Lay the foundation for diagnostics in ty lowering, and implement a first diagnostic

The diagnostic implemented is a simple one (E0109). It serves as a test for the new foundation.

This commit only implements diagnostics for type in bodies and body-carrying signatures; the next commit will include diagnostics in the rest of the things.

Also fix one weird bug that was detected when implementing this that caused `Fn::(A, B) -> C` (which is a valid, if bizarre, alternative syntax to `Fn(A, B) -> C` to lower incorrectly.

And also fix a maybe-bug where parentheses were sneaked into a code string needlessly; this was not detected until now because the parentheses were removed (by the make-AST family API), but with a change in this commit they are now inserted. So fix that too.

Cargo.lock

@@ -509,6 +509,7 @@ dependencies = [
  "base-db",
  "cfg",
  "either",
+ "expect-test",
  "hir-def",
  "hir-expand",
  "hir-ty",
@@ -519,6 +520,9 @@ dependencies = [
  "span",
  "stdx",
  "syntax",
+ "syntax-bridge",
+ "test-fixture",
+ "test-utils",
  "tracing",
  "triomphe",
  "tt",

crates/hir-def/src/body.rs

@@ -141,7 +141,7 @@ pub struct BodySourceMap {
     field_map_back: FxHashMap<ExprId, FieldSource>,
     pat_field_map_back: FxHashMap<PatId, PatFieldSource>,
 
-    types: TypesSourceMap,
+    pub types: TypesSourceMap,
 
     // FIXME: Make this a sane struct.
     template_map: Option<

crates/hir-def/src/hir/type_ref.rs

@@ -219,6 +219,8 @@ pub struct TypesSourceMap {
 }
 
 impl TypesSourceMap {
+    pub const EMPTY: Self = Self { types_map_back: ArenaMap::new() };
+
     pub fn type_syntax(&self, id: TypeRefId) -> Result<TypeSource, SyntheticSyntax> {
         self.types_map_back.get(id).cloned().ok_or(SyntheticSyntax)
     }

crates/hir-def/src/path.rs

@@ -1,5 +1,7 @@
 //! A desugared representation of paths like `crate::foo` or `<Type as Trait>::bar`.
 mod lower;
+#[cfg(test)]
+mod tests;
 
 use std::{
     fmt::{self, Display},
@@ -19,6 +21,8 @@ use syntax::ast;
 
 pub use hir_expand::mod_path::{path, ModPath, PathKind};
 
+pub use lower::hir_segment_to_ast_segment;
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum ImportAlias {
     /// Unnamed alias, as in `use Foo as _;`
@@ -230,7 +234,7 @@ impl Path {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct PathSegment<'a> {
     pub name: &'a Name,
     pub args_and_bindings: Option<&'a GenericArgs>,
@@ -274,6 +278,12 @@ impl<'a> PathSegments<'a> {
             generic_args: self.generic_args.map(|it| it.get(..len).unwrap_or(it)),
         }
     }
+    pub fn strip_last(&self) -> PathSegments<'a> {
+        PathSegments {
+            segments: self.segments.split_last().map_or(&[], |it| it.1),
+            generic_args: self.generic_args.map(|it| it.split_last().map_or(&[][..], |it| it.1)),
+        }
+    }
     pub fn iter(&self) -> impl Iterator<Item = PathSegment<'a>> {
         self.segments
             .iter()

crates/hir-def/src/path/lower.rs

@@ -17,13 +17,31 @@ use crate::{
     type_ref::{LifetimeRef, TypeBound, TypeRef},
 };
 
+#[cfg(test)]
+thread_local! {
+    /// This is used to test `hir_segment_to_ast_segment()`. It's a hack, but it makes testing much easier.
+    pub(super) static SEGMENT_LOWERING_MAP: std::cell::RefCell<rustc_hash::FxHashMap<ast::PathSegment, usize>> = std::cell::RefCell::default();
+}
+
 /// Converts an `ast::Path` to `Path`. Works with use trees.
 /// It correctly handles `$crate` based path from macro call.
+// If you modify the logic of the lowering, make sure to check if `hir_segment_to_ast_segment()`
+// also needs an update.
 pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<Path> {
     let mut kind = PathKind::Plain;
     let mut type_anchor = None;
     let mut segments = Vec::new();
     let mut generic_args = Vec::new();
+    #[cfg(test)]
+    let mut ast_segments = Vec::new();
+    #[cfg(test)]
+    let mut ast_segments_offset = 0;
+    #[allow(unused_mut)]
+    let mut push_segment = |_segment: &ast::PathSegment, segments: &mut Vec<Name>, name| {
+        #[cfg(test)]
+        ast_segments.push(_segment.clone());
+        segments.push(name);
+    };
     loop {
         let segment = path.segment()?;
 
@@ -34,6 +52,10 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
         match segment.kind()? {
             ast::PathSegmentKind::Name(name_ref) => {
                 if name_ref.text() == "$crate" {
+                    if path.qualifier().is_some() {
+                        // FIXME: Report an error.
+                        return None;
+                    }
                     break kind = resolve_crate_root(
                         ctx.db.upcast(),
                         ctx.span_map().span_for_range(name_ref.syntax().text_range()).ctx,
@@ -56,10 +78,10 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
                     generic_args.resize(segments.len(), None);
                     generic_args.push(args);
                 }
-                segments.push(name);
+                push_segment(&segment, &mut segments, name);
             }
             ast::PathSegmentKind::SelfTypeKw => {
-                segments.push(Name::new_symbol_root(sym::Self_.clone()));
+                push_segment(&segment, &mut segments, Name::new_symbol_root(sym::Self_.clone()));
             }
             ast::PathSegmentKind::Type { type_ref, trait_ref } => {
                 assert!(path.qualifier().is_none()); // this can only occur at the first segment
@@ -81,6 +103,10 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
                         kind = mod_path.kind;
 
                         segments.extend(mod_path.segments().iter().cloned().rev());
+                        #[cfg(test)]
+                        {
+                            ast_segments_offset = mod_path.segments().len();
+                        }
                         if let Some(path_generic_args) = path_generic_args {
                             generic_args.resize(segments.len() - num_segments, None);
                             generic_args.extend(Vec::from(path_generic_args).into_iter().rev());
@@ -112,10 +138,18 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
                 }
             }
             ast::PathSegmentKind::CrateKw => {
+                if path.qualifier().is_some() {
+                    // FIXME: Report an error.
+                    return None;
+                }
                 kind = PathKind::Crate;
                 break;
             }
             ast::PathSegmentKind::SelfKw => {
+                if path.qualifier().is_some() {
+                    // FIXME: Report an error.
+                    return None;
+                }
                 // don't break out if `self` is the last segment of a path, this mean we got a
                 // use tree like `foo::{self}` which we want to resolve as `foo`
                 if !segments.is_empty() {
@@ -162,6 +196,13 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
         }
     }
 
