[ty] Improve debuggability of protocol types

crates/ty_python_semantic/resources/mdtest/protocols.md

@@ -382,6 +382,31 @@ class Foo(Protocol):
 reveal_type(get_protocol_members(Foo))  # revealed: frozenset[Literal["method_member", "x", "y", "z"]]
 ```
 
+To see the kinds and types of the protocol members, you can use the debugging aid
+`ty_extensions.reveal_protocol_interface`, meanwhile:
+
+```py
+from ty_extensions import reveal_protocol_interface
+from typing import SupportsIndex, SupportsAbs
+
+# error: [revealed-type] "Revealed protocol interface: `{"method_member": MethodMember(`(self) -> bytes`), "x": AttributeMember(`int`), "y": PropertyMember { getter: `def y(self) -> str` }, "z": PropertyMember { getter: `def z(self) -> int`, setter: `def z(self, z: int) -> None` }}`"
+reveal_protocol_interface(Foo)
+# error: [revealed-type] "Revealed protocol interface: `{"__index__": MethodMember(`(self) -> int`)}`"
+reveal_protocol_interface(SupportsIndex)
+# error: [revealed-type] "Revealed protocol interface: `{"__abs__": MethodMember(`(self) -> _T_co`)}`"
+reveal_protocol_interface(SupportsAbs)
+
+# error: [invalid-argument-type] "Invalid argument to `reveal_protocol_interface`: Only protocol classes can be passed to `reveal_protocol_interface`"
+reveal_protocol_interface(int)
+# error: [invalid-argument-type] "Argument to function `reveal_protocol_interface` is incorrect: Expected `type`, found `Literal["foo"]`"
+reveal_protocol_interface("foo")
+
+# TODO: this should be a `revealed-type` diagnostic rather than `invalid-argument-type`, and it should reveal `{"__abs__": MethodMember(`(self) -> int`)}` for the protocol interface
+#
+# error: [invalid-argument-type] "Invalid argument to `reveal_protocol_interface`: Only protocol classes can be passed to `reveal_protocol_interface`"
+reveal_protocol_interface(SupportsAbs[int])
+```
+
 Certain special attributes and methods are not considered protocol members at runtime, and should
 not be considered protocol members by type checkers either:
 

crates/ty_python_semantic/src/types/diagnostic.rs

@@ -2251,6 +2251,41 @@ pub(crate) fn report_bad_argument_to_get_protocol_members(
     diagnostic.info("See https://typing.python.org/en/latest/spec/protocol.html#");
 }
 
+pub(crate) fn report_bad_argument_to_protocol_interface(
+    context: &InferContext,
+    call: &ast::ExprCall,
+    param_type: Type,
+) {
+    let Some(builder) = context.report_lint(&INVALID_ARGUMENT_TYPE, call) else {
+        return;
+    };
+    let db = context.db();
+    let mut diagnostic = builder.into_diagnostic("Invalid argument to `reveal_protocol_interface`");
+    diagnostic
+        .set_primary_message("Only protocol classes can be passed to `reveal_protocol_interface`");
+
+    if let Some(class) = param_type.to_class_type(context.db()) {
+        let mut class_def_diagnostic = SubDiagnostic::new(
+            SubDiagnosticSeverity::Info,
+            format_args!(
+                "`{}` is declared here, but it is not a protocol class:",
+                class.name(db)
+            ),
+        );
+        class_def_diagnostic.annotate(Annotation::primary(
+            class.class_literal(db).0.header_span(db),
+        ));
+        diagnostic.sub(class_def_diagnostic);
+    }
+
+    diagnostic.info(
+        "A class is only a protocol class if it directly inherits \
+            from `typing.Protocol` or `typing_extensions.Protocol`",
+    );
+    // See TODO in `report_bad_argument_to_get_protocol_members` above
+    diagnostic.info("See https://typing.python.org/en/latest/spec/protocol.html");
+}
+
 pub(crate) fn report_invalid_arguments_to_callable(
     context: &InferContext,
     subscript: &ast::ExprSubscript,

crates/ty_python_semantic/src/types/function.rs

@@ -68,7 +68,7 @@ use crate::types::call::{Binding, CallArguments};
 use crate::types::context::InferContext;
 use crate::types::diagnostic::{
     REDUNDANT_CAST, STATIC_ASSERT_ERROR, TYPE_ASSERTION_FAILURE,
-    report_bad_argument_to_get_protocol_members,
+    report_bad_argument_to_get_protocol_members, report_bad_argument_to_protocol_interface,
     report_runtime_check_against_non_runtime_checkable_protocol,
 };
 use crate::types::generics::{GenericContext, walk_generic_context};
@@ -1088,6 +1088,8 @@ pub enum KnownFunction {
     TopMaterialization,
     /// `ty_extensions.bottom_materialization`
     BottomMaterialization,
+    /// `ty_extensions.reveal_protocol_interface`
+    RevealProtocolInterface,
 }
 
 impl KnownFunction {
@@ -1153,6 +1155,7 @@ impl KnownFunction {
             | Self::EnumMembers
             | Self::StaticAssert
             | Self::HasMember
+            | Self::RevealProtocolInterface
             | Self::AllMembers => module.is_ty_extensions(),
             Self::ImportModule => module.is_importlib(),
         }
@@ -1345,6 +1348,33 @@ impl KnownFunction {
                 report_bad_argument_to_get_protocol_members(context, call_expression, *class);
             }
 
+            KnownFunction::RevealProtocolInterface => {
+                let [Some(param_type)] = parameter_types else {
