Refactor path lowering

And add a new diagnostic for non-`Fn` parenthesized generic args.

Path lowering started to look like a mess, with each function carrying additional parameters for the diagnostic callback (since paths can occur both in type and in expression/pattern position, and their diagnostic handling is different) and the segment index, for the diagnostics report. So I refactored it from stateless functions on `TyLoweringContext` into stateful struct, `PathLoweringContext`, that tracks the process of lowering a path from resolution til assoc types selection.

crates/hir-def/src/data.rs

@@ -250,6 +250,7 @@ bitflags::bitflags! {
         const RUSTC_HAS_INCOHERENT_INHERENT_IMPLS = 1 << 3;
         const SKIP_ARRAY_DURING_METHOD_DISPATCH = 1 << 4;
         const SKIP_BOXED_SLICE_DURING_METHOD_DISPATCH = 1 << 5;
+        const RUSTC_PAREN_SUGAR = 1 << 6;
     }
 }
 
@@ -294,6 +295,9 @@ impl TraitData {
         if attrs.by_key(&sym::rustc_has_incoherent_inherent_impls).exists() {
             flags |= TraitFlags::RUSTC_HAS_INCOHERENT_INHERENT_IMPLS;
         }
+        if attrs.by_key(&sym::rustc_paren_sugar).exists() {
+            flags |= TraitFlags::RUSTC_PAREN_SUGAR;
+        }
 
         let mut skip_array_during_method_dispatch =
             attrs.by_key(&sym::rustc_skip_array_during_method_dispatch).exists();

crates/hir-def/src/path.rs

@@ -173,10 +173,7 @@ impl Path {
                 segments: path.mod_path().segments(),
                 generic_args: Some(path.generic_args()),
             },
-            Path::LangItem(_, seg) => PathSegments {
-                segments: seg.as_ref().map_or(&[], |seg| std::slice::from_ref(seg)),
-                generic_args: None,
-            },
+            Path::LangItem(_, seg) => PathSegments { segments: seg.as_slice(), generic_args: None },
         }
     }
 
@@ -240,6 +237,11 @@ pub struct PathSegment<'a> {
     pub args_and_bindings: Option<&'a GenericArgs>,
 }
 
+impl PathSegment<'_> {
+    pub const MISSING: PathSegment<'static> =
+        PathSegment { name: Name::MISSING, args_and_bindings: None };
+}
+
 #[derive(Debug, Clone, Copy)]
 pub struct PathSegments<'a> {
     segments: &'a [Name],

crates/hir-def/src/resolver.rs

@@ -327,8 +327,9 @@ impl Resolver {
                     | LangItemTarget::ImplDef(_)
                     | LangItemTarget::Static(_) => return None,
                 };
+                // Remaining segments start from 0 because lang paths have no segments other than the remaining.
                 return Some((
-                    ResolveValueResult::Partial(type_ns, 1, None),
+                    ResolveValueResult::Partial(type_ns, 0, None),
                     ResolvePathResultPrefixInfo::default(),
                 ));
             }

crates/hir-expand/src/name.rs

@@ -70,6 +70,8 @@ impl PartialEq<Name> for &Symbol {
 }
 
 impl Name {
+    pub const MISSING: &Name = &Name { symbol: sym::consts::missing, ctx: () };
+
     fn new_text(text: &str) -> Name {
         Name { symbol: Symbol::intern(text), ctx: () }
     }

