[ty] Suppress keyword argument completions unless we're in the "arguments"

Otherwise, given a case like this:

```
(lambda foo: (<CURSOR> + 1))(2)
```

we'll offer _argument_ completions for `foo` at the cursor position.
While we do actually want to offer completions for `foo` in this
context, it is currently difficult to do so. But we definitely don't
want to offer completions for `foo` as an argument to a function here.
Which is what we were doing.

We also add an end-to-end test here to verify that the actual label we
offer in completion suggestions includes the `=` suffix.

Closes https://github.com/astral-sh/ruff/pull/21970

crates/ty_ide/src/completion.rs

@@ -515,6 +515,18 @@ fn detect_function_arg_completions<'db>(
     parsed: &ParsedModuleRef,
     offset: TextSize,
 ) -> Option<Vec<Completion<'db>>> {
+    // But be careful: this isn't as simple as just finding a call
+    // expression. We also have to make sure we are in the "arguments"
+    // portion of the call. Otherwise we risk incorrectly returning
+    // something for `(<CURSOR>)(arg1, arg2)`-style expressions.
+    if !covering_node(parsed.syntax().into(), TextRange::empty(offset))
+        .ancestors()
+        .take_while(|node| !node.is_statement())
+        .any(|node| node.is_arguments())
+    {
+        return None;
+    }
+
     let sig_help = signature_help(db, file, offset)?;
     let set_function_args = detect_set_function_args(parsed, offset);
 
@@ -2594,7 +2606,7 @@ def frob(): ...
 
         assert_snapshot!(
             builder.skip_keywords().skip_builtins().build().snapshot(),
-            @"foo=",
+            @"<No completions found after filtering out completions>",
         );
     }
 
@@ -2608,7 +2620,7 @@ def frob(): ...
 
         assert_snapshot!(
             builder.skip_keywords().skip_builtins().build().snapshot(),
-            @"foo=",
+            @"<No completions found after filtering out completions>",
         );
     }
 

crates/ty_ide/src/signature_help.rs

@@ -124,6 +124,11 @@ fn get_call_expr(
         })?;
 
     // Find the covering node at the given position that is a function call.
+    // Note that we are okay with the range being anywhere within a call
+    // expression, even if it's not in the arguments portion of the call
+    // expression. This is because, e.g., a user can request signature
+    // information at a call site, and this should ideally work anywhere
+    // within the call site, even at the function name.
     let call = covering_node(root_node, token.range())
         .find_first(|node| {
             if !node.is_expr_call() {

crates/ty_server/tests/e2e/completions.rs

@@ -283,3 +283,62 @@ TypedDi<CURSOR>
 
     Ok(())
 }
+
+/// Tests that completions for function arguments will
+/// show a `=` suffix.
+#[test]
+fn function_parameter_shows_equals_suffix() -> Result<()> {
+    let workspace_root = SystemPath::new("src");
+    let foo = SystemPath::new("src/foo.py");
+    let foo_content = "\
+import re
+re.match('', '', fla<CURSOR>
+";
+
+    let mut server = TestServerBuilder::new()?
+        .with_initialization_options(ClientOptions::default().with_auto_import(false))
+        .with_workspace(workspace_root, None)?
+        .with_file(foo, foo_content)?
+        .build()
+        .wait_until_workspaces_are_initialized();
+
+    server.open_text_document(foo, foo_content, 1);
+    let _ = server.await_notification::<PublishDiagnostics>();
+
+    let completions = server.completion_request(&server.file_uri(foo), Position::new(1, 20));
+
+    insta::assert_json_snapshot!(completions, @r#"
+    [
+      {
+        "label": "flags=",
+        "kind": 6,
+        "detail": "int",
+        "sortText": "0",
+        "insertText": "flags="
+      },
+      {
+        "label": "FloatingPointError",
+        "kind": 7,
+        "detail": "<class 'FloatingPointError'>",
+        "documentation": "Floating-point operation failed.\n",
+        "sortText": "1"
+      },
+      {
+        "label": "PythonFinalizationError",
+        "kind": 7,
+        "detail": "<class 'PythonFinalizationError'>",
+        "documentation": "Operation blocked during Python finalization.\n",
+        "sortText": "2"
+      },
+      {
+        "label": "float",
+        "kind": 7,
+        "detail": "<class 'float'>",
+        "documentation": "Convert a string or number to a floating-point number, if possible.\n",
+        "sortText": "3"
+      }
+    ]
+    "#);
+
+    Ok(())
+}

crates/ty_server/tests/e2e/main.rs

@@ -36,6 +36,7 @@ mod notebook;
 mod publish_diagnostics;
 mod pull_diagnostics;
 mod rename;
+mod signature_help;
 
