Add ConstArgKind::Path and make ConstArg its own HIR node

This is a very large commit since a lot needs to be changed in order to
make the tests pass. The salient changes are:

- `ConstArgKind` gets a new `Path` variant, and all const params are now
  represented using it. Non-param paths still use `ConstArgKind::Anon`
  to prevent this change from getting too large, but they will soon use
  the `Path` variant too.

- `ConstArg` gets a distinct `hir_id` field and its own variant in
  `hir::Node`. This affected many parts of the compiler that expected
  the parent of an `AnonConst` to be the containing context (e.g., an
  array repeat expression). They have been changed to check the
  "grandparent" where necessary.

- 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 #127009.

compiler/rustc_middle/src/hir/map/mod.rs

@@ -912,6 +912,7 @@ impl<'hir> Map<'hir> {
             Node::Field(field) => field.span,
             Node::AnonConst(constant) => constant.span,
             Node::ConstBlock(constant) => self.body(constant.body).value.span,
+            Node::ConstArg(const_arg) => const_arg.span(),
             Node::Expr(expr) => expr.span,
             Node::ExprField(field) => field.span,
             Node::Stmt(stmt) => stmt.span,
@@ -962,7 +963,8 @@ impl<'hir> Map<'hir> {
     /// Returns the HirId of `N` in `struct Foo<const N: usize = { ... }>` when
     /// called with the HirId for the `{ ... }` anon const
     pub fn opt_const_param_default_param_def_id(self, anon_const: HirId) -> Option<LocalDefId> {
-        match self.tcx.parent_hir_node(anon_const) {
+        let const_arg = self.tcx.parent_hir_id(anon_const);
+        match self.tcx.parent_hir_node(const_arg) {
             Node::GenericParam(GenericParam {
                 def_id: param_id,
                 kind: GenericParamKind::Const { .. },
@@ -1182,6 +1184,7 @@ fn hir_id_to_string(map: Map<'_>, id: HirId) -> String {
         }
         Node::AnonConst(_) => node_str("const"),
         Node::ConstBlock(_) => node_str("const"),
+        Node::ConstArg(_) => node_str("const"),
         Node::Expr(_) => node_str("expr"),
         Node::ExprField(_) => node_str("expr field"),
         Node::Stmt(_) => node_str("stmt"),

compiler/rustc_middle/src/ty/consts.rs

@@ -202,7 +202,6 @@ pub enum FeedConstTy {
 impl<'tcx> Const<'tcx> {
     /// Convert a [`hir::ConstArg`] to a [`ty::Const`](Self).
     #[instrument(skip(tcx), level = "debug")]
-    #[allow(irrefutable_let_patterns)]
     pub fn from_const_arg(
         tcx: TyCtxt<'tcx>,
         const_arg: &'tcx hir::ConstArg<'tcx>,
@@ -215,6 +214,10 @@ impl<'tcx> Const<'tcx> {
         }
 
         match const_arg.kind {
+            hir::ConstArgKind::Path(qpath) => {
+                // FIXME(min_generic_const_args): for now only params are lowered to ConstArgKind::Path
+                Self::from_param(tcx, qpath, const_arg.hir_id)
+            }
             hir::ConstArgKind::Anon(anon) => Self::from_anon_const(tcx, anon.def_id),
         }
     }
@@ -310,16 +313,12 @@ impl<'tcx> Const<'tcx> {
             }
         }
 
-        if let hir::ExprKind::Path(
-            qpath @ hir::QPath::Resolved(
-                _,
-                &hir::Path { res: Res::Def(DefKind::ConstParam, _), .. },
-            ),
-        ) = expr.kind
+        if let hir::ExprKind::Path(hir::QPath::Resolved(
+            _,
+            &hir::Path { res: Res::Def(DefKind::ConstParam, _), .. },
+        )) = expr.kind
         {
-            // FIXME: once ConstArgKind::Path added, uncomment span_bug and delete return
-            // span_bug!(expr.span, "try_from_lit: received const param which shouldn't be possible")
-            return Some(Self::from_param(tcx, qpath, expr.hir_id));
+            span_bug!(expr.span, "try_from_lit: received const param which shouldn't be possible")
         }
 
         None

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -607,7 +607,9 @@ impl<'a, V> ::std::ops::Index<HirId> for LocalTableInContext<'a, V> {
     type Output = V;
 
     fn index(&self, key: HirId) -> &V {
-        self.get(key).expect("LocalTableInContext: key not found")
+        self.get(key).unwrap_or_else(|| {
+            bug!("LocalTableInContext({:?}): key {:?} not found", self.hir_owner, key)
+        })
     }
 }