crates/hir-ty/src/infer.rs

@@ -16,7 +16,7 @@
 pub(crate) mod cast;
 pub(crate) mod closure;
 mod coerce;
-mod diagnostics;
+pub(crate) mod diagnostics;
 mod expr;
 mod mutability;
 mod pat;
@@ -1480,21 +1480,22 @@ impl<'a> InferenceContext<'a> {
             &self.diagnostics,
             InferenceTyDiagnosticSource::Body,
         );
+        let mut path_ctx = ctx.at_path(path, node);
         let (resolution, unresolved) = if value_ns {
-            let Some(res) = ctx.resolve_path_in_value_ns(path, node, HygieneId::ROOT) else {
+            let Some(res) = path_ctx.resolve_path_in_value_ns(HygieneId::ROOT) else {
                 return (self.err_ty(), None);
             };
             match res {
                 ResolveValueResult::ValueNs(value, _) => match value {
                     ValueNs::EnumVariantId(var) => {
-                        let substs = ctx.substs_from_path(path, var.into(), true);
+                        let substs = path_ctx.substs_from_path(var.into(), true);
                         drop(ctx);
                         let ty = self.db.ty(var.lookup(self.db.upcast()).parent.into());
                         let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
                         return (ty, Some(var.into()));
                     }
                     ValueNs::StructId(strukt) => {
-                        let substs = ctx.substs_from_path(path, strukt.into(), true);
+                        let substs = path_ctx.substs_from_path(strukt.into(), true);
                         drop(ctx);
                         let ty = self.db.ty(strukt.into());
                         let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
@@ -1509,7 +1510,7 @@ impl<'a> InferenceContext<'a> {
                 ResolveValueResult::Partial(typens, unresolved, _) => (typens, Some(unresolved)),
             }
         } else {
-            match ctx.resolve_path_in_type_ns(path, node) {
+            match path_ctx.resolve_path_in_type_ns() {
                 Some((it, idx)) => (it, idx),
                 None => return (self.err_ty(), None),
             }
@@ -1520,21 +1521,21 @@ impl<'a> InferenceContext<'a> {
         };
         return match resolution {
             TypeNs::AdtId(AdtId::StructId(strukt)) => {
-                let substs = ctx.substs_from_path(path, strukt.into(), true);
+                let substs = path_ctx.substs_from_path(strukt.into(), true);
                 drop(ctx);
                 let ty = self.db.ty(strukt.into());
                 let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
                 forbid_unresolved_segments((ty, Some(strukt.into())), unresolved)
             }
             TypeNs::AdtId(AdtId::UnionId(u)) => {
-                let substs = ctx.substs_from_path(path, u.into(), true);
+                let substs = path_ctx.substs_from_path(u.into(), true);
                 drop(ctx);
                 let ty = self.db.ty(u.into());
                 let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
                 forbid_unresolved_segments((ty, Some(u.into())), unresolved)
             }
             TypeNs::EnumVariantId(var) => {
-                let substs = ctx.substs_from_path(path, var.into(), true);
+                let substs = path_ctx.substs_from_path(var.into(), true);
                 drop(ctx);
                 let ty = self.db.ty(var.lookup(self.db.upcast()).parent.into());
                 let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
@@ -1545,31 +1546,32 @@ impl<'a> InferenceContext<'a> {
                 let substs = generics.placeholder_subst(self.db);
                 let mut ty = self.db.impl_self_ty(impl_id).substitute(Interner, &substs);
 
-                let Some(mut remaining_idx) = unresolved else {
+                let Some(remaining_idx) = unresolved else {
                     drop(ctx);
                     return self.resolve_variant_on_alias(ty, None, mod_path);
                 };
 
                 let mut remaining_segments = path.segments().skip(remaining_idx);
 
+                if remaining_segments.len() >= 2 {
+                    path_ctx.ignore_last_segment();
+                }
+
                 // We need to try resolving unresolved segments one by one because each may resolve
                 // to a projection, which `TyLoweringContext` cannot handle on its own.
                 let mut tried_resolving_once = false;
-                while !remaining_segments.is_empty() {
-                    let resolved_segment = path.segments().get(remaining_idx - 1).unwrap();
-                    let current_segment = remaining_segments.take(1);
-
+                while let Some(current_segment) = remaining_segments.first() {
                     // If we can resolve to an enum variant, it takes priority over associated type
                     // of the same name.
                     if let Some((AdtId::EnumId(id), _)) = ty.as_adt() {
                         let enum_data = self.db.enum_data(id);
-                        let name = current_segment.first().unwrap().name;
-                        if let Some(variant) = enum_data.variant(name) {
+                        if let Some(variant) = enum_data.variant(current_segment.name) {
                             return if remaining_segments.len() == 1 {
                                 (ty, Some(variant.into()))
                             } else {
                                 // We still have unresolved paths, but enum variants never have
                                 // associated types!
+                                // FIXME: Report an error.
                                 (self.err_ty(), None)
                             };
                         }
@@ -1578,23 +1580,13 @@ impl<'a> InferenceContext<'a> {
                     if tried_resolving_once {
                         // FIXME: with `inherent_associated_types` this is allowed, but our `lower_partly_resolved_path()`
                         // will need to be updated to err at the correct segment.
-                        //
-                        // We need to stop here because otherwise the segment index passed to `lower_partly_resolved_path()`
-                        // will be incorrect, and that can mess up error reporting.
                         break;
                     }
 