+    #[cfg(test)]
+    {
+        ast_segments.reverse();
+        SEGMENT_LOWERING_MAP
+            .with_borrow_mut(|map| map.extend(ast_segments.into_iter().zip(ast_segments_offset..)));
+    }
+
     let mod_path = Interned::new(ModPath::from_segments(kind, segments));
     if type_anchor.is_none() && generic_args.is_empty() {
         return Some(Path::BarePath(mod_path));
@@ -181,6 +222,41 @@ pub(super) fn lower_path(ctx: &mut LowerCtx<'_>, mut path: ast::Path) -> Option<
     }
 }
 
+/// This function finds the AST segment that corresponds to the HIR segment
+/// with index `segment_idx` on the path that is lowered from `path`.
+pub fn hir_segment_to_ast_segment(path: &ast::Path, segment_idx: u32) -> Option<ast::PathSegment> {
+    // Too tightly coupled to `lower_path()`, but unfortunately we cannot decouple them,
+    // as keeping source maps for all paths segments will have a severe impact on memory usage.
+
+    let mut segments = path.segments();
+    if let Some(ast::PathSegmentKind::Type { trait_ref: Some(trait_ref), .. }) =
+        segments.clone().next().and_then(|it| it.kind())
+    {
+        segments.next();
+        return find_segment(trait_ref.path()?.segments().chain(segments), segment_idx);
+    }
+    return find_segment(segments, segment_idx);
+
+    fn find_segment(
+        segments: impl Iterator<Item = ast::PathSegment>,
+        segment_idx: u32,
+    ) -> Option<ast::PathSegment> {
+        segments
+            .filter(|segment| match segment.kind() {
+                Some(
+                    ast::PathSegmentKind::CrateKw
+                    | ast::PathSegmentKind::SelfKw
+                    | ast::PathSegmentKind::SuperKw
+                    | ast::PathSegmentKind::Type { .. },
+                )
+                | None => false,
+                Some(ast::PathSegmentKind::Name(name)) => name.text() != "$crate",
+                Some(ast::PathSegmentKind::SelfTypeKw) => true,
+            })
+            .nth(segment_idx as usize)
+    }
+}
+
 pub(super) fn lower_generic_args(
     lower_ctx: &mut LowerCtx<'_>,
     node: ast::GenericArgList,

crates/hir-def/src/path/tests.rs

@@ -0,0 +1,126 @@
+use expect_test::{expect, Expect};
+use span::Edition;
+use syntax::ast::{self, make};
+use test_fixture::WithFixture;
+
+use crate::{
+    lower::LowerCtx,
+    path::{
+        lower::{hir_segment_to_ast_segment, SEGMENT_LOWERING_MAP},
+        Path,
+    },
+    pretty,
+    test_db::TestDB,
+    type_ref::{TypesMap, TypesSourceMap},
+};
+
+fn lower_path(path: ast::Path) -> (TestDB, TypesMap, Option<Path>) {
+    let (db, file_id) = TestDB::with_single_file("");
+    let mut types_map = TypesMap::default();
+    let mut types_source_map = TypesSourceMap::default();
+    let mut ctx = LowerCtx::new(&db, file_id.into(), &mut types_map, &mut types_source_map);
+    let lowered_path = ctx.lower_path(path);
+    (db, types_map, lowered_path)
+}
+
+#[track_caller]
+fn check_hir_to_ast(path: &str, ignore_segments: &[&str]) {
+    let path = make::path_from_text(path);
+    SEGMENT_LOWERING_MAP.with_borrow_mut(|map| map.clear());
+    let _ = lower_path(path.clone()).2.expect("failed to lower path");
+    SEGMENT_LOWERING_MAP.with_borrow(|map| {
+        for (segment, segment_idx) in map {
+            if ignore_segments.contains(&&*segment.to_string()) {
+                continue;
+            }
+
+            let restored_segment = hir_segment_to_ast_segment(&path, *segment_idx as u32)
+                .unwrap_or_else(|| {
+                    panic!(
+                        "failed to map back segment `{segment}` \
+                        numbered {segment_idx} in HIR from path `{path}`"
+                    )
+                });
+            assert_eq!(
+                segment, &restored_segment,
+                "mapping back `{segment}` numbered {segment_idx} in HIR \
+                from path `{path}` produced incorrect segment `{restored_segment}`"
+            );
+        }
+    });
+}
+
+#[test]
+fn hir_to_ast_trait_ref() {
+    check_hir_to_ast("<A as B::C::D>::E::F", &["A"]);
+}
+
+#[test]
+fn hir_to_ast_plain_path() {
+    check_hir_to_ast("A::B::C::D::E::F", &[]);
+}
+
+#[test]
+fn hir_to_ast_crate_path() {
+    check_hir_to_ast("crate::A::B::C", &[]);
+    check_hir_to_ast("crate::super::super::A::B::C", &[]);
+}
+
+#[test]
+fn hir_to_ast_self_path() {
+    check_hir_to_ast("self::A::B::C", &[]);
+    check_hir_to_ast("self::super::super::A::B::C", &[]);
+}
+
+#[test]
+fn hir_to_ast_super_path() {
+    check_hir_to_ast("super::A::B::C", &[]);
+    check_hir_to_ast("super::super::super::A::B::C", &[]);
+}
+
+#[test]
+fn hir_to_ast_type_anchor_path() {
+    check_hir_to_ast("<A::B>::C::D", &["A", "B"]);
+}
+
+#[test]
+fn hir_to_ast_path_super_in_middle() {
+    check_hir_to_ast("A::super::B::super::super::C::D", &[]);
+}
+
+#[track_caller]
+fn check_fail_lowering(path: &str) {
+    let (_, _, lowered_path) = lower_path(make::path_from_text(path));
+    assert!(lowered_path.is_none(), "path `{path}` should fail lowering");
+}
+
+#[test]
+fn keywords_in_middle_fail_lowering1() {
+    check_fail_lowering("self::A::self::B::super::C::crate::D");
+}
+
+#[test]
+fn keywords_in_middle_fail_lowering2() {
+    check_fail_lowering("A::super::self::C::D");
+}
+
+#[test]
+fn keywords_in_middle_fail_lowering3() {
+    check_fail_lowering("A::crate::B::C::D");
+}
+
+#[track_caller]
+fn check_path_lowering(path: &str, expected: Expect) {
+    let (db, types_map, lowered_path) = lower_path(make::path_from_text(path));
+    let lowered_path = lowered_path.expect("failed to lower path");
+    let mut buf = String::new();
+    pretty::print_path(&db, &lowered_path, &types_map, &mut buf, Edition::CURRENT)
+        .expect("failed to pretty-print path");
+    expected.assert_eq(&buf);
+}
+
+#[test]
+fn fn_like_path_with_coloncolon() {
+    check_path_lowering("Fn::(A, B) -> C", expect![[r#"Fn::<(A, B), Output = C>"#]]);
+    check_path_lowering("Fn::(A, B)", expect![[r#"Fn::<(A, B), Output = ()>"#]]);
+}

crates/hir-expand/src/files.rs

@@ -180,6 +180,13 @@ impl<FileId: FileIdToSyntax, T> InFileWrapper<FileId, T> {
     }
 }
 
