Auto merge of rust-lang#135186 - camelid:const-path-multi, r=BoxyUwU

mgca: Lower all const paths as `ConstArgKind::Path`

When `#![feature(min_generic_const_args)]` is enabled, we now lower all
const paths in generic arg position to `hir::ConstArgKind::Path`. We
then lower assoc const paths to `ty::ConstKind::Unevaluated` since we
can no longer use the anon const expression lowering machinery. In the
process of implementing this, I factored out `hir_ty_lowering` code that
is now shared between lowering assoc types and assoc consts.

This PR also introduces a `#[type_const]` attribute for trait assoc
consts that are allowed as const args. However, we still need to
implement code to check that assoc const definitions satisfy
`#[type_const]` if present (basically is it a const path or a
monomorphic anon const).

r? `@BoxyUwU`

Author: bors

Diff for: compiler/rustc_ast/src/ast.rs

@@ -124,10 +124,19 @@ impl Path {
         self.segments.first().is_some_and(|segment| segment.ident.name == kw::PathRoot)
     }
 
-    /// If this path is a single identifier with no arguments, does not ensure
-    /// that the path resolves to a const param, the caller should check this.
-    pub fn is_potential_trivial_const_arg(&self) -> bool {
-        matches!(self.segments[..], [PathSegment { args: None, .. }])
+    /// Check if this path is potentially a trivial const arg, i.e., one that can _potentially_
+    /// be represented without an anon const in the HIR.
+    ///
+    /// If `allow_mgca_arg` is true (as should be the case in most situations when
+    /// `#![feature(min_generic_const_args)]` is enabled), then this always returns true
+    /// because all paths are valid.
+    ///
+    /// Otherwise, it returns true iff the path has exactly one segment, and it has no generic args
+    /// (i.e., it is _potentially_ a const parameter).
+    #[tracing::instrument(level = "debug", ret)]
+    pub fn is_potential_trivial_const_arg(&self, allow_mgca_arg: bool) -> bool {
+        allow_mgca_arg
+            || self.segments.len() == 1 && self.segments.iter().all(|seg| seg.args.is_none())
     }
 }
 
@@ -1208,22 +1217,31 @@ pub struct Expr {
 }
 
 impl Expr {
-    /// Could this expr be either `N`, or `{ N }`, where `N` is a const parameter.
+    /// Check if this expression is potentially a trivial const arg, i.e., one that can _potentially_
+    /// be represented without an anon const in the HIR.
+    ///
+    /// This will unwrap at most one block level (curly braces). After that, if the expression
+    /// is a path, it mostly dispatches to [`Path::is_potential_trivial_const_arg`].
+    /// See there for more info about `allow_mgca_arg`.
     ///
-    /// If this is not the case, name resolution does not resolve `N` when using
-    /// `min_const_generics` as more complex expressions are not supported.
+    /// The only additional thing to note is that when `allow_mgca_arg` is false, this function
+    /// will only allow paths with no qself, before dispatching to the `Path` function of
+    /// the same name.
     ///
-    /// Does not ensure that the path resolves to a const param, the caller should check this.
+    /// Does not ensure that the path resolves to a const param/item, the caller should check this.
     /// This also does not consider macros, so it's only correct after macro-expansion.
-    pub fn is_potential_trivial_const_arg(&self) -> bool {
+    pub fn is_potential_trivial_const_arg(&self, allow_mgca_arg: bool) -> bool {
         let this = self.maybe_unwrap_block();
-
-        if let ExprKind::Path(None, path) = &this.kind
-            && path.is_potential_trivial_const_arg()
-        {
-            true
+        if allow_mgca_arg {
+            matches!(this.kind, ExprKind::Path(..))
         } else {
-            false
+            if let ExprKind::Path(None, path) = &this.kind
+                && path.is_potential_trivial_const_arg(allow_mgca_arg)
+            {
+                true
+            } else {
+                false
+            }
         }
     }
 