                     // `lower_partly_resolved_path()` returns `None` as type namespace unless
                     // `remaining_segments` is empty, which is never the case here. We don't know
                     // which namespace the new `ty` is in until normalized anyway.
-                    (ty, _) = ctx.lower_partly_resolved_path(
-                        node,
-                        resolution,
-                        resolved_segment,
-                        current_segment,
-                        (remaining_idx - 1) as u32,
-                        false,
-                    );
+                    (ty, _) = path_ctx.lower_partly_resolved_path(resolution, false);
                     tried_resolving_once = true;
 
                     ty = self.table.insert_type_vars(ty);
@@ -1604,8 +1596,6 @@ impl<'a> InferenceContext<'a> {
                         return (self.err_ty(), None);
                     }
 
-                    // FIXME(inherent_associated_types): update `resolution` based on `ty` here.
-                    remaining_idx += 1;
                     remaining_segments = remaining_segments.skip(1);
                 }
                 drop(ctx);
@@ -1621,11 +1611,7 @@ impl<'a> InferenceContext<'a> {
                 (ty, variant)
             }
             TypeNs::TypeAliasId(it) => {
-                let resolved_seg = match unresolved {
-                    None => path.segments().last().unwrap(),
-                    Some(n) => path.segments().get(path.segments().len() - n - 1).unwrap(),
-                };
-                let substs = ctx.substs_from_path_segment(resolved_seg, it.into(), true, None);
+                let substs = path_ctx.substs_from_path_segment(it.into(), true, None);
                 drop(ctx);
                 let ty = self.db.ty(it.into());
                 let ty = self.insert_type_vars(ty.substitute(Interner, &substs));

crates/hir-ty/src/infer/diagnostics.rs

@@ -5,16 +5,13 @@
 use std::cell::RefCell;
 use std::ops::{Deref, DerefMut};
 
-use hir_def::expr_store::HygieneId;
-use hir_def::hir::ExprOrPatId;
-use hir_def::path::{Path, PathSegment, PathSegments};
-use hir_def::resolver::{ResolveValueResult, Resolver, TypeNs};
-use hir_def::type_ref::TypesMap;
-use hir_def::TypeOwnerId;
+use either::Either;
+use hir_def::{hir::ExprOrPatId, path::Path, resolver::Resolver, type_ref::TypesMap, TypeOwnerId};
 
-use crate::db::HirDatabase;
 use crate::{
-    InferenceDiagnostic, InferenceTyDiagnosticSource, Ty, TyLoweringContext, TyLoweringDiagnostic,
+    db::HirDatabase,
+    lower::path::{PathDiagnosticCallback, PathLoweringContext},
+    InferenceDiagnostic, InferenceTyDiagnosticSource, TyLoweringContext, TyLoweringDiagnostic,
 };
 
 // Unfortunately, this struct needs to use interior mutability (but we encapsulate it)
@@ -44,13 +41,19 @@ impl Diagnostics {
     }
 }
 
+pub(crate) struct PathDiagnosticCallbackData<'a> {
+    node: ExprOrPatId,
+    diagnostics: &'a Diagnostics,
+}
+
 pub(super) struct InferenceTyLoweringContext<'a> {
     ctx: TyLoweringContext<'a>,
     diagnostics: &'a Diagnostics,
     source: InferenceTyDiagnosticSource,
 }
 
 impl<'a> InferenceTyLoweringContext<'a> {
+    #[inline]
     pub(super) fn new(
         db: &'a dyn HirDatabase,
         resolver: &'a Resolver,
@@ -62,65 +65,42 @@ impl<'a> InferenceTyLoweringContext<'a> {
         Self { ctx: TyLoweringContext::new(db, resolver, types_map, owner), diagnostics, source }
     }
 
