Auto merge of #15349 - lowr:fix/fn-def-display-source-code, r=lnicola

Show anonymous fn def type as a fn pointer in source code

Fixes #15346

The second commit is an unrelated change. I can remove it if not desired.

crates/hir-ty/src/display.rs

@@ -48,22 +48,15 @@
 };
 
 pub trait HirWrite: fmt::Write {
-    fn start_location_link(&mut self, location: ModuleDefId);
-    fn end_location_link(&mut self);
+    fn start_location_link(&mut self, _location: ModuleDefId) {}
+    fn end_location_link(&mut self) {}
 }
 
 // String will ignore link metadata
-impl HirWrite for String {
-    fn start_location_link(&mut self, _: ModuleDefId) {}
-
-    fn end_location_link(&mut self) {}
-}
+impl HirWrite for String {}
 
 // `core::Formatter` will ignore metadata
-impl HirWrite for fmt::Formatter<'_> {
-    fn start_location_link(&mut self, _: ModuleDefId) {}
-    fn end_location_link(&mut self) {}
-}
+impl HirWrite for fmt::Formatter<'_> {}
 
 pub struct HirFormatter<'a> {
     pub db: &'a dyn HirDatabase,
@@ -885,6 +878,13 @@ fn hir_fmt(
             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) => {

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
+}
+"#,
+    );
+}

crates/ide-assists/src/handlers/generate_function.rs

@@ -1878,7 +1878,6 @@ fn bar<U: A<()>>(t: T, u: U) ${0:-> _} where T: A<S<i32>> {
 
     #[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!()
 }
 ",