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`

7 files changed, 71 insertions, 31 deletions

diff --git a/clippy_lints/src/large_stack_arrays.rs b/clippy_lints/src/large_stack_arrays.rs
index c9bfc9c85d9..d94b0cce948 100644
--- a/clippy_lints/src/large_stack_arrays.rs
+++ b/clippy_lints/src/large_stack_arrays.rs
@@ -106,13 +106,12 @@ fn might_be_expanded<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'tcx>) -> bool {
     ///
     /// This is a fail-safe to a case where even the `is_from_proc_macro` is unable to determain the
     /// correct result.
-    fn repeat_expr_might_be_expanded<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'tcx>) -> bool {
-        let ExprKind::Repeat(_, ArrayLen::Body(anon_const)) = expr.kind else {
+    fn repeat_expr_might_be_expanded<'tcx>(expr: &Expr<'tcx>) -> bool {
+        let ExprKind::Repeat(_, ArrayLen::Body(len_ct)) = expr.kind else {
             return false;
         };
-        let len_span = cx.tcx.def_span(anon_const.def_id);
-        !expr.span.contains(len_span)
+        !expr.span.contains(len_ct.span())
     }
 
-    expr.span.from_expansion() || is_from_proc_macro(cx, expr) || repeat_expr_might_be_expanded(cx, expr)
+    expr.span.from_expansion() || is_from_proc_macro(cx, expr) || repeat_expr_might_be_expanded(expr)
 }
diff --git a/clippy_lints/src/trailing_empty_array.rs b/clippy_lints/src/trailing_empty_array.rs
index 462084e96a8..db8c63892b8 100644
--- a/clippy_lints/src/trailing_empty_array.rs
+++ b/clippy_lints/src/trailing_empty_array.rs
@@ -2,7 +2,7 @@ use clippy_utils::diagnostics::span_lint_and_help;
 use clippy_utils::has_repr_attr;
 use rustc_hir::{Item, ItemKind};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::ty::Const;
+use rustc_middle::ty::{Const, FeedConstTy};
 use rustc_session::declare_lint_pass;
 
 declare_clippy_lint! {
@@ -53,14 +53,14 @@ impl<'tcx> LateLintPass<'tcx> for TrailingEmptyArray {
     }
 }
 
-fn is_struct_with_trailing_zero_sized_array(cx: &LateContext<'_>, item: &Item<'_>) -> bool {
+fn is_struct_with_trailing_zero_sized_array<'tcx>(cx: &LateContext<'tcx>, item: &Item<'tcx>) -> bool {
     if let ItemKind::Struct(data, _) = &item.kind
         // First check if last field is an array
         && let Some(last_field) = data.fields().last()
         && let rustc_hir::TyKind::Array(_, rustc_hir::ArrayLen::Body(length)) = last_field.ty.kind
 
         // Then check if that array is zero-sized
-        && let length = Const::from_anon_const(cx.tcx, length.def_id)
+        && let length = Const::from_const_arg(cx.tcx, length, FeedConstTy::No)
         && let length = length.try_eval_target_usize(cx.tcx, cx.param_env)
         && let Some(length) = length
     {
diff --git a/clippy_lints/src/utils/author.rs b/clippy_lints/src/utils/author.rs
index e9d69407df8..316c1f32d3a 100644
--- a/clippy_lints/src/utils/author.rs
+++ b/clippy_lints/src/utils/author.rs
@@ -5,10 +5,9 @@ use clippy_utils::{get_attr, higher};
 use rustc_ast::ast::{LitFloatType, LitKind};
 use rustc_ast::LitIntType;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_hir as hir;
 use rustc_hir::{
-    ArrayLen, BindingMode, CaptureBy, Closure, ClosureKind, CoroutineKind, ExprKind, FnRetTy, HirId, Lit, PatKind,
-    QPath, StmtKind, TyKind,
+    self as hir, ArrayLen, BindingMode, CaptureBy, Closure, ClosureKind, ConstArg, ConstArgKind, CoroutineKind,
+    ExprKind, FnRetTy, HirId, Lit, PatKind, QPath, StmtKind, TyKind,
 };
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_session::declare_lint_pass;
@@ -270,6 +269,21 @@ impl<'a, 'tcx> PrintVisitor<'a, 'tcx> {
         }
     }
 