-    pub(super) fn resolve_path_in_type_ns(
-        &mut self,
-        path: &Path,
-        node: ExprOrPatId,
-    ) -> Option<(TypeNs, Option<usize>)> {
-        let diagnostics = self.diagnostics;
-        self.ctx.resolve_path_in_type_ns(path, &mut |_, diag| {
-            diagnostics.push(InferenceDiagnostic::PathDiagnostic { node, diag })
-        })
-    }
-
-    pub(super) fn resolve_path_in_value_ns(
-        &mut self,
-        path: &Path,
-        node: ExprOrPatId,
-        hygiene_id: HygieneId,
-    ) -> Option<ResolveValueResult> {
-        let diagnostics = self.diagnostics;
-        self.ctx.resolve_path_in_value_ns(path, hygiene_id, &mut |_, diag| {
-            diagnostics.push(InferenceDiagnostic::PathDiagnostic { node, diag })
-        })
-    }
-
-    pub(super) fn lower_partly_resolved_path(
-        &mut self,
+    #[inline]
+    pub(super) fn at_path<'b>(
+        &'b mut self,
+        path: &'b Path,
         node: ExprOrPatId,
-        resolution: TypeNs,
-        resolved_segment: PathSegment<'_>,
-        remaining_segments: PathSegments<'_>,
-        resolved_segment_idx: u32,
-        infer_args: bool,
-    ) -> (Ty, Option<TypeNs>) {
-        let diagnostics = self.diagnostics;
-        self.ctx.lower_partly_resolved_path(
-            resolution,
-            resolved_segment,
-            remaining_segments,
-            resolved_segment_idx,
-            infer_args,
-            &mut |_, diag| diagnostics.push(InferenceDiagnostic::PathDiagnostic { node, diag }),
-        )
+    ) -> PathLoweringContext<'b, 'a> {
+        let on_diagnostic = PathDiagnosticCallback {
+            data: Either::Right(PathDiagnosticCallbackData { diagnostics: self.diagnostics, node }),
+            callback: |data, _, diag| {
+                let data = data.as_ref().right().unwrap();
+                data.diagnostics
+                    .push(InferenceDiagnostic::PathDiagnostic { node: data.node, diag });
+            },
+        };
+        PathLoweringContext::new(&mut self.ctx, on_diagnostic, path)
     }
 }
 
 impl<'a> Deref for InferenceTyLoweringContext<'a> {
     type Target = TyLoweringContext<'a>;
 
+    #[inline]
     fn deref(&self) -> &Self::Target {
         &self.ctx
     }
 }
 
 impl DerefMut for InferenceTyLoweringContext<'_> {
+    #[inline]
     fn deref_mut(&mut self) -> &mut Self::Target {
         &mut self.ctx
     }
 }
 
 impl Drop for InferenceTyLoweringContext<'_> {
+    #[inline]
     fn drop(&mut self) {
         self.diagnostics
             .push_ty_diagnostics(self.source, std::mem::take(&mut self.ctx.diagnostics));

crates/hir-ty/src/infer/pat.rs

@@ -564,9 +564,17 @@ impl InferenceContext<'_> {
             | Pat::Range { .. }
             | Pat::Slice { .. } => true,
             Pat::Or(pats) => pats.iter().all(|p| self.is_non_ref_pat(body, *p)),
-            Pat::Path(p) => {
-                let v = self.resolve_value_path_inner(p, pat.into());
-                v.is_some_and(|x| !matches!(x.0, hir_def::resolver::ValueNs::ConstId(_)))
+            Pat::Path(path) => {
+                // A const is a reference pattern, but other value ns things aren't (see #16131). We don't need more than
+                // the hir-def resolver for this, because if there are segments left, this can only be an (associated) const.
+                //
+                // Do not use `TyLoweringContext`'s resolution, we want to ignore errors here (they'll be reported elsewhere).
+                let resolution = self.resolver.resolve_path_in_value_ns_fully(
+                    self.db.upcast(),
+                    path,
+                    body.pat_path_hygiene(pat),
+                );
+                resolution.is_some_and(|it| !matches!(it, hir_def::resolver::ValueNs::ConstId(_)))
             }
             Pat::ConstBlock(..) => false,
             Pat::Lit(expr) => !matches!(