 use std::collections::{BTreeMap, HashMap, VecDeque};
 use std::num::NonZeroUsize;
@@ -54,7 +55,8 @@ use lsp_types::notification::{
 };
 use lsp_types::request::{
     Completion, DocumentDiagnosticRequest, HoverRequest, Initialize, InlayHintRequest,
-    PrepareRenameRequest, Request, Shutdown, WorkspaceConfiguration, WorkspaceDiagnosticRequest,
+    PrepareRenameRequest, Request, Shutdown, SignatureHelpRequest, WorkspaceConfiguration,
+    WorkspaceDiagnosticRequest,
 };
 use lsp_types::{
     ClientCapabilities, CompletionItem, CompletionParams, CompletionResponse,
@@ -64,11 +66,11 @@ use lsp_types::{
     DocumentDiagnosticParams, DocumentDiagnosticReportResult, FileEvent, Hover, HoverParams,
     InitializeParams, InitializeResult, InitializedParams, InlayHint, InlayHintClientCapabilities,
     InlayHintParams, NumberOrString, PartialResultParams, Position, PreviousResultId,
-    PublishDiagnosticsClientCapabilities, Range, TextDocumentClientCapabilities,
-    TextDocumentContentChangeEvent, TextDocumentIdentifier, TextDocumentItem,
-    TextDocumentPositionParams, Url, VersionedTextDocumentIdentifier, WorkDoneProgressParams,
-    WorkspaceClientCapabilities, WorkspaceDiagnosticParams, WorkspaceDiagnosticReportResult,
-    WorkspaceEdit, WorkspaceFolder,
+    PublishDiagnosticsClientCapabilities, Range, SignatureHelp, SignatureHelpParams,
+    SignatureHelpTriggerKind, TextDocumentClientCapabilities, TextDocumentContentChangeEvent,
+    TextDocumentIdentifier, TextDocumentItem, TextDocumentPositionParams, Url,
+    VersionedTextDocumentIdentifier, WorkDoneProgressParams, WorkspaceClientCapabilities,
+    WorkspaceDiagnosticParams, WorkspaceDiagnosticReportResult, WorkspaceEdit, WorkspaceFolder,
 };
 use ruff_db::system::{OsSystem, SystemPath, SystemPathBuf, TestSystem};
 use rustc_hash::FxHashMap;
@@ -940,6 +942,28 @@ impl TestServer {
             None => vec![],
         }
     }
+
+    /// Sends a `textDocument/signatureHelp` request for the document at the given URL and position.
+    pub(crate) fn signature_help_request(
+        &mut self,
+        uri: &Url,
+        position: Position,
+    ) -> Option<SignatureHelp> {
+        let signature_help_id = self.send_request::<SignatureHelpRequest>(SignatureHelpParams {
+            text_document_position_params: TextDocumentPositionParams {
+                text_document: TextDocumentIdentifier { uri: uri.clone() },
+                position,
+            },
+            work_done_progress_params: lsp_types::WorkDoneProgressParams::default(),
+            context: Some(lsp_types::SignatureHelpContext {
+                trigger_kind: SignatureHelpTriggerKind::INVOKED,
+                trigger_character: None,
+                is_retrigger: false,
+                active_signature_help: None,
+            }),
+        });
+        self.await_response::<SignatureHelpRequest>(&signature_help_id)
+    }
 }
 
 impl fmt::Debug for TestServer {

crates/ty_server/tests/e2e/signature_help.rs

@@ -0,0 +1,37 @@
+use anyhow::Result;
+use lsp_types::{Position, notification::PublishDiagnostics};
+use ruff_db::system::SystemPath;
+use ty_server::ClientOptions;
+
+use crate::TestServerBuilder;
+
+/// Tests that we get signature help even when the cursor
+/// is on the function name.
+///
+/// This is a regression test to ensure we don't accidentally
+/// cause this case to stop working.
+#[test]
+fn works_in_function_name() -> Result<()> {
+    let workspace_root = SystemPath::new("src");
+    let foo = SystemPath::new("src/foo.py");
+    let foo_content = "\
+import re
+re.match('', '')
+";
+
+    let mut server = TestServerBuilder::new()?
+        .with_initialization_options(ClientOptions::default())
+        .with_workspace(workspace_root, None)?
+        .with_file(foo, foo_content)?
+        .build()
+        .wait_until_workspaces_are_initialized();
+
+    server.open_text_document(foo, foo_content, 1);
+    let _ = server.await_notification::<PublishDiagnostics>();
+
+    let signature_help = server.signature_help_request(&server.file_uri(foo), Position::new(1, 6));
+
+    insta::assert_json_snapshot!(signature_help);
+
+    Ok(())
+}

crates/ty_server/tests/e2e/snapshots/e2e__signature_help__works_in_function_name.snap

@@ -0,0 +1,39 @@
+---
+source: crates/ty_server/tests/e2e/signature_help.rs
+expression: signature_help
+---
+{
+  "signatures": [
+    {
+      "label": "(pattern: str | Pattern[str], string: str, flags: int = Literal[0]) -> Match[str] | None",
+      "documentation": "Try to apply the pattern at the start of the string, returning/na Match object, or None if no match was found.\n",
+      "parameters": [
+        {
+          "label": "pattern: str | Pattern[str]"
+        },
+        {
+          "label": "string: str"
+        },
+        {
+          "label": "flags: int = Literal[0]"
+        }
+      ]
+    },
+    {
+      "label": "(pattern: bytes | Pattern[bytes], string: Buffer, flags: int = Literal[0]) -> Match[bytes] | None",
+      "parameters": [
+        {
+          "label": "pattern: bytes | Pattern[bytes]"
+        },
+        {
+          "label": "string: Buffer"
+        },
+        {
+          "label": "flags: int = Literal[0]"
+        }
+      ]
+    }
+  ],
+  "activeSignature": 0,
+  "activeParameter": 0
+}