Diff for: compiler/rustc_ast_lowering/src/lib.rs

@@ -1094,7 +1094,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             .and_then(|partial_res| partial_res.full_res())
                         {
                             if !res.matches_ns(Namespace::TypeNS)
-                                && path.is_potential_trivial_const_arg()
+                                && path.is_potential_trivial_const_arg(false)
                             {
                                 debug!(
                                     "lower_generic_arg: Lowering type argument as const argument: {:?}",
@@ -2061,8 +2061,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     ) -> &'hir hir::ConstArg<'hir> {
         let tcx = self.tcx;
 
-        // FIXME(min_generic_const_args): we only allow one-segment const paths for now
-        let ct_kind = if path.is_potential_trivial_const_arg()
+        let ct_kind = if path
+            .is_potential_trivial_const_arg(tcx.features().min_generic_const_args())
             && (tcx.features().min_generic_const_args()
                 || matches!(res, Res::Def(DefKind::ConstParam, _)))
         {
@@ -2072,7 +2072,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 path,
                 ParamMode::Optional,
                 AllowReturnTypeNotation::No,
-                // FIXME(min_generic_const_args): update for `fn foo() -> Bar<FOO<impl Trait>>` support
+                // FIXME(mgca): update for `fn foo() -> Bar<FOO<impl Trait>>` support
                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 None,
             );
@@ -2136,19 +2136,18 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         };
         let maybe_res =
             self.resolver.get_partial_res(expr.id).and_then(|partial_res| partial_res.full_res());
-        // FIXME(min_generic_const_args): we only allow one-segment const paths for now
-        if let ExprKind::Path(None, path) = &expr.kind
-            && path.is_potential_trivial_const_arg()
+        if let ExprKind::Path(qself, path) = &expr.kind
+            && path.is_potential_trivial_const_arg(tcx.features().min_generic_const_args())
             && (tcx.features().min_generic_const_args()
                 || matches!(maybe_res, Some(Res::Def(DefKind::ConstParam, _))))
         {
             let qpath = self.lower_qpath(
                 expr.id,
-                &None,
+                qself,
                 path,
                 ParamMode::Optional,
                 AllowReturnTypeNotation::No,
-                // FIXME(min_generic_const_args): update for `fn foo() -> Bar<FOO<impl Trait>>` support
+                // FIXME(mgca): update for `fn foo() -> Bar<FOO<impl Trait>>` support
                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 None,
             );

Diff for: compiler/rustc_builtin_macros/src/format.rs

@@ -190,7 +190,8 @@ fn make_format_args(
                                 && let [stmt] = block.stmts.as_slice()
                                 && let StmtKind::Expr(expr) = &stmt.kind
                                 && let ExprKind::Path(None, path) = &expr.kind
-                                && path.is_potential_trivial_const_arg()
+                                && path.segments.len() == 1
+                                && path.segments[0].args.is_none()
                             {
                                 err.multipart_suggestion(
                                     "quote your inlined format argument to use as string literal",

Diff for: compiler/rustc_feature/src/builtin_attrs.rs

@@ -576,6 +576,13 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         EncodeCrossCrate::Yes, experimental!(patchable_function_entry)
     ),
 
+    // Probably temporary component of min_generic_const_args.
+    // `#[type_const] const ASSOC: usize;`
+    gated!(
+        type_const, Normal, template!(Word), ErrorFollowing,
+        EncodeCrossCrate::Yes, min_generic_const_args, experimental!(type_const),
+    ),
+
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
     // ==========================================================================

Diff for: compiler/rustc_hir_analysis/src/collect.rs

@@ -45,6 +45,7 @@ use tracing::{debug, instrument};
 
 use crate::check::intrinsic::intrinsic_operation_unsafety;
 use crate::errors;
+use crate::hir_ty_lowering::errors::assoc_kind_str;
 use crate::hir_ty_lowering::{FeedConstTy, HirTyLowerer, RegionInferReason};
 
 pub(crate) mod dump;
@@ -443,21 +444,22 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
         self.tcx.at(span).type_param_predicates((self.item_def_id, def_id, assoc_name))
     }
 
