InferArg { hir_id, .. }) | ArrayLen::Body(AnonConst { hir_id, .. }) => {
+            ArrayLen::Infer(InferArg { hir_id, .. }) | ArrayLen::Body(ConstArg { hir_id, .. }) => {
                 *hir_id
             }
         }
@@ -2434,7 +2474,7 @@ impl<'hir> AssocItemConstraint<'hir> {
     }
 
     /// Obtain the const on the RHS of an assoc const equality constraint if applicable.
-    pub fn ct(self) -> Option<&'hir AnonConst> {
+    pub fn ct(self) -> Option<&'hir ConstArg<'hir>> {
         match self.kind {
             AssocItemConstraintKind::Equality { term: Term::Const(ct) } => Some(ct),
             _ => None,
@@ -2445,7 +2485,7 @@ impl<'hir> AssocItemConstraint<'hir> {
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub enum Term<'hir> {
     Ty(&'hir Ty<'hir>),
-    Const(&'hir AnonConst),
+    Const(&'hir ConstArg<'hir>),
 }
 
 impl<'hir> From<&'hir Ty<'hir>> for Term<'hir> {
@@ -2454,8 +2494,8 @@ impl<'hir> From<&'hir Ty<'hir>> for Term<'hir> {
     }
 }
 
-impl<'hir> From<&'hir AnonConst> for Term<'hir> {
-    fn from(c: &'hir AnonConst) -> Self {
+impl<'hir> From<&'hir ConstArg<'hir>> for Term<'hir> {
+    fn from(c: &'hir ConstArg<'hir>) -> Self {
         Term::Const(c)
     }
 }
@@ -3689,6 +3729,7 @@ pub enum Node<'hir> {
     Field(&'hir FieldDef<'hir>),
     AnonConst(&'hir AnonConst),
     ConstBlock(&'hir ConstBlock),
+    ConstArg(&'hir ConstArg<'hir>),
     Expr(&'hir Expr<'hir>),
     ExprField(&'hir ExprField<'hir>),
     Stmt(&'hir Stmt<'hir>),
@@ -3750,6 +3791,7 @@ impl<'hir> Node<'hir> {
             Node::Param(..)
             | Node::AnonConst(..)
             | Node::ConstBlock(..)
+            | Node::ConstArg(..)
             | Node::Expr(..)
             | Node::Stmt(..)
             | Node::Block(..)
@@ -3966,7 +4008,7 @@ mod size_asserts {
     static_assert_size!(FnDecl<'_>, 40);
     static_assert_size!(ForeignItem<'_>, 72);
     static_assert_size!(ForeignItemKind<'_>, 40);
-    static_assert_size!(GenericArg<'_>, 24);
+    static_assert_size!(GenericArg<'_>, 16);
     static_assert_size!(GenericBound<'_>, 48);
     static_assert_size!(Generics<'_>, 56);
     static_assert_size!(Impl<'_>, 80);
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)