+    fn const_arg(&self, const_arg: &Binding<&ConstArg<'_>>) {
+        match const_arg.value.kind {
+            ConstArgKind::Path(ref qpath) => {
+                bind!(self, qpath);
+                chain!(self, "let ConstArgKind::Path(ref {qpath}) = {const_arg}.kind");
+                self.qpath(qpath);
+            },
+            ConstArgKind::Anon(anon_const) => {
+                bind!(self, anon_const);
+                chain!(self, "let ConstArgKind::Anon({anon_const}) = {const_arg}.kind");
+                self.body(field!(anon_const.body));
+            },
+        }
+    }
+
     fn lit(&self, lit: &Binding<&Lit>) {
         let kind = |kind| chain!(self, "let LitKind::{kind} = {lit}.node");
         macro_rules! kind {
@@ -602,10 +616,10 @@ impl<'a, 'tcx> PrintVisitor<'a, 'tcx> {
                 self.expr(value);
                 match length.value {
                     ArrayLen::Infer(..) => chain!(self, "let ArrayLen::Infer(..) = length"),
-                    ArrayLen::Body(anon_const) => {
-                        bind!(self, anon_const);
-                        chain!(self, "let ArrayLen::Body({anon_const}) = {length}");
-                        self.body(field!(anon_const.body));
+                    ArrayLen::Body(const_arg) => {
+                        bind!(self, const_arg);
+                        chain!(self, "let ArrayLen::Body({const_arg}) = {length}");
+                        self.const_arg(const_arg);
                     },
                 }
             },
diff --git a/clippy_utils/src/hir_utils.rs b/clippy_utils/src/hir_utils.rs
index 8706cec5d38..6c6a237a8b1 100644
--- a/clippy_utils/src/hir_utils.rs
+++ b/clippy_utils/src/hir_utils.rs
@@ -7,9 +7,9 @@ use rustc_data_structures::fx::FxHasher;
 use rustc_hir::def::Res;
 use rustc_hir::MatchSource::TryDesugar;
 use rustc_hir::{
-    ArrayLen, AssocItemConstraint, BinOpKind, BindingMode, Block, BodyId, Closure, Expr, ExprField, ExprKind, FnRetTy,
-    GenericArg, GenericArgs, HirId, HirIdMap, InlineAsmOperand, LetExpr, Lifetime, LifetimeName, Pat, PatField,
-    PatKind, Path, PathSegment, PrimTy, QPath, Stmt, StmtKind, Ty, TyKind,
+    ArrayLen, AssocItemConstraint, BinOpKind, BindingMode, Block, BodyId, Closure, ConstArg, ConstArgKind, Expr,
+    ExprField, ExprKind, FnRetTy, GenericArg, GenericArgs, HirId, HirIdMap, InlineAsmOperand, LetExpr, Lifetime,
+    LifetimeName, Pat, PatField, PatKind, Path, PathSegment, PrimTy, QPath, Stmt, StmtKind, Ty, TyKind,
 };
 use rustc_lexer::{tokenize, TokenKind};
 use rustc_lint::LateContext;
@@ -227,7 +227,7 @@ impl HirEqInterExpr<'_, '_, '_> {
     pub fn eq_array_length(&mut self, left: ArrayLen<'_>, right: ArrayLen<'_>) -> bool {
         match (left, right) {
             (ArrayLen::Infer(..), ArrayLen::Infer(..)) => true,
-            (ArrayLen::Body(l_ct), ArrayLen::Body(r_ct)) => self.eq_body(l_ct.body, r_ct.body),
+            (ArrayLen::Body(l_ct), ArrayLen::Body(r_ct)) => self.eq_const_arg(l_ct, r_ct),
             (_, _) => false,
         }
     }
@@ -411,7 +411,7 @@ impl HirEqInterExpr<'_, '_, '_> {
 
