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`

9 files changed, 100 insertions, 55 deletions

diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
index e0aad299163..5e23d473274 100644
--- a/compiler/rustc_hir_analysis/src/collect.rs
+++ b/compiler/rustc_hir_analysis/src/collect.rs
@@ -304,7 +304,9 @@ impl<'tcx> Visitor<'tcx> for CollectItemTypesVisitor<'tcx> {
                     self.tcx.ensure().type_of(param.def_id);
                     if let Some(default) = default {
                         // need to store default and type of default
-                        self.tcx.ensure().type_of(default.def_id);
+                        if let hir::ConstArgKind::Anon(ac) = default.kind {
+                            self.tcx.ensure().type_of(ac.def_id);
+                        }
                         self.tcx.ensure().const_param_default(param.def_id);
                     }
                 }
diff --git a/compiler/rustc_hir_analysis/src/collect/generics_of.rs b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
index 22d465c8e62..690423421b9 100644
--- a/compiler/rustc_hir_analysis/src/collect/generics_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
@@ -13,7 +13,7 @@ use rustc_session::lint;
 use rustc_span::symbol::{kw, Symbol};
 use rustc_span::Span;
 
-#[instrument(level = "debug", skip(tcx))]
+#[instrument(level = "debug", skip(tcx), ret)]
 pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
     use rustc_hir::*;
 
@@ -102,6 +102,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 None
             } else if tcx.features().generic_const_exprs {
                 let parent_node = tcx.parent_hir_node(hir_id);
+                debug!(?parent_node);
                 if let Node::Variant(Variant { disr_expr: Some(constant), .. }) = parent_node
                     && constant.hir_id == hir_id
                 {
@@ -164,13 +165,17 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 }
             } else {
                 let parent_node = tcx.parent_hir_node(hir_id);
+                let parent_node = match parent_node {
+                    Node::ConstArg(ca) => tcx.parent_hir_node(ca.hir_id),
+                    _ => parent_node,
+                };
                 match parent_node {
                     // HACK(eddyb) this provides the correct generics for repeat
                     // expressions' count (i.e. `N` in `[x; N]`), and explicit
                     // `enum` discriminants (i.e. `D` in `enum Foo { Bar = D }`),
                     // as they shouldn't be able to cause query cycle errors.
-                    Node::Expr(Expr { kind: ExprKind::Repeat(_, constant), .. })
-                        if constant.hir_id() == hir_id =>
+                    Node::Expr(Expr { kind: ExprKind::Repeat(_, ArrayLen::Body(ct)), .. })
+                        if ct.anon_const_hir_id() == Some(hir_id) =>
                     {
                         Some(parent_did)
                     }
diff --git a/compiler/rustc_hir_analysis/src/collect/predicates_of.rs b/compiler/rustc_hir_analysis/src/collect/predicates_of.rs
index b89d034fc2e..9e430c83e20 100644
--- a/compiler/rustc_hir_analysis/src/collect/predicates_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/predicates_of.rs
@@ -388,7 +388,7 @@ fn const_evaluatable_predicates_of(
             }
         }
 
-        fn visit_const_param_default(&mut self, _param: HirId, _ct: &'tcx hir::AnonConst) {
+        fn visit_const_param_default(&mut self, _param: HirId, _ct: &'tcx hir::ConstArg<'tcx>) {
             // Do not look into const param defaults,
             // these get checked when they are actually instantiated.
             //
diff --git a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
index cad7870a0a1..7930f54038d 100644
--- a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
+++ b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
@@ -954,7 +954,7 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
             GenericParamKind::Const { ty, default, .. } => {
                 self.visit_ty(ty);
                 if let Some(default) = default {
-                    self.visit_body(self.tcx.hir().body(default.body));
+                    self.visit_const_arg(default);
                 }
             }
         }
@@ -1594,7 +1594,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                     i += 1;
                 }
                 GenericArg::Const(ct) => {
-                    self.visit_anon_const(&ct.value);
+                    self.visit_const_arg(ct);
                     i += 1;
                 }
                 GenericArg::Infer(inf) => {
diff --git a/compiler/rustc_hir_analysis/src/collect/type_of.rs b/compiler/rustc_hir_analysis/src/collect/type_of.rs
index 974dd415f46..9affd654366 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of.rs
@@ -35,16 +35,32 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
     let parent_node_id = tcx.parent_hir_id(hir_id);
     let parent_node = tcx.hir_node(parent_node_id);
 
