[red-knot] support reveal_type as pseudo-builtin (#13403)

Support using `reveal_type` without importing it, as implied by the type
spec and supported by existing type checkers.

We use `typing_extensions.reveal_type` for the implicit built-in; this
way it exists on all Python versions. (It imports from `typing` on newer
Python versions.)

Emits an "undefined name" diagnostic whenever `reveal_type` is
referenced in this way (in addition to the revealed-type diagnostic when
it is called). This follows the mypy example (with `--enable-error-code
unimported-reveal`) and I think provides a good (and easily
understandable) balance for user experience. If you are using
`reveal_type` for quick temporary debugging, the additional
undefined-name diagnostic doesn't hinder that use case. If we make the
revealed-type diagnostic a non-failing one, the undefined-name
diagnostic can still be a failing diagnostic, helping prevent
accidentally leaving it in place. For any use cases where you want to
leave it in place, you can always import it to avoid the undefined-name
diagnostic.

In the future, we can easily provide configuration options to a) turn
off builtin-reveal_type altogether, and/or b) silence the undefined-name
diagnostic when using it, if we have users on either side (loving or
hating pseudo-builtin `reveal_type`) who are dissatisfied with this
compromise.

crates/red_knot_python_semantic/src/stdlib.rs

@@ -11,6 +11,7 @@ enum CoreStdlibModule {
     Builtins,
     Types,
     Typeshed,
+    TypingExtensions,
 }
 
 impl CoreStdlibModule {
@@ -19,6 +20,7 @@ impl CoreStdlibModule {
             Self::Builtins => "builtins",
             Self::Types => "types",
             Self::Typeshed => "_typeshed",
+            Self::TypingExtensions => "typing_extensions",
         };
         ModuleName::new_static(module_name)
             .unwrap_or_else(|| panic!("{module_name} should be a valid module name!"))
@@ -62,6 +64,14 @@ pub(crate) fn typeshed_symbol_ty<'db>(db: &'db dyn Db, symbol: &str) -> Type<'db
     core_module_symbol_ty(db, CoreStdlibModule::Typeshed, symbol)
 }
 
+/// Lookup the type of `symbol` in the `typing_extensions` module namespace.
+///
+/// Returns `Unbound` if the `typing_extensions` module isn't available for some reason.
+#[inline]
+pub(crate) fn typing_extensions_symbol_ty<'db>(db: &'db dyn Db, symbol: &str) -> Type<'db> {
+    core_module_symbol_ty(db, CoreStdlibModule::TypingExtensions, symbol)
+}
+
 /// Get the scope of a core stdlib module.
 ///
 /// Can return `None` if a custom typeshed is used that is missing the core module in question.

crates/red_knot_python_semantic/src/types.rs

@@ -10,7 +10,9 @@ use crate::semantic_index::{
     global_scope, semantic_index, symbol_table, use_def_map, BindingWithConstraints,
     BindingWithConstraintsIterator, DeclarationsIterator,
 };
-use crate::stdlib::{builtins_symbol_ty, types_symbol_ty, typeshed_symbol_ty};
+use crate::stdlib::{
+    builtins_symbol_ty, types_symbol_ty, typeshed_symbol_ty, typing_extensions_symbol_ty,
+};
 use crate::types::narrow::narrowing_constraint;
 use crate::{Db, FxOrderSet};
 

crates/red_knot_python_semantic/src/types/infer.rs

@@ -51,8 +51,8 @@ use crate::stdlib::builtins_module_scope;
 use crate::types::diagnostic::{TypeCheckDiagnostic, TypeCheckDiagnostics};
 use crate::types::{
     bindings_ty, builtins_symbol_ty, declarations_ty, global_symbol_ty, symbol_ty,
-    BytesLiteralType, ClassType, FunctionType, StringLiteralType, TupleType, Type,
-    TypeArrayDisplay, UnionType,
+    typing_extensions_symbol_ty, BytesLiteralType, ClassType, FunctionType, StringLiteralType,
+    TupleType, Type, TypeArrayDisplay, UnionType,
 };
 use crate::Db;
 
@@ -2081,7 +2081,8 @@ impl<'db> TypeInferenceBuilder<'db> {
     }
 
     /// Look up a name reference that isn't bound in the local scope.
-    fn lookup_name(&self, name: &ast::name::Name) -> Type<'db> {
+    fn lookup_name(&mut self, name_node: &ast::ExprName) -> Type<'db> {
+        let ast::ExprName { id: name, .. } = name_node;
         let file_scope_id = self.scope.file_scope_id(self.db);
         let is_bound = self
             .index
@@ -2126,7 +2127,17 @@ impl<'db> TypeInferenceBuilder<'db> {
             };
             // Fallback to builtins (without infinite recursion if we're already in builtins.)
             if ty.may_be_unbound(self.db) && Some(self.scope) != builtins_module_scope(self.db) {
-                ty.replace_unbound_with(self.db, builtins_symbol_ty(self.db, name))
+                let mut builtin_ty = builtins_symbol_ty(self.db, name);
+                if builtin_ty.is_unbound() && name == "reveal_type" {
+                    self.add_diagnostic(
+                        name_node.into(),
+                        "undefined-reveal",
+                        format_args!(
+                            "'reveal_type' used without importing it; this is allowed for debugging convenience but will fail at runtime."),
+                    );
+                    builtin_ty = typing_extensions_symbol_ty(self.db, name);
+                }
+                ty.replace_unbound_with(self.db, builtin_ty)
             } else {
                 ty
             }
@@ -2162,7 +2173,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                 };
 
                 let unbound_ty = if may_be_unbound {
-                    Some(self.lookup_name(id))
+                    Some(self.lookup_name(name))
                 } else {
                     None
                 };
@@ -2804,6 +2815,30 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn reveal_type_builtin() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+            x = 1
+            reveal_type(x)
+            ",
+        )?;
+
+        assert_file_diagnostics(
+            &db,
+            "/src/a.py",
+            &[
+                "'reveal_type' used without importing it; this is allowed for debugging convenience but will fail at runtime.",
+                "Revealed type is 'Literal[1]'.",
+            ],
+        );
+
+        Ok(())
+    }
+
     #[test]
     fn follow_import_to_class() -> anyhow::Result<()> {
         let mut db = setup_db();