+#[allow(private_bounds)]
+impl<FileId: FileIdToSyntax, N: AstNode> InFileWrapper<FileId, AstPtr<N>> {
+    pub fn to_node(&self, db: &dyn ExpandDatabase) -> N {
+        self.value.to_node(&self.file_syntax(db))
+    }
+}
+
 impl<FileId: Copy, N: AstNode> InFileWrapper<FileId, N> {
     pub fn syntax(&self) -> InFileWrapper<FileId, &SyntaxNode> {
         self.with_value(self.value.syntax())

crates/hir-ty/src/infer.rs

@@ -58,7 +58,7 @@ use crate::{
     fold_tys,
     generics::Generics,
     infer::{coerce::CoerceMany, expr::ExprIsRead, unify::InferenceTable},
-    lower::ImplTraitLoweringMode,
+    lower::{ImplTraitLoweringMode, TyLoweringDiagnostic},
     mir::MirSpan,
     to_assoc_type_id,
     traits::FnTrait,
@@ -191,6 +191,14 @@ impl<T> InferOk<T> {
     }
 }
 
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum InferenceTyDiagnosticSource {
+    /// Diagnostics that come from types in the body.
+    Body,
+    /// Diagnostics that come from types in fn parameters/return type, or static & const types.
+    Signature,
+}
+
 #[derive(Debug)]
 pub(crate) struct TypeError;
 pub(crate) type InferResult<T> = Result<InferOk<T>, TypeError>;
@@ -264,6 +272,10 @@ pub enum InferenceDiagnostic {
         expr_ty: Ty,
         cast_ty: Ty,
     },
+    TyDiagnostic {
+        source: InferenceTyDiagnosticSource,
+        diag: TyLoweringDiagnostic,
+    },
 }
 
 /// A mismatch between an expected and an inferred type.
@@ -858,7 +870,8 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn collect_const(&mut self, data: &ConstData) {
-        let return_ty = self.make_ty(data.type_ref, &data.types_map);
+        let return_ty =
+            self.make_ty(data.type_ref, &data.types_map, InferenceTyDiagnosticSource::Signature);
 
         // Constants might be defining usage sites of TAITs.
         self.make_tait_coercion_table(iter::once(&return_ty));
@@ -867,7 +880,8 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn collect_static(&mut self, data: &StaticData) {
-        let return_ty = self.make_ty(data.type_ref, &data.types_map);
+        let return_ty =
+            self.make_ty(data.type_ref, &data.types_map, InferenceTyDiagnosticSource::Signature);
 
         // Statics might be defining usage sites of TAITs.
         self.make_tait_coercion_table(iter::once(&return_ty));
@@ -877,7 +891,8 @@ impl<'a> InferenceContext<'a> {
 
     fn collect_fn(&mut self, func: FunctionId) {
         let data = self.db.function_data(func);
-        let mut param_tys = self.with_ty_lowering(&data.types_map, |ctx| {
+        let mut param_tys =
+            self.with_ty_lowering(&data.types_map, InferenceTyDiagnosticSource::Signature, |ctx| {
                 ctx.type_param_mode(ParamLoweringMode::Placeholder)
                     .impl_trait_mode(ImplTraitLoweringMode::Param);
                 data.params.iter().map(|&type_ref| ctx.lower_ty(type_ref)).collect::<Vec<_>>()
@@ -918,7 +933,8 @@ impl<'a> InferenceContext<'a> {
         }
         let return_ty = data.ret_type;
 
-        let return_ty = self.with_ty_lowering(&data.types_map, |ctx| {
+        let return_ty =
+            self.with_ty_lowering(&data.types_map, InferenceTyDiagnosticSource::Signature, |ctx| {
                 ctx.type_param_mode(ParamLoweringMode::Placeholder)
                     .impl_trait_mode(ImplTraitLoweringMode::Opaque)
                     .lower_ty(return_ty)
@@ -1226,9 +1242,20 @@ impl<'a> InferenceContext<'a> {
         self.result.diagnostics.push(diagnostic);
     }
 
+    fn push_ty_diagnostics(
+        &mut self,
+        source: InferenceTyDiagnosticSource,
+        diagnostics: Vec<TyLoweringDiagnostic>,
+    ) {
+        self.result.diagnostics.extend(
+            diagnostics.into_iter().map(|diag| InferenceDiagnostic::TyDiagnostic { source, diag }),
+        );
+    }
+
     fn with_ty_lowering<R>(
-        &self,
+        &mut self,
         types_map: &TypesMap,
+        types_source: InferenceTyDiagnosticSource,
         f: impl FnOnce(&mut crate::lower::TyLoweringContext<'_>) -> R,
     ) -> R {
         let mut ctx = crate::lower::TyLoweringContext::new(
@@ -1237,32 +1264,41 @@ impl<'a> InferenceContext<'a> {
             types_map,
             self.owner.into(),
         );
-        f(&mut ctx)
+        let result = f(&mut ctx);
+        self.push_ty_diagnostics(types_source, ctx.diagnostics);
+        result
     }
 
     fn with_body_ty_lowering<R>(
-        &self,
+        &mut self,
         f: impl FnOnce(&mut crate::lower::TyLoweringContext<'_>) -> R,
     ) -> R {
-        self.with_ty_lowering(&self.body.types, f)
+        self.with_ty_lowering(&self.body.types, InferenceTyDiagnosticSource::Body, f)
     }
 
-    fn make_ty(&mut self, type_ref: TypeRefId, types_map: &TypesMap) -> Ty {
+    fn make_ty(
-        let ty = self.with_ty_lowering(types_map, |ctx| ctx.lower_ty(type_ref));
+        &mut self,
+        type_ref: TypeRefId,
+        types_map: &TypesMap,
+        type_source: InferenceTyDiagnosticSource,
+    ) -> Ty {
+        let ty = self.with_ty_lowering(types_map, type_source, |ctx| ctx.lower_ty(type_ref));
         let ty = self.insert_type_vars(ty);
         self.normalize_associated_types_in(ty)
     }
 
     fn make_body_ty(&mut self, type_ref: TypeRefId) -> Ty {
-        self.make_ty(type_ref, &self.body.types)
+        self.make_ty(type_ref, &self.body.types, InferenceTyDiagnosticSource::Body)
     }
 
     fn err_ty(&self) -> Ty {
         self.result.standard_types.unknown.clone()
     }
 
