Repository: rust-lang/rust-analyzer (rust)

Issue:
Code action to toggle `async fn` and `impl Future`
It's better to have a code action that toggles a function or trait method between `async fn method() -> Ret` and `fn method() -> impl Future<Output = Ret>`. Conversion from `async fn` to `impl Future` should be lossless, but, of course, the opposite is not.

It's convenient in cases like:
- like to convert an existing `-> impl Future` function to `async fn` especially it was generated (e.g. https://github.com/rust-lang/rust-analyzer/issues/16195)
- like to add `Send` bound to an existing `async fn` to assert the implementation is Send.

Pull request:
Add `toggle_async_sugar` assist code action
Implement code action for sugaring and de-sugaring asynchronous functions.

This code action does not import `Future` trait when de-sugaring and does not touch function boby, I guess this can be implemented later if needed. This action also does not take into consideration other bounds because IMO it's usually "let me try to use sugared version here".

Feel free to request changes, that's my first code action implementation 😄 

Closes #17010 
Relates to #16195

Patch:
diff --git a/crates/ide-assists/src/handlers/toggle_async_sugar.rs b/crates/ide-assists/src/handlers/toggle_async_sugar.rs
new file mode 100644
index 0000000000..30e09648ea
--- /dev/null
+++ b/crates/ide-assists/src/handlers/toggle_async_sugar.rs
@@ -0,0 +1,601 @@
+use hir::{ImportPathConfig, ModuleDef};
+use ide_db::{
+    assists::{AssistId, AssistKind},
+    famous_defs::FamousDefs,
+};
+use syntax::{
+    ast::{self, HasVisibility},
+    AstNode, NodeOrToken, SyntaxKind, SyntaxNode, SyntaxToken, TextRange,
+};
+
+use crate::{AssistContext, Assists};
+
+// Assist: sugar_impl_future_into_async
+//
+// Rewrites asynchronous function from `-> impl Future` into `async fn`.
+// This action does not touch the function body and therefore `async { 0 }`
+// block does not transform to just `0`.
+//
+// ```
+// # //- minicore: future
+// pub fn foo() -> impl core::future::F$0uture<Output = usize> {
+//     async { 0 }
+// }
+// ```
+// ->
+// ```
+// pub async fn foo() -> usize {
+//     async { 0 }
+// }
+// ```
+pub(crate) fn sugar_impl_future_into_async(
+    acc: &mut Assists,
+    ctx: &AssistContext<'_>,
+) -> Option<()> {
+    let ret_type: ast::RetType = ctx.find_node_at_offset()?;
+    let function = ret_type.syntax().parent().and_then(ast::Fn::cast)?;
+
+    if function.async_token().is_some() || function.const_token().is_some() {
+        return None;
+    }
+
+    let ast::Type::ImplTraitType(return_impl_trait) = ret_type.ty()? else {
+        return None;
+    };
+
+    let main_trait_path = return_impl_trait
+        .type_bound_list()?
+        .bounds()
+        .filter_map(|bound| match bound.ty() {
+            Some(ast::Type::PathType(trait_path)) => trait_path.path(),
+            _ => None,
+        })
+        .next()?;
+
+    let trait_type = ctx.sema.resolve_trait(&main_trait_path)?;
+    let scope = ctx.sema.scope(main_trait_path.syntax())?;
+    if trait_type != FamousDefs(&ctx.sema, scope.krate()).core_future_Future()? {
+        return None;
+    }
+    let future_output = unwrap_future_output(main_trait_path)?;
+
+    acc.add(
+        AssistId("sugar_impl_future_into_async", AssistKind::RefactorRewrite),
+        "Convert `impl Future` into async",
+        function.syntax().text_range(),
+        |builder| {
+            match future_output {
+                // Empty tuple
+                ast::Type::TupleType(t) if t.fields().next().is_none() => {
+                    let mut ret_type_range = ret_type.syntax().text_range();
+
+                    // find leftover whitespace
+                    let whitespace_range = function
+                        .param_list()
+                        .as_ref()
+                        .map(|params| NodeOrToken::Node(params.syntax()))
+                        .and_then(following_whitespace);