-    let (generics, arg_idx) = match parent_node {
-        // Easy case: arrays repeat expressions.
-        Node::Ty(&hir::Ty { kind: TyKind::Array(_, ref constant), .. })
-        | Node::Expr(&Expr { kind: ExprKind::Repeat(_, ref constant), .. })
-            if constant.hir_id() == hir_id =>
+    match parent_node {
+        // Anon consts "inside" the type system.
+        Node::ConstArg(&ConstArg {
+            hir_id: arg_hir_id,
+            kind: ConstArgKind::Anon(&AnonConst { hir_id: anon_hir_id, .. }),
+            ..
+        }) if anon_hir_id == hir_id => const_arg_anon_type_of(tcx, arg_hir_id, span),
+
+        // Anon consts outside the type system.
+        Node::Expr(&Expr { kind: ExprKind::InlineAsm(asm), .. })
+        | Node::Item(&Item { kind: ItemKind::GlobalAsm(asm), .. })
+            if asm.operands.iter().any(|(op, _op_sp)| match op {
+                hir::InlineAsmOperand::Const { anon_const }
+                | hir::InlineAsmOperand::SymFn { anon_const } => anon_const.hir_id == hir_id,
+                _ => false,
+            }) =>
         {
-            return tcx.types.usize;
+            tcx.typeck(def_id).node_type(hir_id)
+        }
+        Node::Variant(Variant { disr_expr: Some(ref e), .. }) if e.hir_id == hir_id => {
+            tcx.adt_def(tcx.hir().get_parent_item(hir_id)).repr().discr_type().to_ty(tcx)
         }
+        // Sort of affects the type system, but only for the purpose of diagnostics
+        // so no need for ConstArg.
         Node::Ty(&hir::Ty { kind: TyKind::Typeof(ref e), span, .. }) if e.hir_id == hir_id => {
-            let ty = tcx.typeck(def_id).node_type(e.hir_id);
+            let ty = tcx.typeck(def_id).node_type(tcx.local_def_id_to_hir_id(def_id));
             let ty = tcx.fold_regions(ty, |r, _| {
                 if r.is_erased() { ty::Region::new_error_misc(tcx) } else { r }
             });
@@ -56,24 +72,35 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
             tcx.dcx().emit_err(TypeofReservedKeywordUsed { span, ty, opt_sugg });
             return ty;
         }
-        Node::Expr(&Expr { kind: ExprKind::InlineAsm(asm), .. })
-        | Node::Item(&Item { kind: ItemKind::GlobalAsm(asm), .. })
-            if asm.operands.iter().any(|(op, _op_sp)| match op {
-                hir::InlineAsmOperand::Const { anon_const }
-                | hir::InlineAsmOperand::SymFn { anon_const } => anon_const.hir_id == hir_id,
-                _ => false,
-            }) =>
+
+        _ => Ty::new_error_with_message(
+            tcx,
+            span,
+            format!("unexpected anon const parent in type_of(): {parent_node:?}"),
+        ),
+    }
+}
+
+fn const_arg_anon_type_of<'tcx>(tcx: TyCtxt<'tcx>, arg_hir_id: HirId, span: Span) -> Ty<'tcx> {
+    use hir::*;
+    use rustc_middle::ty::Ty;
+
+    let parent_node_id = tcx.parent_hir_id(arg_hir_id);
+    let parent_node = tcx.hir_node(parent_node_id);
+
+    let (generics, arg_idx) = match parent_node {
+        // Easy case: arrays repeat expressions.
+        Node::Ty(&hir::Ty { kind: TyKind::Array(_, ref constant), .. })
+        | Node::Expr(&Expr { kind: ExprKind::Repeat(_, ref constant), .. })
+            if constant.hir_id() == arg_hir_id =>
         {
-            return tcx.typeck(def_id).node_type(hir_id);
-        }
-        Node::Variant(Variant { disr_expr: Some(ref e), .. }) if e.hir_id == hir_id => {
-            return tcx.adt_def(tcx.hir().get_parent_item(hir_id)).repr().discr_type().to_ty(tcx);
+            return tcx.types.usize;
         }
         Node::GenericParam(&GenericParam {
             def_id: param_def_id,
             kind: GenericParamKind::Const { default: Some(ct), .. },
             ..
-        }) if ct.hir_id == hir_id => {
+        }) if ct.hir_id == arg_hir_id => {
             return tcx
                 .type_of(param_def_id)
                 .no_bound_vars()
@@ -104,7 +131,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
             // to a ty::Alias(ty::Projection, `<Self as Foo>::Assoc<3>`).
             let item_def_id = tcx
                 .hir()
-                .parent_owner_iter(hir_id)
+                .parent_owner_iter(arg_hir_id)
                 .find(|(_, node)| matches!(node, OwnerNode::Item(_)))
                 .unwrap()
                 .0
@@ -124,7 +151,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                         args.args
                             .iter()
                             .filter(|arg| arg.is_ty_or_const())
-                            .position(|arg| arg.hir_id() == hir_id)
+                            .position(|arg| arg.hir_id() == arg_hir_id)
                     })
                     .unwrap_or_else(|| {
                         bug!("no arg matching AnonConst in segment");
@@ -145,7 +172,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                 ExprKind::MethodCall(segment, ..) | ExprKind::Path(QPath::TypeRelative(_, segment)),
             ..
         }) => {
-            let body_owner = tcx.hir().enclosing_body_owner(hir_id);
+            let body_owner = tcx.hir().enclosing_body_owner(arg_hir_id);
             let tables = tcx.typeck(body_owner);
             // This may fail in case the method/path does not actually exist.
             // As there is no relevant param for `def_id`, we simply return
@@ -163,10 +190,10 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                     args.args
                         .iter()
                         .filter(|arg| arg.is_ty_or_const())
-                        .position(|arg| arg.hir_id() == hir_id)
+                        .position(|arg| arg.hir_id() == arg_hir_id)
                 })
                 .unwrap_or_else(|| {
-                    bug!("no arg matching AnonConst in segment");
+                    bug!("no arg matching ConstArg in segment");
                 });
 