-    fn make_lifetime(&mut self, lifetime_ref: &LifetimeRef) -> Lifetime {
+    fn make_body_lifetime(&mut self, lifetime_ref: &LifetimeRef) -> Lifetime {
-        let lt = self.with_ty_lowering(TypesMap::EMPTY, |ctx| ctx.lower_lifetime(lifetime_ref));
+        let lt = self.with_ty_lowering(TypesMap::EMPTY, InferenceTyDiagnosticSource::Body, |ctx| {
+            ctx.lower_lifetime(lifetime_ref)
+        });
         self.insert_type_vars(lt)
     }
 
@@ -1431,12 +1467,20 @@ impl<'a> InferenceContext<'a> {
                 Some(ResolveValueResult::ValueNs(value, _)) => match value {
                     ValueNs::EnumVariantId(var) => {
                         let substs = ctx.substs_from_path(path, var.into(), true);
+                        self.push_ty_diagnostics(
+                            InferenceTyDiagnosticSource::Body,
+                            ctx.diagnostics,
+                        );
                         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);
+                        self.push_ty_diagnostics(
+                            InferenceTyDiagnosticSource::Body,
+                            ctx.diagnostics,
+                        );
                         let ty = self.db.ty(strukt.into());
                         let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
                         return (ty, Some(strukt.into()));
@@ -1462,18 +1506,21 @@ impl<'a> InferenceContext<'a> {
         return match resolution {
             TypeNs::AdtId(AdtId::StructId(strukt)) => {
                 let substs = ctx.substs_from_path(path, strukt.into(), true);
+                self.push_ty_diagnostics(InferenceTyDiagnosticSource::Body, ctx.diagnostics);
                 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);
+                self.push_ty_diagnostics(InferenceTyDiagnosticSource::Body, ctx.diagnostics);
                 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);
+                self.push_ty_diagnostics(InferenceTyDiagnosticSource::Body, ctx.diagnostics);
                 let ty = self.db.ty(var.lookup(self.db.upcast()).parent.into());
                 let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
                 forbid_unresolved_segments((ty, Some(var.into())), unresolved)
@@ -1519,6 +1566,9 @@ impl<'a> InferenceContext<'a> {
                         resolved_segment,
                         current_segment,
                         false,
+                        &mut |_, _reason| {
+                            // FIXME: Report an error.
+                        },
                     );
 
                     ty = self.table.insert_type_vars(ty);
@@ -1532,6 +1582,7 @@ impl<'a> InferenceContext<'a> {
                     remaining_idx += 1;
                     remaining_segments = remaining_segments.skip(1);
                 }
+                self.push_ty_diagnostics(InferenceTyDiagnosticSource::Body, ctx.diagnostics);
 
                 let variant = ty.as_adt().and_then(|(id, _)| match id {
                     AdtId::StructId(s) => Some(VariantId::StructId(s)),
@@ -1550,6 +1601,7 @@ impl<'a> InferenceContext<'a> {
                 };
                 let substs =
                     ctx.substs_from_path_segment(resolved_seg, Some(it.into()), true, None);
+                self.push_ty_diagnostics(InferenceTyDiagnosticSource::Body, ctx.diagnostics);
                 let ty = self.db.ty(it.into());
                 let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
 

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

@@ -2155,7 +2155,7 @@ impl InferenceContext<'_> {
                                 DebruijnIndex::INNERMOST,
                             )
                         },
-                        |this, lt_ref| this.make_lifetime(lt_ref),
+                        |this, lt_ref| this.make_body_lifetime(lt_ref),
                     ),
                 };
 

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

@@ -19,7 +19,7 @@ use crate::{
     TyBuilder, TyExt, TyKind, ValueTyDefId,
 };
 
-use super::{ExprOrPatId, InferenceContext};
+use super::{ExprOrPatId, InferenceContext, InferenceTyDiagnosticSource};
 
 impl InferenceContext<'_> {
     pub(super) fn infer_path(&mut self, path: &Path, id: ExprOrPatId) -> Option<Ty> {
@@ -163,6 +163,7 @@ impl InferenceContext<'_> {
 
             let remaining_segments_for_ty = path.segments().take(path.segments().len() - 1);
             let (ty, _) = ctx.lower_ty_relative_path(ty, orig_ns, remaining_segments_for_ty);
+            self.push_ty_diagnostics(InferenceTyDiagnosticSource::Body, ctx.diagnostics);
             let ty = self.table.insert_type_vars(ty);
             let ty = self.table.normalize_associated_types_in(ty);
             self.resolve_ty_assoc_item(ty, last.name, id).map(|(it, substs)| (it, Some(substs)))?
@@ -265,6 +266,9 @@ impl InferenceContext<'_> {
                         resolved_segment,
                         remaining_segments_for_ty,
                         true,
+                        &mut |_, _reason| {
+                            // FIXME: Report an error.
+                        },
                     )
                 });
                 if ty.is_unknown() {

crates/hir-ty/src/lib.rs

@@ -84,12 +84,13 @@ pub use infer::{
     cast::CastError,
     closure::{CaptureKind, CapturedItem},
     could_coerce, could_unify, could_unify_deeply, Adjust, Adjustment, AutoBorrow, BindingMode,
-    InferenceDiagnostic, InferenceResult, OverloadedDeref, PointerCast,
+    InferenceDiagnostic, InferenceResult, InferenceTyDiagnosticSource, OverloadedDeref,
+    PointerCast,
 };
 pub use interner::Interner;
 pub use lower::{
-    associated_type_shorthand_candidates, ImplTraitLoweringMode, ParamLoweringMode, TyDefId,
+    associated_type_shorthand_candidates, GenericArgsProhibitedReason, ImplTraitLoweringMode,
-    TyLoweringContext, ValueTyDefId,
+    ParamLoweringMode, TyDefId, TyLoweringContext, TyLoweringDiagnosticKind, ValueTyDefId,
 };
 pub use mapping::{
     from_assoc_type_id, from_chalk_trait_id, from_foreign_def_id, from_placeholder_idx,

