[ty] Improve diagnostic when callable is used in a type expression instead of collections.abc.Callable or typing.Callable

crates/ty_python_semantic/resources/mdtest/annotations/invalid.md

@@ -266,3 +266,12 @@ def _(
 ) -> (int, str):  # error: [invalid-type-form]
     return x
 ```
+
+### Special-cased diagnostic for `callable` used in a type expression
+
+```py
+# error: [invalid-type-form]
+# error: [invalid-type-form]
+def decorator(fn: callable) -> callable:
+    return fn
+```

crates/ty_python_semantic/resources/mdtest/snapshots/invalid.md_-_Tests_for_invalid_ty…_-_Diagnostics_for_comm…_-_Special-cased_diagno…_(a4b698196d337a3f).snap

@@ -0,0 +1,49 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: invalid.md - Tests for invalid types in type expressions - Diagnostics for common errors - Special-cased diagnostic for `callable` used in a type expression
+mdtest path: crates/ty_python_semantic/resources/mdtest/annotations/invalid.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | # error: [invalid-type-form]
+2 | # error: [invalid-type-form]
+3 | def decorator(fn: callable) -> callable:
+4 |     return fn
+```
+
+# Diagnostics
+
+```
+error[invalid-type-form]: Function `callable` is not valid in a type expression
+ --> src/mdtest_snippet.py:3:19
+  |
+1 | # error: [invalid-type-form]
+2 | # error: [invalid-type-form]
+3 | def decorator(fn: callable) -> callable:
+  |                   ^^^^^^^^ Did you mean `collections.abc.Callable`?
+4 |     return fn
+  |
+info: rule `invalid-type-form` is enabled by default
+
+```
+
+```
+error[invalid-type-form]: Function `callable` is not valid in a type expression
+ --> src/mdtest_snippet.py:3:32
+  |
+1 | # error: [invalid-type-form]
+2 | # error: [invalid-type-form]
+3 | def decorator(fn: callable) -> callable:
+  |                                ^^^^^^^^ Did you mean `collections.abc.Callable`?
+4 |     return fn
+  |
+info: rule `invalid-type-form` is enabled by default
+
+```

crates/ty_python_semantic/src/types.rs

@@ -36,8 +36,8 @@ pub(crate) use self::signatures::{CallableSignature, Signature};
 pub(crate) use self::subclass_of::{SubclassOfInner, SubclassOfType};
 pub use crate::diagnostic::add_inferred_python_version_hint_to_diagnostic;
 use crate::place::{
-    Definedness, Place, PlaceAndQualifiers, TypeOrigin, Widening, imported_symbol,
-    known_module_symbol,
+    Definedness, Place, PlaceAndQualifiers, TypeOrigin, Widening, builtins_module_scope,
+    imported_symbol, known_module_symbol,
 };
 use crate::semantic_index::definition::{Definition, DefinitionKind};
 use crate::semantic_index::place::ScopedPlaceId;
@@ -9459,6 +9459,13 @@ impl<'db> InvalidTypeExpression<'db> {
                         "Type qualifier `{qualifier}` is not allowed in type expressions \
                         (only in annotation expressions, and only with exactly one argument)",
                     ),
+                    InvalidTypeExpression::InvalidType(Type::FunctionLiteral(function), _) => {
+                        write!(
+                            f,
+                            "Function `{function}` is not valid in a type expression",
+                            function = function.name(self.db)
+                        )
+                    }
                     InvalidTypeExpression::InvalidType(Type::ModuleLiteral(module), _) => write!(
                         f,
                         "Module `{module}` is not valid in a type expression",
@@ -9520,6 +9527,20 @@ impl<'db> InvalidTypeExpression<'db> {
                 "You might have meant to use a concrete TypedDict \
                 or `collections.abc.Mapping[str, object]`",
             );
+        // It would be nice if we could register `builtins.callable` as a known function,
+        // but currently doing this would require reimplementing the signature "manually"
+        // in `Type::bindings()`, which isn't worth it given that we have no other special
+        // casing for this function.
+        } else if let InvalidTypeExpression::InvalidType(Type::FunctionLiteral(function), _) = self
+            && function.name(db) == "callable"
+            && let function_body_scope = function.literal(db).last_definition(db).body_scope(db)
+            && function_body_scope
+                .scope(db)
+                .parent()
+                .map(|parent| parent.to_scope_id(db, function_body_scope.file(db)))
+                == builtins_module_scope(db)
+        {
+            diagnostic.set_primary_message("Did you mean `collections.abc.Callable`?");
         }
     }
 }