+                    return;
+                };
+                let Some(protocol_class) = param_type
+                    .into_class_literal()
+                    .and_then(|class| class.into_protocol_class(db))
+                else {
+                    report_bad_argument_to_protocol_interface(
+                        context,
+                        call_expression,
+                        *param_type,
+                    );
+                    return;
+                };
+                if let Some(builder) =
+                    context.report_diagnostic(DiagnosticId::RevealedType, Severity::Info)
+                {
+                    let mut diag = builder.into_diagnostic("Revealed protocol interface");
+                    let span = context.span(&call_expression.arguments.args[0]);
+                    diag.annotate(Annotation::primary(span).message(format_args!(
+                        "`{}`",
+                        protocol_class.interface(db).display(db)
+                    )));
+                }
+            }
+
             KnownFunction::IsInstance | KnownFunction::IsSubclass => {
                 let [Some(first_arg), Some(Type::ClassLiteral(class))] = parameter_types else {
                     return;
@@ -1458,6 +1488,7 @@ pub(crate) mod tests {
                 | KnownFunction::TopMaterialization
                 | KnownFunction::BottomMaterialization
                 | KnownFunction::HasMember
+                | KnownFunction::RevealProtocolInterface
                 | KnownFunction::AllMembers => KnownModule::TyExtensions,
 
                 KnownFunction::ImportModule => KnownModule::ImportLib,

crates/ty_python_semantic/src/types/protocol_class.rs

@@ -1,3 +1,4 @@
+use std::fmt::Write;
 use std::{collections::BTreeMap, ops::Deref};
 
 use itertools::Itertools;
@@ -215,6 +216,31 @@ impl<'db> ProtocolInterface<'db> {
             data.find_legacy_typevars(db, typevars);
         }
     }
+
+    pub(super) fn display(self, db: &'db dyn Db) -> impl std::fmt::Display {
+        struct ProtocolInterfaceDisplay<'db> {
+            db: &'db dyn Db,
+            interface: ProtocolInterface<'db>,
+        }
+
+        impl std::fmt::Display for ProtocolInterfaceDisplay<'_> {
+            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                f.write_char('{')?;
+                for (i, (name, data)) in self.interface.inner(self.db).iter().enumerate() {
+                    write!(f, "\"{name}\": {data}", data = data.display(self.db))?;
+                    if i < self.interface.inner(self.db).len() - 1 {
+                        f.write_str(", ")?;
+                    }
+                }
+                f.write_char('}')
+            }
+        }
+
+        ProtocolInterfaceDisplay {
+            db,
+            interface: self,
+        }
+    }
 }
 
 #[derive(Debug, PartialEq, Eq, Clone, Hash, salsa::Update)]
@@ -256,6 +282,41 @@ impl<'db> ProtocolMemberData<'db> {
             qualifiers: self.qualifiers,
         }
     }
+
+    fn display(&self, db: &'db dyn Db) -> impl std::fmt::Display {
+        struct ProtocolMemberDataDisplay<'db> {
+            db: &'db dyn Db,
+            data: ProtocolMemberKind<'db>,
+        }
+
+        impl std::fmt::Display for ProtocolMemberDataDisplay<'_> {
+            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                match self.data {
+                    ProtocolMemberKind::Method(callable) => {
+                        write!(f, "MethodMember(`{}`)", callable.display(self.db))
+                    }
+                    ProtocolMemberKind::Property(property) => {
+                        let mut d = f.debug_struct("PropertyMember");
+                        if let Some(getter) = property.getter(self.db) {
+                            d.field("getter", &format_args!("`{}`", &getter.display(self.db)));
+                        }
+                        if let Some(setter) = property.setter(self.db) {
+                            d.field("setter", &format_args!("`{}`", &setter.display(self.db)));
+                        }
+                        d.finish()
+                    }
+                    ProtocolMemberKind::Other(ty) => {
+                        write!(f, "AttributeMember(`{}`)", ty.display(self.db))
+                    }
+                }
+            }
+        }
+
+        ProtocolMemberDataDisplay {
+            db,
+            data: self.kind,
+        }
+    }
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, salsa::Update, Hash)]

crates/ty_vendored/ty_extensions/ty_extensions.pyi

@@ -64,3 +64,8 @@ def all_members(obj: Any) -> tuple[str, ...]: ...
 
 # Returns `True` if the given object has a member with the given name.
 def has_member(obj: Any, name: str) -> bool: ...
+
+# Passing a protocol type to this function will cause ty to emit an info-level
+# diagnostic describing the protocol's interface. Passing a non-protocol type
+# will cause ty to emit an error diagnostic.
+def reveal_protocol_interface(protocol: type) -> None: ...