crates/hir-ty/src/lower.rs

@@ -102,6 +102,31 @@ impl ImplTraitLoweringState {
     }
 }
 
+type TypeSource = Either<TypeRefId, hir_def::type_ref::TypeSource>;
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct TyLoweringDiagnostic {
+    pub source: TypeSource,
+    pub kind: TyLoweringDiagnosticKind,
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum TyLoweringDiagnosticKind {
+    GenericArgsProhibited { segment: u32, reason: GenericArgsProhibitedReason },
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum GenericArgsProhibitedReason {
+    Module,
+    TyParam,
+    SelfTy,
+    PrimitiveTy,
+    /// When there is a generic enum, within the expression `Enum::Variant`,
+    /// either `Enum` or `Variant` are allowed to have generic arguments, but not both.
+    // FIXME: This is not used now but it should be.
+    EnumVariant,
+}
+
 #[derive(Debug)]
 pub struct TyLoweringContext<'a> {
     pub db: &'a dyn HirDatabase,
@@ -125,6 +150,7 @@ pub struct TyLoweringContext<'a> {
     expander: Option<Expander>,
     /// Tracks types with explicit `?Sized` bounds.
     pub(crate) unsized_types: FxHashSet<Ty>,
+    pub(crate) diagnostics: Vec<TyLoweringDiagnostic>,
 }
 
 impl<'a> TyLoweringContext<'a> {
@@ -159,6 +185,7 @@ impl<'a> TyLoweringContext<'a> {
             type_param_mode,
             expander: None,
             unsized_types: FxHashSet::default(),
+            diagnostics: Vec::new(),
         }
     }
 
@@ -198,6 +225,20 @@ impl<'a> TyLoweringContext<'a> {
         self.type_param_mode = type_param_mode;
         self
     }
+
+    pub fn push_diagnostic(&mut self, type_ref: TypeRefId, kind: TyLoweringDiagnosticKind) {
+        let source = match self.types_source_map {
+            Some(source_map) => {
+                let Ok(source) = source_map.type_syntax(type_ref) else {
+                    stdx::never!("error in synthetic type");
+                    return;
+                };
+                Either::Right(source)
+            }
+            None => Either::Left(type_ref),
+        };
+        self.diagnostics.push(TyLoweringDiagnostic { source, kind });
+    }
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Default)]
@@ -464,6 +505,7 @@ impl<'a> TyLoweringContext<'a> {
                                 impl_trait_mode: mem::take(&mut self.impl_trait_mode),
                                 expander: self.expander.take(),
                                 unsized_types: mem::take(&mut self.unsized_types),
+                                diagnostics: mem::take(&mut self.diagnostics),
                             };
 
                             let ty = inner_ctx.lower_ty(type_ref);
@@ -471,6 +513,7 @@ impl<'a> TyLoweringContext<'a> {
                             self.impl_trait_mode = inner_ctx.impl_trait_mode;
                             self.expander = inner_ctx.expander;
                             self.unsized_types = inner_ctx.unsized_types;
+                            self.diagnostics = inner_ctx.diagnostics;
 
                             self.expander.as_mut().unwrap().exit(mark);
                             Some(ty)
@@ -542,6 +585,10 @@ impl<'a> TyLoweringContext<'a> {
         resolved_segment: PathSegment<'_>,
         remaining_segments: PathSegments<'_>,
         infer_args: bool,
+        on_prohibited_generics_for_resolved_segment: &mut dyn FnMut(
+            &mut Self,
+            GenericArgsProhibitedReason,
+        ),
     ) -> (Ty, Option<TypeNs>) {
         let ty = match resolution {
             TypeNs::TraitId(trait_) => {
@@ -608,7 +655,15 @@ impl<'a> TyLoweringContext<'a> {
                 // FIXME(trait_alias): Implement trait alias.
                 return (TyKind::Error.intern(Interner), None);
             }
-            TypeNs::GenericParam(param_id) => match self.type_param_mode {
+            TypeNs::GenericParam(param_id) => {
+                if resolved_segment.args_and_bindings.is_some() {
+                    on_prohibited_generics_for_resolved_segment(
+                        self,
+                        GenericArgsProhibitedReason::TyParam,
+                    );
+                }
+
+                match self.type_param_mode {
                     ParamLoweringMode::Placeholder => {
                         TyKind::Placeholder(to_placeholder_idx(self.db, param_id.into()))
                     }
@@ -628,8 +683,16 @@ impl<'a> TyLoweringContext<'a> {
                         TyKind::BoundVar(BoundVar::new(self.in_binders, idx))
                     }
                 }
-            .intern(Interner),
+                .intern(Interner)
+            }
             TypeNs::SelfType(impl_id) => {
+                if resolved_segment.args_and_bindings.is_some() {
+                    on_prohibited_generics_for_resolved_segment(
+                        self,
+                        GenericArgsProhibitedReason::SelfTy,
+                    );
+                }
+
                 let generics = self.generics().expect("impl should have generic param scope");
 
                 match self.type_param_mode {
@@ -655,6 +718,13 @@ impl<'a> TyLoweringContext<'a> {
                 }
             }
             TypeNs::AdtSelfType(adt) => {
+                if resolved_segment.args_and_bindings.is_some() {
+                    on_prohibited_generics_for_resolved_segment(
+                        self,
+                        GenericArgsProhibitedReason::SelfTy,
+                    );
+                }
+
                 let generics = generics(self.db.upcast(), adt.into());
                 let substs = match self.type_param_mode {
                     ParamLoweringMode::Placeholder => generics.placeholder_subst(self.db),
@@ -667,6 +737,12 @@ impl<'a> TyLoweringContext<'a> {
 
             TypeNs::AdtId(it) => self.lower_path_inner(resolved_segment, it.into(), infer_args),
             TypeNs::BuiltinType(it) => {
+                if resolved_segment.args_and_bindings.is_some() {
+                    on_prohibited_generics_for_resolved_segment(
+                        self,
+                        GenericArgsProhibitedReason::PrimitiveTy,
+                    );
+                }
                 self.lower_path_inner(resolved_segment, it.into(), infer_args)
             }
             TypeNs::TypeAliasId(it) => {
@@ -698,14 +774,39 @@ impl<'a> TyLoweringContext<'a> {
             return (ty, None);
         }
 