-    fn lower_assoc_ty(
+    fn lower_assoc_shared(
         &self,
         span: Span,
         item_def_id: DefId,
-        item_segment: &hir::PathSegment<'tcx>,
+        item_segment: &rustc_hir::PathSegment<'tcx>,
         poly_trait_ref: ty::PolyTraitRef<'tcx>,
-    ) -> Ty<'tcx> {
+        kind: ty::AssocKind,
+    ) -> Result<(DefId, ty::GenericArgsRef<'tcx>), ErrorGuaranteed> {
         if let Some(trait_ref) = poly_trait_ref.no_bound_vars() {
             let item_args = self.lowerer().lower_generic_args_of_assoc_item(
                 span,
                 item_def_id,
                 item_segment,
                 trait_ref.args,
             );
-            Ty::new_projection_from_args(self.tcx(), item_def_id, item_args)
+            Ok((item_def_id, item_args))
         } else {
             // There are no late-bound regions; we can just ignore the binder.
             let (mut mpart_sugg, mut inferred_sugg) = (None, None);
@@ -518,16 +520,14 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
                 }
                 _ => {}
             }
-            Ty::new_error(
-                self.tcx(),
-                self.tcx().dcx().emit_err(errors::AssociatedItemTraitUninferredGenericParams {
-                    span,
-                    inferred_sugg,
-                    bound,
-                    mpart_sugg,
-                    what: "type",
-                }),
-            )
+
+            Err(self.tcx().dcx().emit_err(errors::AssociatedItemTraitUninferredGenericParams {
+                span,
+                inferred_sugg,
+                bound,
+                mpart_sugg,
+                what: assoc_kind_str(kind),
+            }))
         }
     }
 

Diff for: compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -468,11 +468,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
                 // Good error for `where Trait::method(..): Send`.
                 let Some(self_ty) = opt_self_ty else {
-                    return self.error_missing_qpath_self_ty(
+                    let guar = self.error_missing_qpath_self_ty(
                         trait_def_id,
                         hir_ty.span,
                         item_segment,
+                        ty::AssocKind::Type,
                     );
+                    return Ty::new_error(tcx, guar);
                 };
                 let self_ty = self.lower_ty(self_ty);
 

Diff for: compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -385,14 +385,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         })
     }
 
-    pub(super) fn report_ambiguous_assoc_ty(
+    pub(super) fn report_ambiguous_assoc(
         &self,
         span: Span,
         types: &[String],
         traits: &[String],
         name: Symbol,
+        kind: ty::AssocKind,
     ) -> ErrorGuaranteed {
-        let mut err = struct_span_code_err!(self.dcx(), span, E0223, "ambiguous associated type");
+        let kind_str = assoc_kind_str(kind);
+        let mut err =
+            struct_span_code_err!(self.dcx(), span, E0223, "ambiguous associated {kind_str}");
         if self
             .tcx()
             .resolutions(())
@@ -417,7 +420,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         span,
                         format!(
                             "if there were a type named `Type` that implements a trait named \
-                             `Trait` with associated type `{name}`, you could use the \
+                             `Trait` with associated {kind_str} `{name}`, you could use the \
                              fully-qualified path",
                         ),
                         format!("<Type as Trait>::{name}"),
@@ -440,7 +443,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         span,
                         format!(
                             "if there were a type named `Example` that implemented one of the \
-                             traits with associated type `{name}`, you could use the \
+                             traits with associated {kind_str} `{name}`, you could use the \
                              fully-qualified path",
                         ),
                         traits.iter().map(|trait_str| format!("<Example as {trait_str}>::{name}")),
@@ -451,7 +454,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     err.span_suggestion_verbose(
                         span,
                         format!(
-                            "if there were a trait named `Example` with associated type `{name}` \
+                            "if there were a trait named `Example` with associated {kind_str} `{name}` \
                              implemented for `{type_str}`, you could use the fully-qualified path",
                         ),
                         format!("<{type_str} as Example>::{name}"),
@@ -462,7 +465,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     err.span_suggestions(
                         span,
                         format!(
-                            "if there were a trait named `Example` with associated type `{name}` \
+                            "if there were a trait named `Example` with associated {kind_str} `{name}` \
                              implemented for one of the types, you could use the fully-qualified \
                              path",
                         ),
@@ -491,7 +494,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         err.emit()
     }
 
