feat: Generate static method using Self::assoc() syntax

This change improves the `generate_function` assist to support generating static methods/associated functions using the `Self::assoc()` syntax. Previously, one could generate a static method, but only when specifying the type name directly (like `Foo::assoc()`). After this change, `Self` is supported as well as the type name.

Fixes rust-lang#13012

Author: DorianListens

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

@@ -61,7 +61,7 @@ fn gen_fn(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
     }
 
     let fn_name = &*name_ref.text();
-    let target_module;
+    let mut target_module = None;
     let mut adt_name = None;
 
     let (target, file, insert_offset) = match path.qualifier() {
@@ -78,16 +78,11 @@ fn gen_fn(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
                     }
                 }
 
-                let current_module = ctx.sema.scope(call.syntax())?.module();
-                let module = adt.module(ctx.sema.db);
-                target_module = if current_module == module { None } else { Some(module) };
-                if current_module.krate() != module.krate() {
-                    return None;
-                }
-                let (impl_, file) = get_adt_source(ctx, &adt, fn_name)?;
-                let (target, insert_offset) = get_method_target(ctx, &module, &impl_)?;
-                adt_name = if impl_.is_none() { Some(adt.name(ctx.sema.db)) } else { None };
-                (target, file, insert_offset)
+                static_method_target(ctx, &call, adt, &mut target_module, fn_name, &mut adt_name)?
+            }
+            Some(hir::PathResolution::SelfType(impl_)) => {
+                let adt = impl_.self_ty(ctx.db()).as_adt()?;
+                static_method_target(ctx, &call, adt, &mut target_module, fn_name, &mut adt_name)?
             }
             _ => {
                 return None;
@@ -399,6 +394,26 @@ fn get_method_target(
     Some((target.clone(), get_insert_offset(&target)))
 }
 
+fn static_method_target(
+    ctx: &AssistContext<'_>,
+    call: &CallExpr,
+    adt: hir::Adt,
+    target_module: &mut Option<Module>,
+    fn_name: &str,
+    adt_name: &mut Option<hir::Name>,
+) -> Option<(GeneratedFunctionTarget, FileId, TextSize)> {
+    let current_module = ctx.sema.scope(call.syntax())?.module();
+    let module = adt.module(ctx.sema.db);
+    *target_module = if current_module == module { None } else { Some(module) };
+    if current_module.krate() != module.krate() {
+        return None;
+    }
+    let (impl_, file) = get_adt_source(ctx, &adt, fn_name)?;
+    let (target, insert_offset) = get_method_target(ctx, &module, &impl_)?;
+    *adt_name = if impl_.is_none() { Some(adt.name(ctx.sema.db)) } else { None };
+    Some((target, file, insert_offset))
+}
+
 fn get_insert_offset(target: &GeneratedFunctionTarget) -> TextSize {
     match &target {
         GeneratedFunctionTarget::BehindItem(it) => it.text_range().end(),
@@ -1633,6 +1648,33 @@ fn bar() ${0:-> _} {
         )
     }
 
+    #[test]
+    fn create_static_method_within_an_impl_with_self_syntax() {
+        check_assist(
+            generate_function,
+            r"
+struct S;
+impl S {
+    fn foo(&self) {
+        Self::bar$0();
+    }
+}
+",
+            r"
+struct S;
+impl S {
+    fn foo(&self) {
+        Self::bar();
+    }
+
+    fn bar() ${0:-> _} {
+        todo!()
+    }
+}
+",
+        )
+    }
+
     #[test]
     fn no_panic_on_invalid_global_path() {
         check_assist(