-        let (resolved_segment, remaining_segments) = match remaining_index {
+        let (module_segments, resolved_segment_idx, resolved_segment, remaining_segments) =
+            match remaining_index {
                 None => (
+                    path.segments().strip_last(),
+                    path.segments().len() - 1,
                     path.segments().last().expect("resolved path has at least one element"),
                     PathSegments::EMPTY,
                 ),
-            Some(i) => (path.segments().get(i - 1).unwrap(), path.segments().skip(i)),
+                Some(i) => (
+                    path.segments().take(i - 1),
+                    i - 1,
+                    path.segments().get(i - 1).unwrap(),
+                    path.segments().skip(i),
+                ),
             };
-        self.lower_partly_resolved_path(resolution, resolved_segment, remaining_segments, false)
+
+        self.prohibit_generics(path_id, 0, module_segments, GenericArgsProhibitedReason::Module);
+
+        self.lower_partly_resolved_path(
+            resolution,
+            resolved_segment,
+            remaining_segments,
+            false,
+            &mut |this, reason| {
+                this.push_diagnostic(
+                    path_id.type_ref(),
+                    TyLoweringDiagnosticKind::GenericArgsProhibited {
+                        segment: resolved_segment_idx as u32,
+                        reason,
+                    },
+                )
+            },
+        )
     }
 
     fn select_associated_type(&mut self, res: Option<TypeNs>, segment: PathSegment<'_>) -> Ty {
@@ -742,12 +843,8 @@ impl<'a> TyLoweringContext<'a> {
                 // generic params. It's inefficient to splice the `Substitution`s, so we may want
                 // that method to optionally take parent `Substitution` as we already know them at
                 // this point (`t.substitution`).
-                let substs = self.substs_from_path_segment(
+                let substs =
-                    segment.clone(),
+                    self.substs_from_path_segment(segment, Some(associated_ty.into()), false, None);
-                    Some(associated_ty.into()),
-                    false,
-                    None,
-                );
 
                 let len_self =
                     crate::generics::generics(self.db.upcast(), associated_ty.into()).len_self();
@@ -999,6 +1096,23 @@ impl<'a> TyLoweringContext<'a> {
         TraitRef { trait_id: to_chalk_trait_id(resolved), substitution: substs }
     }
 
+    fn prohibit_generics(
+        &mut self,
+        path_id: PathId,
+        idx: u32,
+        segments: PathSegments<'_>,
+        reason: GenericArgsProhibitedReason,
+    ) {
+        segments.iter().zip(idx..).for_each(|(segment, idx)| {
+            if segment.args_and_bindings.is_some() {
+                self.push_diagnostic(
+                    path_id.type_ref(),
+                    TyLoweringDiagnosticKind::GenericArgsProhibited { segment: idx, reason },
+                );
+            }
+        });
+    }
+
     fn lower_trait_ref_from_path(
         &mut self,
         path_id: PathId,
@@ -1010,6 +1124,13 @@ impl<'a> TyLoweringContext<'a> {
             TypeNs::TraitId(tr) => tr,
             _ => return None,
         };
+        // Do this after we verify it's indeed a trait to not confuse the user if they're not modules.
+        self.prohibit_generics(
+            path_id,
+            0,
+            path.segments().strip_last(),
+            GenericArgsProhibitedReason::Module,
+        );
         let segment = path.segments().last().expect("path should have at least one segment");
         Some(self.lower_trait_ref_from_resolved_path(resolved, segment, explicit_self_ty))
     }
@@ -1233,7 +1354,9 @@ impl<'a> TyLoweringContext<'a> {
                                     }
                                     _ => unreachable!(),
                                 }
-                                ext.lower_ty(type_ref)
+                                let ty = ext.lower_ty(type_ref);
+                                self.diagnostics.extend(ext.diagnostics);
+                                ty
                             } else {
                                 self.lower_ty(type_ref)
                             };

crates/hir/Cargo.toml

@@ -33,6 +33,14 @@ syntax.workspace = true
 tt.workspace = true
 span.workspace = true
 
+[dev-dependencies]
+expect-test.workspace = true
+
+# local deps
+test-utils.workspace = true
+test-fixture.workspace = true
+syntax-bridge.workspace = true
+
 [features]
 in-rust-tree = ["hir-expand/in-rust-tree"]
 

crates/hir/src/diagnostics.rs

@@ -3,23 +3,34 @@
 //!
 //! This probably isn't the best way to do this -- ideally, diagnostics should
 //! be expressed in terms of hir types themselves.
-pub use hir_ty::diagnostics::{CaseType, IncorrectCase};
+use cfg::{CfgExpr, CfgOptions};
+use either::Either;
+use hir_def::{
+    hir::ExprOrPatId,
+    path::{hir_segment_to_ast_segment, ModPath},
+    type_ref::TypesSourceMap,
+    AssocItemId, DefWithBodyId, SyntheticSyntax,
+};
+use hir_expand::{name::Name, HirFileId, InFile};
 use hir_ty::{
     db::HirDatabase,
     diagnostics::{BodyValidationDiagnostic, UnsafetyReason},
-    CastError, InferenceDiagnostic,
+    CastError, InferenceDiagnostic, InferenceTyDiagnosticSource, TyLoweringDiagnosticKind,
+};
+use syntax::{
+    ast::{self, HasGenericArgs},
+    AstPtr, SyntaxError, SyntaxNodePtr, TextRange,
 };
-
-use cfg::{CfgExpr, CfgOptions};
-use either::Either;
-pub use hir_def::VariantId;
-use hir_def::{hir::ExprOrPatId, path::ModPath, AssocItemId, DefWithBodyId, SyntheticSyntax};
-use hir_expand::{name::Name, HirFileId, InFile};
-use syntax::{ast, AstPtr, SyntaxError, SyntaxNodePtr, TextRange};
 use triomphe::Arc;
 
 use crate::{AssocItem, Field, Local, Trait, Type};
 
+pub use hir_def::VariantId;
+pub use hir_ty::{
+    diagnostics::{CaseType, IncorrectCase},
+    GenericArgsProhibitedReason,
+};
+
 macro_rules! diagnostics {
     ($($diag:ident,)*) => {
         #[derive(Debug)]
@@ -98,6 +109,7 @@ diagnostics![
     UnresolvedIdent,
     UnusedMut,
     UnusedVariable,
+    GenericArgsProhibited,
 ];
 
 #[derive(Debug)]
@@ -388,6 +400,12 @@ pub struct InvalidCast {
     pub cast_ty: Type,
 }
 
+#[derive(Debug)]
+pub struct GenericArgsProhibited {
+    pub args: InFile<AstPtr<Either<ast::GenericArgList, ast::ParamList>>>,
+    pub reason: GenericArgsProhibitedReason,
+}
+
 impl AnyDiagnostic {
     pub(crate) fn body_validation_diagnostic(
         db: &dyn HirDatabase,
@@ -527,6 +545,7 @@ impl AnyDiagnostic {
         db: &dyn HirDatabase,
         def: DefWithBodyId,
         d: &InferenceDiagnostic,
+        outer_types_source_map: &TypesSourceMap,
         source_map: &hir_def::body::BodySourceMap,
     ) -> Option<AnyDiagnostic> {
         let expr_syntax = |expr| {
@@ -640,6 +659,36 @@ impl AnyDiagnostic {
                 let cast_ty = Type::new(db, def, cast_ty.clone());
                 InvalidCast { expr, error: *error, expr_ty, cast_ty }.into()
             }
+            InferenceDiagnostic::TyDiagnostic { source, diag } => {
+                let source_map = match source {
+                    InferenceTyDiagnosticSource::Body => &source_map.types,
+                    InferenceTyDiagnosticSource::Signature => outer_types_source_map,
+                };
+                let source = match diag.source {
+                    Either::Left(type_ref_id) => {
+                        let Ok(source) = source_map.type_syntax(type_ref_id) else {
+                            stdx::never!("error on synthetic type syntax");
+                            return None;
+                        };
+                        source
+                    }
+                    Either::Right(source) => source,
+                };
+                let syntax = || source.value.to_node(&db.parse_or_expand(source.file_id));
+                match diag.kind {
+                    TyLoweringDiagnosticKind::GenericArgsProhibited { segment, reason } => {
+                        let ast::Type::PathType(syntax) = syntax() else { return None };
+                        let segment = hir_segment_to_ast_segment(&syntax.path()?, segment)?;
+                        let args = if let Some(generics) = segment.generic_arg_list() {
+                            AstPtr::new(&generics).wrap_left()
+                        } else {
+                            AstPtr::new(&segment.param_list()?).wrap_right()
+                        };
+                        let args = source.with_value(args);
+                        GenericArgsProhibited { args, reason }.into()
+                    }
+                }
+            }
         })
     }
 }

