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, 15 insertions, 6 deletions

diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index 5105d60ae18..db5eba0d9eb 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -12,8 +12,9 @@ use rustc_ast_pretty::pp::{self, Breaks};
 use rustc_ast_pretty::pprust::{Comments, PrintState};
 use rustc_hir as hir;
 use rustc_hir::{
-    BindingMode, ByRef, GenericArg, GenericBound, GenericParam, GenericParamKind, HirId,
-    LifetimeParamKind, Node, PatKind, PreciseCapturingArg, RangeEnd, Term, TraitBoundModifier,
+    BindingMode, ByRef, ConstArgKind, GenericArg, GenericBound, GenericParam, GenericParamKind,
+    HirId, LifetimeParamKind, Node, PatKind, PreciseCapturingArg, RangeEnd, Term,
+    TraitBoundModifier,
 };
 use rustc_span::source_map::SourceMap;
 use rustc_span::symbol::{kw, Ident, Symbol};
@@ -87,6 +88,7 @@ impl<'a> State<'a> {
             Node::Variant(a) => self.print_variant(a),
             Node::AnonConst(a) => self.print_anon_const(a),
             Node::ConstBlock(a) => self.print_inline_const(a),
+            Node::ConstArg(a) => self.print_const_arg(a),
             Node::Expr(a) => self.print_expr(a),
             Node::ExprField(a) => self.print_expr_field(a),
             Node::Stmt(a) => self.print_stmt(a),
@@ -983,7 +985,7 @@ impl<'a> State<'a> {
     fn print_array_length(&mut self, len: &hir::ArrayLen<'_>) {
         match len {
             hir::ArrayLen::Infer(..) => self.word("_"),
-            hir::ArrayLen::Body(ct) => self.print_anon_const(ct),
+            hir::ArrayLen::Body(ct) => self.print_const_arg(ct),
         }
     }
 
@@ -991,6 +993,13 @@ impl<'a> State<'a> {
         self.ann.nested(self, Nested::Body(constant.body))
     }
 
+    fn print_const_arg(&mut self, const_arg: &hir::ConstArg<'_>) {
+        match &const_arg.kind {
+            ConstArgKind::Path(qpath) => self.print_qpath(qpath, true),
+            ConstArgKind::Anon(anon) => self.print_anon_const(anon),
+        }
+    }
+
     fn print_call_post(&mut self, args: &[hir::Expr<'_>]) {
         self.popen();
         self.commasep_exprs(Inconsistent, args);
@@ -1679,7 +1688,7 @@ impl<'a> State<'a> {
                             GenericArg::Lifetime(lt) if !elide_lifetimes => s.print_lifetime(lt),
                             GenericArg::Lifetime(_) => {}
                             GenericArg::Type(ty) => s.print_type(ty),
-                            GenericArg::Const(ct) => s.print_anon_const(&ct.value),
+                            GenericArg::Const(ct) => s.print_const_arg(ct),
                             GenericArg::Infer(_inf) => s.word("_"),
                         }
                     });
@@ -1720,7 +1729,7 @@ impl<'a> State<'a> {
                 self.word_space("=");
                 match term {
                     Term::Ty(ty) => self.print_type(ty),
-                    Term::Const(ref c) => self.print_anon_const(c),
+                    Term::Const(ref c) => self.print_const_arg(c),
                 }
             }
             hir::AssocItemConstraintKind::Bound { bounds } => {
@@ -2155,7 +2164,7 @@ impl<'a> State<'a> {
                 if let Some(default) = default {
                     self.space();
                     self.word_space("=");
-                    self.print_anon_const(default);
+                    self.print_const_arg(default);
                 }
             }
         }