-    pub(crate) fn complain_about_ambiguous_inherent_assoc_ty(
+    pub(crate) fn complain_about_ambiguous_inherent_assoc(
         &self,
         name: Ident,
         candidates: Vec<DefId>,
@@ -552,13 +555,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     }
 
     // FIXME(inherent_associated_types): Find similarly named associated types and suggest them.
-    pub(crate) fn complain_about_inherent_assoc_ty_not_found(
+    pub(crate) fn complain_about_inherent_assoc_not_found(
         &self,
         name: Ident,
         self_ty: Ty<'tcx>,
         candidates: Vec<(DefId, (DefId, DefId))>,
         fulfillment_errors: Vec<FulfillmentError<'tcx>>,
         span: Span,
+        kind: ty::AssocKind,
     ) -> ErrorGuaranteed {
         // FIXME(fmease): This was copied in parts from an old version of `rustc_hir_typeck::method::suggest`.
         // Either
@@ -568,12 +572,16 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let tcx = self.tcx();
 
+        let kind_str = assoc_kind_str(kind);
         let adt_did = self_ty.ty_adt_def().map(|def| def.did());
         let add_def_label = |err: &mut Diag<'_>| {
             if let Some(did) = adt_did {
                 err.span_label(
                     tcx.def_span(did),
-                    format!("associated item `{name}` not found for this {}", tcx.def_descr(did)),
+                    format!(
+                        "associated {kind_str} `{name}` not found for this {}",
+                        tcx.def_descr(did)
+                    ),
                 );
             }
         };
@@ -600,11 +608,11 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 self.dcx(),
                 name.span,
                 E0220,
-                "associated type `{name}` not found for `{self_ty}` in the current scope"
+                "associated {kind_str} `{name}` not found for `{self_ty}` in the current scope"
             );
             err.span_label(name.span, format!("associated item not found in `{self_ty}`"));
             err.note(format!(
-                "the associated type was found for\n{type_candidates}{additional_types}",
+                "the associated {kind_str} was found for\n{type_candidates}{additional_types}",
             ));
             add_def_label(&mut err);
             return err.emit();
@@ -685,7 +693,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let mut err = self.dcx().struct_span_err(
             name.span,
-            format!("the associated type `{name}` exists for `{self_ty}`, but its trait bounds were not satisfied")
+            format!("the associated {kind_str} `{name}` exists for `{self_ty}`, but its trait bounds were not satisfied")
         );
         if !bounds.is_empty() {
             err.note(format!(
@@ -695,7 +703,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
         err.span_label(
             name.span,
-            format!("associated type cannot be referenced on `{self_ty}` due to unsatisfied trait bounds")
+            format!("associated {kind_str} cannot be referenced on `{self_ty}` due to unsatisfied trait bounds")
         );
 
         for (span, mut bounds) in bound_spans {
@@ -1614,7 +1622,7 @@ fn generics_args_err_extend<'a>(
     }
 }
 
-pub(super) fn assoc_kind_str(kind: ty::AssocKind) -> &'static str {
+pub(crate) fn assoc_kind_str(kind: ty::AssocKind) -> &'static str {
     match kind {
         ty::AssocKind::Fn => "function",
         ty::AssocKind::Const => "constant",

Diff for: compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -115,7 +115,7 @@ fn generic_arg_mismatch_err(
                     body.value.kind
                 && let Res::Def(DefKind::Fn { .. }, id) = path.res
             {
-                // FIXME(min_generic_const_args): this branch is dead once new const path lowering
+                // FIXME(mgca): this branch is dead once new const path lowering
                 // (for single-segment paths) is no longer gated
                 err.help(format!("`{}` is a function item, not a type", tcx.item_name(id)));
                 err.help("function item types cannot be named directly");