crates/hir/src/lib.rs

@@ -20,12 +20,11 @@
 #![cfg_attr(feature = "in-rust-tree", feature(rustc_private))]
 #![recursion_limit = "512"]
 
-mod semantics;
-mod source_analyzer;
-
 mod attrs;
 mod from_id;
 mod has_source;
+mod semantics;
+mod source_analyzer;
 
 pub mod db;
 pub mod diagnostics;
@@ -54,6 +53,7 @@ use hir_def::{
     path::ImportAlias,
     per_ns::PerNs,
     resolver::{HasResolver, Resolver},
+    type_ref::TypesSourceMap,
     AssocItemId, AssocItemLoc, AttrDefId, CallableDefId, ConstId, ConstParamId, CrateRootModuleId,
     DefWithBodyId, EnumId, EnumVariantId, ExternCrateId, FunctionId, GenericDefId, GenericParamId,
     HasModule, ImplId, InTypeConstId, ItemContainerId, LifetimeParamId, LocalFieldId, Lookup,
@@ -1802,6 +1802,25 @@ impl DefWithBody {
         let krate = self.module(db).id.krate();
 
         let (body, source_map) = db.body_with_source_map(self.into());
+        let item_tree_source_maps;
+        let outer_types_source_map = match self {
+            DefWithBody::Function(function) => {
+                let function = function.id.lookup(db.upcast()).id;
+                item_tree_source_maps = function.item_tree_with_source_map(db.upcast()).1;
+                item_tree_source_maps.function(function.value).item()
+            }
+            DefWithBody::Static(statik) => {
+                let statik = statik.id.lookup(db.upcast()).id;
+                item_tree_source_maps = statik.item_tree_with_source_map(db.upcast()).1;
+                item_tree_source_maps.statik(statik.value)
+            }
+            DefWithBody::Const(konst) => {
+                let konst = konst.id.lookup(db.upcast()).id;
+                item_tree_source_maps = konst.item_tree_with_source_map(db.upcast()).1;
+                item_tree_source_maps.konst(konst.value)
+            }
+            DefWithBody::Variant(_) | DefWithBody::InTypeConst(_) => &TypesSourceMap::EMPTY,
+        };
 
         for (_, def_map) in body.blocks(db.upcast()) {
             Module { id: def_map.module_id(DefMap::ROOT) }.diagnostics(db, acc, style_lints);
@@ -1861,7 +1880,13 @@ impl DefWithBody {
 
         let infer = db.infer(self.into());
         for d in &infer.diagnostics {
-            acc.extend(AnyDiagnostic::inference_diagnostic(db, self.into(), d, &source_map));
+            acc.extend(AnyDiagnostic::inference_diagnostic(
+                db,
+                self.into(),
+                d,
+                outer_types_source_map,
+                &source_map,
+            ));
         }
 