             (tcx.generics_of(type_dependent_def), idx)
@@ -185,18 +212,18 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                         | ExprKind::Struct(&QPath::Resolved(_, path), ..),
                     ..
                 }) => {
-                    let body_owner = tcx.hir().enclosing_body_owner(hir_id);
+                    let body_owner = tcx.hir().enclosing_body_owner(arg_hir_id);
                     let _tables = tcx.typeck(body_owner);
                     &*path
                 }
                 Node::Pat(pat) => {
-                    if let Some(path) = get_path_containing_arg_in_pat(pat, hir_id) {
+                    if let Some(path) = get_path_containing_arg_in_pat(pat, arg_hir_id) {
                         path
                     } else {
                         return Ty::new_error_with_message(
                             tcx,
                             span,
-                            format!("unable to find const parent for {hir_id} in pat {pat:?}"),
+                            format!("unable to find const parent for {arg_hir_id} in pat {pat:?}"),
                         );
                     }
                 }
@@ -217,14 +244,14 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                 args.args
                     .iter()
                     .filter(|arg| arg.is_ty_or_const())
-                    .position(|arg| arg.hir_id() == hir_id)
+                    .position(|arg| arg.hir_id() == arg_hir_id)
                     .map(|index| (index, seg))
                     .or_else(|| {
                         args.constraints
                             .iter()
                             .copied()
                             .filter_map(AssocItemConstraint::ct)
-                            .position(|ct| ct.hir_id == hir_id)
+                            .position(|ct| ct.hir_id == arg_hir_id)
                             .map(|idx| (idx, seg))
                     })
             }) else {
@@ -249,7 +276,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
             return Ty::new_error_with_message(
                 tcx,
                 span,
-                format!("unexpected const parent in type_of(): {parent_node:?}"),
+                format!("unexpected const arg parent in type_of(): {parent_node:?}"),
             );
         }
     };
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
index a1feef9e15b..6f9c481650b 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
@@ -413,12 +413,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             });
 
             // Provide the resolved type of the associated constant to `type_of(AnonConst)`.
-            if let Some(anon_const) = constraint.ct() {
-                let ty = alias_term
-                    .map_bound(|alias| tcx.type_of(alias.def_id).instantiate(tcx, alias.args));
-                let ty =
-                    check_assoc_const_binding_type(self, constraint.ident, ty, constraint.hir_id);
-                tcx.feed_anon_const_type(anon_const.def_id, ty::EarlyBinder::bind(ty));
+            if let Some(const_arg) = constraint.ct() {
+                if let hir::ConstArgKind::Anon(anon_const) = const_arg.kind {
+                    let ty = alias_term
+                        .map_bound(|alias| tcx.type_of(alias.def_id).instantiate(tcx, alias.args));
+                    let ty = check_assoc_const_binding_type(
+                        self,
+                        constraint.ident,
+                        ty,
+                        constraint.hir_id,
+                    );
+                    tcx.feed_anon_const_type(anon_const.def_id, ty::EarlyBinder::bind(ty));
+                }
             }
 
