From b517aeeca5976955f8f49e8ce3de4bc63cf629c9 Mon Sep 17 00:00:00 2001
From: Ryo Yoshida <low.ryoshida@gmail.com>
Date: Fri, 28 Jul 2023 19:09:38 +0900
Subject: [PATCH] Show `TyKind::FnDef` as a fn pointer in source code

---
 crates/hir-ty/src/display.rs                  |  7 +++++++
 .../hir-ty/src/tests/display_source_code.rs   | 19 +++++++++++++++++++
 .../src/handlers/generate_function.rs         |  3 +--
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/crates/hir-ty/src/display.rs b/crates/hir-ty/src/display.rs
index 96787959e1f2..f53e24c238d8 100644
--- a/crates/hir-ty/src/display.rs
+++ b/crates/hir-ty/src/display.rs
@@ -885,6 +885,13 @@ impl HirDisplay for Ty {
             TyKind::FnDef(def, parameters) => {
                 let def = from_chalk(db, *def);
                 let sig = db.callable_item_signature(def).substitute(Interner, parameters);
+
+                if f.display_target.is_source_code() {
+                    // `FnDef` is anonymous and there's no surface syntax for it. Show it as a
+                    // function pointer type.
+                    return sig.hir_fmt(f);
+                }
+
                 f.start_location_link(def.into());
                 match def {
                     CallableDefId::FunctionId(ff) => {
diff --git a/crates/hir-ty/src/tests/display_source_code.rs b/crates/hir-ty/src/tests/display_source_code.rs
index 425432479e81..e75b037e38d3 100644
--- a/crates/hir-ty/src/tests/display_source_code.rs
+++ b/crates/hir-ty/src/tests/display_source_code.rs
@@ -227,3 +227,22 @@ fn f(a: impl Foo<i8, Assoc<i16> = i32>) {
         "#,
     );
 }
+
+#[test]
+fn fn_def_is_shown_as_fn_ptr() {
+    check_types_source_code(
+        r#"
+fn foo(_: i32) -> i64 { 42 }
+struct S<T>(T);
+enum E { A(usize) }
+fn test() {
+    let f = foo;
+      //^ fn(i32) -> i64
+    let f = S::<i8>;
+      //^ fn(i8) -> S<i8>
+    let f = E::A;
+      //^ fn(usize) -> E
+}
+"#,
+    );
+}
diff --git a/crates/ide-assists/src/handlers/generate_function.rs b/crates/ide-assists/src/handlers/generate_function.rs
index 8085572497aa..5b13e01b1330 100644
--- a/crates/ide-assists/src/handlers/generate_function.rs
+++ b/crates/ide-assists/src/handlers/generate_function.rs
@@ -1878,7 +1878,6 @@ where
 
     #[test]
     fn add_function_with_fn_arg() {
-        // FIXME: The argument in `bar` is wrong.
         check_assist(
             generate_function,
             r"
@@ -1899,7 +1898,7 @@ fn foo() {
     bar(Baz::new);
 }
 
-fn bar(new: fn) ${0:-> _} {
+fn bar(new: fn() -> Baz) ${0:-> _} {
     todo!()
 }
 ",