     fn eq_generic_arg(&mut self, left: &GenericArg<'_>, right: &GenericArg<'_>) -> bool {
         match (left, right) {
-            (GenericArg::Const(l), GenericArg::Const(r)) => self.eq_body(l.value.body, r.value.body),
+            (GenericArg::Const(l), GenericArg::Const(r)) => self.eq_const_arg(l, r),
             (GenericArg::Lifetime(l_lt), GenericArg::Lifetime(r_lt)) => Self::eq_lifetime(l_lt, r_lt),
             (GenericArg::Type(l_ty), GenericArg::Type(r_ty)) => self.eq_ty(l_ty, r_ty),
             (GenericArg::Infer(l_inf), GenericArg::Infer(r_inf)) => self.eq_ty(&l_inf.to_ty(), &r_inf.to_ty()),
@@ -419,6 +419,17 @@ impl HirEqInterExpr<'_, '_, '_> {
         }
     }
 
+    fn eq_const_arg(&mut self, left: &ConstArg<'_>, right: &ConstArg<'_>) -> bool {
+        match (&left.kind, &right.kind) {
+            (ConstArgKind::Path(l_p), ConstArgKind::Path(r_p)) => self.eq_qpath(l_p, r_p),
+            (ConstArgKind::Anon(l_an), ConstArgKind::Anon(r_an)) => self.eq_body(l_an.body, r_an.body),
+            // Use explicit match for now since ConstArg is undergoing flux.
+            (ConstArgKind::Path(..), ConstArgKind::Anon(..)) | (ConstArgKind::Anon(..), ConstArgKind::Path(..)) => {
+                false
+            },
+        }
+    }
+
     fn eq_lifetime(left: &Lifetime, right: &Lifetime) -> bool {
         left.res == right.res
     }
@@ -1123,7 +1134,7 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
     pub fn hash_array_length(&mut self, length: ArrayLen<'_>) {
         match length {
             ArrayLen::Infer(..) => {},
-            ArrayLen::Body(anon_const) => self.hash_body(anon_const.body),
+            ArrayLen::Body(ct) => self.hash_const_arg(ct),
         }
     }
 
@@ -1134,12 +1145,19 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
         self.maybe_typeck_results = old_maybe_typeck_results;
     }
 