             alias_term
@@ -435,7 +441,9 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             hir::AssocItemConstraintKind::Equality { term } => {
                 let term = match term {
                     hir::Term::Ty(ty) => self.lower_ty(ty).into(),
-                    hir::Term::Const(ct) => ty::Const::from_anon_const(tcx, ct.def_id).into(),
+                    hir::Term::Const(ct) => {
+                        ty::Const::from_const_arg(tcx, ct, ty::FeedConstTy::No).into()
+                    }
                 };
 
                 // Find any late-bound regions declared in `ty` that are not
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
index 968f38cf05d..8ff6ced8b39 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
@@ -340,7 +340,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         {
             let span = match term {
                 hir::Term::Ty(ty) => ty.span,
-                hir::Term::Const(ct) => tcx.def_span(ct.def_id),
+                hir::Term::Const(ct) => ct.span(),
             };
             (span, Some(ident.span), assoc_item.kind, assoc_kind)
         } else {
@@ -1294,8 +1294,7 @@ pub fn prohibit_assoc_item_constraint(
                     hir::AssocItemConstraintKind::Equality { term: hir::Term::Const(c) },
                     GenericParamDefKind::Const { .. },
                 ) => {
-                    let span = tcx.hir().span(c.hir_id);
-                    suggest_direct_use(&mut err, span);
+                    suggest_direct_use(&mut err, c.span());
                 }
                 (hir::AssocItemConstraintKind::Bound { bounds }, _) => {
                     // Suggest `impl<T: Bound> Trait<T> for Foo` when finding
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs
index b1c77db9f37..abe2cff321f 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs
@@ -113,8 +113,12 @@ fn generic_arg_mismatch_err(
             }
         }
         (GenericArg::Const(cnst), GenericParamDefKind::Type { .. }) => {
-            let body = tcx.hir().body(cnst.value.body);
-            if let rustc_hir::ExprKind::Path(rustc_hir::QPath::Resolved(_, path)) = body.value.kind
+            // FIXME(min_generic_const_args): once ConstArgKind::Path is used for non-params too,
+            // this should match against that instead of ::Anon
+            if let hir::ConstArgKind::Anon(anon) = cnst.kind
+                && let body = tcx.hir().body(anon.body)
+                && let rustc_hir::ExprKind::Path(rustc_hir::QPath::Resolved(_, path)) =
+                    body.value.kind
             {
                 if let Res::Def(DefKind::Fn { .. }, id) = path.res {
                     err.help(format!("`{}` is a function item, not a type", tcx.item_name(id)));
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index a665306f2c6..7b8a03def86 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -471,11 +471,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         handle_ty_args(has_default, &inf.to_ty())
                     }
                     (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => {
-                        let did = ct.value.def_id;
-                        tcx.feed_anon_const_type(did, tcx.type_of(param.def_id));
-                        ty::Const::from_anon_const(tcx, did).into()
+                        ty::Const::from_const_arg(tcx, ct, ty::FeedConstTy::Param(param.def_id))
+                            .into()
                     }
-                    (&GenericParamDefKind::Const { .. }, hir::GenericArg::Infer(inf)) => {
+                    (&GenericParamDefKind::Const { .. }, GenericArg::Infer(inf)) => {
                         self.lowerer.ct_infer(Some(param), inf.span).into()
                     }
                     (kind, arg) => span_bug!(
@@ -912,7 +911,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                                 let term: ty::Term<'_> = match term {
                                     hir::Term::Ty(ty) => self.lower_ty(ty).into(),
                                     hir::Term::Const(ct) => {
-                                        ty::Const::from_anon_const(tcx, ct.def_id).into()
+                                        ty::Const::from_const_arg(tcx, ct, ty::FeedConstTy::No)
+                                            .into()
                                     }
                                 };
                                 // FIXME(#97583): This isn't syntactically well-formed!
@@ -2140,7 +2140,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 let length = match length {
                     hir::ArrayLen::Infer(inf) => self.ct_infer(None, inf.span),
                     hir::ArrayLen::Body(constant) => {
-                        ty::Const::from_anon_const(tcx, constant.def_id)
+                        ty::Const::from_const_arg(tcx, constant, ty::FeedConstTy::No)
                     }
                 };