         for (pat_or_expr, mismatch) in infer.type_mismatches() {

crates/ide-diagnostics/src/handlers/generic_args_prohibited.rs

@@ -0,0 +1,242 @@
+use either::Either;
+use hir::GenericArgsProhibitedReason;
+use ide_db::assists::Assist;
+use ide_db::source_change::SourceChange;
+use ide_db::text_edit::TextEdit;
+use syntax::{ast, AstNode, TextRange};
+
+use crate::{fix, Diagnostic, DiagnosticCode, DiagnosticsContext};
+
+// Diagnostic: generic-args-prohibited
+//
+// This diagnostic is shown when generic arguments are provided for a type that does not accept
+// generic arguments.
+pub(crate) fn generic_args_prohibited(
+    ctx: &DiagnosticsContext<'_>,
+    d: &hir::GenericArgsProhibited,
+) -> Diagnostic {
+    Diagnostic::new_with_syntax_node_ptr(
+        ctx,
+        DiagnosticCode::RustcHardError("E0109"),
+        describe_reason(d.reason),
+        d.args.map(Into::into),
+    )
+    .with_fixes(fixes(ctx, d))
+}
+
+fn describe_reason(reason: GenericArgsProhibitedReason) -> String {
+    let kind = match reason {
+        GenericArgsProhibitedReason::Module => "modules",
+        GenericArgsProhibitedReason::TyParam => "type parameters",
+        GenericArgsProhibitedReason::SelfTy => "`Self`",
+        GenericArgsProhibitedReason::PrimitiveTy => "builtin types",
+        GenericArgsProhibitedReason::EnumVariant => {
+            return "you can specify generic arguments on either the enum or the variant, but not both"
+                .to_owned();
+        }
+    };
+    format!("generic arguments are not allowed on {kind}")
+}
+
+fn fixes(ctx: &DiagnosticsContext<'_>, d: &hir::GenericArgsProhibited) -> Option<Vec<Assist>> {
+    let file_id = d.args.file_id.file_id()?;
+    let syntax = d.args.to_node(ctx.sema.db);
+    let range = match &syntax {
+        Either::Left(_) => syntax.syntax().text_range(),
+        Either::Right(param_list) => {
+            let path_segment = ast::PathSegment::cast(param_list.syntax().parent()?)?;
+            let start = if let Some(coloncolon) = path_segment.coloncolon_token() {
+                coloncolon.text_range().start()
+            } else {
+                param_list.syntax().text_range().start()
+            };
+            let end = if let Some(ret_type) = path_segment.ret_type() {
+                ret_type.syntax().text_range().end()
+            } else {
+                param_list.syntax().text_range().end()
+            };
+            TextRange::new(start, end)
+        }
+    };
+    Some(vec![fix(
+        "remove_generic_args",
+        "Remove these generics",
+        SourceChange::from_text_edit(file_id, TextEdit::delete(range)),
+        syntax.syntax().text_range(),
+    )])
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_diagnostics, check_fix};
+
+    #[test]
+    fn primitives() {
+        check_diagnostics(
+            r#"
+//- /core.rs crate:core library
+#![rustc_coherence_is_core]
+impl str {
+    pub fn trim() {}
+}
+
+//- /lib.rs crate:foo deps:core
+fn bar<T>() {}
+
+fn foo() {
+    let _: (bool<()>, ());
+             // ^^^^ 💡 error: generic arguments are not allowed on builtin types
+    let _ = <str<'_>>::trim;
+             // ^^^^ 💡 error: generic arguments are not allowed on builtin types
+    bar::<u32<{ const { 1 + 1 } }>>();
+          // ^^^^^^^^^^^^^^^^^^^^^ 💡 error: generic arguments are not allowed on builtin types
+}
+            "#,
+        );
+    }
+
+    #[test]
+    fn modules() {
+        check_diagnostics(
+            r#"
+pub mod foo {
+    pub mod bar {
+        pub struct Baz;
+
+        impl Baz {
+            pub fn qux() {}
+        }
+    }
+}
+
+fn foo() {
+    let _: foo::<'_>::bar::Baz;
+           // ^^^^^^ 💡 error: generic arguments are not allowed on modules
+    let _ = <foo::bar<()>::Baz>::qux;
+                  // ^^^^ 💡 error: generic arguments are not allowed on modules
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn type_parameters() {
+        check_diagnostics(
+            r#"
+fn foo<T, U>() {
+    let _: T<'a>;
+         // ^^^^ 💡 error: generic arguments are not allowed on type parameters
+    let _: U::<{ 1 + 2 }>;
+         // ^^^^^^^^^^^^^ 💡 error: generic arguments are not allowed on type parameters
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn fn_like_generic_args() {
+        check_diagnostics(
+            r#"
+fn foo() {
+    let _: bool(bool, i32) -> ();
+            // ^^^^^^^^^^^ 💡 error: generic arguments are not allowed on builtin types
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn fn_signature() {
+        check_diagnostics(
+            r#"
+fn foo(
+    _a: bool<'_>,
+         // ^^^^ 💡 error: generic arguments are not allowed on builtin types
+    _b: i32::<i64>,
+        // ^^^^^^^ 💡 error: generic arguments are not allowed on builtin types
+    _c: &(&str<1>)
+           // ^^^ 💡 error: generic arguments are not allowed on builtin types
+) -> ((), i32<bool>) {
+          // ^^^^^^ 💡 error: generic arguments are not allowed on builtin types
+    ((), 0)
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn const_static_type() {
+        check_diagnostics(
+            r#"
+const A: i32<bool> = 0;
+         // ^^^^^^ 💡 error: generic arguments are not allowed on builtin types
+static A: i32::<{ 1 + 3 }> = 0;
+          // ^^^^^^^^^^^^^ 💡 error: generic arguments are not allowed on builtin types
+        "#,
+        );
+    }
+
+    #[test]
+    fn fix() {
+        check_fix(
+            r#"
+fn foo() {
+    let _: bool<'_, (), { 1 + 1 }>$0;
+}"#,
+            r#"
+fn foo() {
+    let _: bool;
+}"#,
+        );
+        check_fix(
+            r#"
+fn foo() {
+    let _: bool::$0<'_, (), { 1 + 1 }>;
+}"#,
+            r#"
+fn foo() {
+    let _: bool;
+}"#,
+        );
+        check_fix(
+            r#"
+fn foo() {
+    let _: bool(i$032);
+}"#,
+            r#"
+fn foo() {
+    let _: bool;
+}"#,
+        );
+        check_fix(
+            r#"
+fn foo() {
+    let _: bool$0(i32) -> i64;
+}"#,
+            r#"
+fn foo() {
+    let _: bool;
+}"#,
+        );
+        check_fix(
+            r#"
+fn foo() {
+    let _: bool::(i$032) -> i64;
+}"#,
+            r#"
+fn foo() {
+    let _: bool;
+}"#,
+        );
+        check_fix(
+            r#"
+fn foo() {
+    let _: bool::(i32)$0;
+}"#,
+            r#"
+fn foo() {
+    let _: bool;
+}"#,
+        );
+    }
+}

crates/ide-diagnostics/src/handlers/unresolved_method.rs

@@ -167,9 +167,9 @@ fn assoc_func_fix(ctx: &DiagnosticsContext<'_>, d: &hir::UnresolvedMethodCall) -
         }
 
         let method_name = call.name_ref()?;
-        let assoc_func_call = format!("{receiver_type_adt_name}::{method_name}()");
+        let assoc_func_path = format!("{receiver_type_adt_name}::{method_name}");
 
-        let assoc_func_call = make::expr_path(make::path_from_text(&assoc_func_call));
+        let assoc_func_path = make::expr_path(make::path_from_text(&assoc_func_path));
 
         let args: Vec<_> = if need_to_take_receiver_as_first_arg {
             std::iter::once(receiver).chain(call.arg_list()?.args()).collect()
@@ -178,7 +178,7 @@ fn assoc_func_fix(ctx: &DiagnosticsContext<'_>, d: &hir::UnresolvedMethodCall) -
         };
         let args = make::arg_list(args);
 
-        let assoc_func_call_expr_string = make::expr_call(assoc_func_call, args).to_string();
+        let assoc_func_call_expr_string = make::expr_call(assoc_func_path, args).to_string();
 
         let file_id = ctx.sema.original_range_opt(call.receiver()?.syntax())?.file_id;
 

crates/ide-diagnostics/src/lib.rs

@@ -27,6 +27,7 @@ mod handlers {
     pub(crate) mod await_outside_of_async;
     pub(crate) mod break_outside_of_loop;
     pub(crate) mod expected_function;
+    pub(crate) mod generic_args_prohibited;
     pub(crate) mod inactive_code;
     pub(crate) mod incoherent_impl;
     pub(crate) mod incorrect_case;
@@ -468,6 +469,7 @@ pub fn semantic_diagnostics(
                 Some(it) => it,
                 None => continue,
             },
+            AnyDiagnostic::GenericArgsProhibited(d) => handlers::generic_args_prohibited::generic_args_prohibited(&ctx, &d)
         };
         res.push(d)
     }

crates/syntax/src/ast/make.rs

@@ -402,7 +402,7 @@ pub fn join_paths(paths: impl IntoIterator<Item = ast::Path>) -> ast::Path {
 
 // FIXME: should not be pub
 pub fn path_from_text(text: &str) -> ast::Path {
-    ast_from_text(&format!("fn main() {{ let test = {text}; }}"))
+    ast_from_text(&format!("fn main() {{ let test: {text}; }}"))
 }
 
 pub fn use_tree_glob() -> ast::UseTree {