+    fn hash_const_arg(&mut self, const_arg: &ConstArg<'_>) {
+        match &const_arg.kind {
+            ConstArgKind::Path(path) => self.hash_qpath(path),
+            ConstArgKind::Anon(anon) => self.hash_body(anon.body),
+        }
+    }
+
     fn hash_generic_args(&mut self, arg_list: &[GenericArg<'_>]) {
         for arg in arg_list {
             match *arg {
                 GenericArg::Lifetime(l) => self.hash_lifetime(l),
                 GenericArg::Type(ty) => self.hash_ty(ty),
-                GenericArg::Const(ref ca) => self.hash_body(ca.value.body),
+                GenericArg::Const(ref ca) => self.hash_const_arg(ca),
                 GenericArg::Infer(ref inf) => self.hash_ty(&inf.to_ty()),
             }
         }
diff --git a/clippy_utils/src/lib.rs b/clippy_utils/src/lib.rs
index bdb3b5e45c4..8c33c34fa1c 100644
--- a/clippy_utils/src/lib.rs
+++ b/clippy_utils/src/lib.rs
@@ -102,11 +102,11 @@ use rustc_hir::hir_id::{HirIdMap, HirIdSet};
 use rustc_hir::intravisit::{walk_expr, FnKind, Visitor};
 use rustc_hir::LangItem::{OptionNone, OptionSome, ResultErr, ResultOk};
 use rustc_hir::{
-    self as hir, def, Arm, ArrayLen, BindingMode, Block, BlockCheckMode, Body, ByRef, Closure, ConstContext,
-    Destination, Expr, ExprField, ExprKind, FnDecl, FnRetTy, GenericArgs, HirId, Impl, ImplItem, ImplItemKind,
-    ImplItemRef, Item, ItemKind, LangItem, LetStmt, MatchSource, Mutability, Node, OwnerId, OwnerNode, Param, Pat,
-    PatKind, Path, PathSegment, PrimTy, QPath, Stmt, StmtKind, TraitItem, TraitItemKind, TraitItemRef, TraitRef,
-    TyKind, UnOp,
+    self as hir, def, Arm, ArrayLen, BindingMode, Block, BlockCheckMode, Body, ByRef, Closure, ConstArgKind,
+    ConstContext, Destination, Expr, ExprField, ExprKind, FnDecl, FnRetTy, GenericArgs, HirId, Impl, ImplItem,
+    ImplItemKind, ImplItemRef, Item, ItemKind, LangItem, LetStmt, MatchSource, Mutability, Node, OwnerId, OwnerNode,
+    Param, Pat, PatKind, Path, PathSegment, PrimTy, QPath, Stmt, StmtKind, TraitItem, TraitItemKind, TraitItemRef,
+    TraitRef, TyKind, UnOp,
 };
 use rustc_lexer::{tokenize, TokenKind};
 use rustc_lint::{LateContext, Level, Lint, LintContext};
@@ -904,7 +904,8 @@ pub fn is_default_equivalent(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
         },
         ExprKind::Tup(items) | ExprKind::Array(items) => items.iter().all(|x| is_default_equivalent(cx, x)),
         ExprKind::Repeat(x, ArrayLen::Body(len)) => {
-            if let ExprKind::Lit(const_lit) = cx.tcx.hir().body(len.body).value.kind
+            if let ConstArgKind::Anon(anon_const) = len.kind
+                && let ExprKind::Lit(const_lit) = cx.tcx.hir().body(anon_const.body).value.kind
                 && let LitKind::Int(v, _) = const_lit.node
                 && v <= 32
                 && is_default_equivalent(cx, x)
@@ -933,7 +934,8 @@ fn is_default_equivalent_from(cx: &LateContext<'_>, from_func: &Expr<'_>, arg: &
             }) => return sym.is_empty() && is_path_lang_item(cx, ty, LangItem::String),
             ExprKind::Array([]) => return is_path_diagnostic_item(cx, ty, sym::Vec),
             ExprKind::Repeat(_, ArrayLen::Body(len)) => {
-                if let ExprKind::Lit(const_lit) = cx.tcx.hir().body(len.body).value.kind
+                if let ConstArgKind::Anon(anon_const) = len.kind
+                    && let ExprKind::Lit(const_lit) = cx.tcx.hir().body(anon_const.body).value.kind
                     && let LitKind::Int(v, _) = const_lit.node
                 {
                     return v == 0 && is_path_diagnostic_item(cx, ty, sym::Vec);
diff --git a/tests/ui/author/repeat.stdout b/tests/ui/author/repeat.stdout
index c2a369610cc..d9e3f864f12 100644
--- a/tests/ui/author/repeat.stdout
+++ b/tests/ui/author/repeat.stdout
@@ -1,7 +1,8 @@
 if let ExprKind::Repeat(value, length) = expr.kind
     && let ExprKind::Lit(ref lit) = value.kind
     && let LitKind::Int(1, LitIntType::Unsigned(UintTy::U8)) = lit.node
-    && let ArrayLen::Body(anon_const) = length
+    && let ArrayLen::Body(const_arg) = length
+    && let ConstArgKind::Anon(anon_const) = const_arg.kind
     && expr1 = &cx.tcx.hir().body(anon_const.body).value
     && let ExprKind::Lit(ref lit1) = expr1.kind
     && let LitKind::Int(5, LitIntType::Unsuffixed) = lit1.node
diff --git a/tests/ui/min_ident_chars.stderr b/tests/ui/min_ident_chars.stderr
index 3dd5c9561fd..7fbd8462fdc 100644
--- a/tests/ui/min_ident_chars.stderr
+++ b/tests/ui/min_ident_chars.stderr
@@ -193,5 +193,11 @@ error: this ident consists of a single char
 LL | fn wrong_pythagoras(a: f32, b: f32) -> f32 {
    |                             ^
 
-error: aborting due to 32 previous errors
+error: this ident consists of a single char
+  --> tests/ui/min_ident_chars.rs:93:41
+   |
+LL |     struct Array<T, const N: usize>([T; N]);
+   |                                         ^
+
+error: aborting due to 33 previous errors