+
+                    if let Some(whitespace_range) = whitespace_range {
+                        ret_type_range =
+                            TextRange::new(whitespace_range.start(), ret_type_range.end());
+                    }
+
+                    builder.delete(ret_type_range);
+                }
+                _ => {
+                    builder.replace(
+                        return_impl_trait.syntax().text_range(),
+                        future_output.syntax().text(),
+                    );
+                }
+            }
+
+            let (place_for_async, async_kw) = match function.visibility() {
+                Some(vis) => (vis.syntax().text_range().end(), " async"),
+                None => (function.syntax().text_range().start(), "async "),
+            };
+            builder.insert(place_for_async, async_kw);
+        },
+    )
+}
+
+// Assist: desugar_async_into_impl_future
+//
+// Rewrites asynchronous function from `async fn` into `-> impl Future`.
+// This action does not touch the function body and therefore `0`
+// block does not transform to `async { 0 }`.
+//
+// ```
+// # //- minicore: future
+// pub as$0ync fn foo() -> usize {
+//     0
+// }
+// ```
+// ->
+// ```
+// pub fn foo() -> impl core::future::Future<Output = usize> {
+//     0
+// }
+// ```
+pub(crate) fn desugar_async_into_impl_future(
+    acc: &mut Assists,
+    ctx: &AssistContext<'_>,
+) -> Option<()> {
+    let async_token = ctx.find_token_syntax_at_offset(SyntaxKind::ASYNC_KW)?;
+    let function = async_token.parent().and_then(ast::Fn::cast)?;
+
+    let rparen = function.param_list()?.r_paren_token()?;
+    let return_type = match function.ret_type() {
+        // unable to get a `ty` makes the action unapplicable
+        Some(ret_type) => Some(ret_type.ty()?),
+        // No type means `-> ()`
+        None => None,
+    };
+
+    let scope = ctx.sema.scope(function.syntax())?;
+    let module = scope.module();
+    let future_trait = FamousDefs(&ctx.sema, scope.krate()).core_future_Future()?;
+    let trait_path = module.find_path(
+        ctx.db(),
+        ModuleDef::Trait(future_trait),
+        ImportPathConfig {
+            prefer_no_std: ctx.config.prefer_no_std,
+            prefer_prelude: ctx.config.prefer_prelude,
+        },
+    )?;
+    let trait_path = trait_path.display(ctx.db());
+
+    acc.add(
+        AssistId("desugar_async_into_impl_future", AssistKind::RefactorRewrite),
+        "Convert async into `impl Future`",
+        function.syntax().text_range(),
+        |builder| {
+            let mut async_range = async_token.text_range();
+
+            if let Some(whitespace_range) = following_whitespace(NodeOrToken::Token(async_token)) {
+                async_range = TextRange::new(async_range.start(), whitespace_range.end());
+            }
+            builder.delete(async_range);
+
+            match return_type {
+                Some(ret_type) => builder.replace(
+                    ret_type.syntax().text_range(),
+                    format!("impl {trait_path}<Output = {ret_type}>"),
+                ),
+                None => builder.insert(
+                    rparen.text_range().end(),
+                    format!(" -> impl {trait_path}<Output = ()>"),
+                ),
+            }
+        },
+    )
+}
+
+fn unwrap_future_output(path: ast::Path) -> Option<ast::Type> {
+    let future_trait = path.segments().last()?;
+    let assoc_list = future_trait.generic_arg_list()?;
+    let future_assoc = assoc_list.generic_args().next()?;
+    match future_assoc {
+        ast::GenericArg::AssocTypeArg(output_type) => output_type.ty(),
+        _ => None,
+    }
+}
+
+fn following_whitespace(nt: NodeOrToken<&SyntaxNode, SyntaxToken>) -> Option<TextRange> {
+    let next_token = match nt {
+        NodeOrToken::Node(node) => node.next_sibling_or_token(),
+        NodeOrToken::Token(token) => token.next_sibling_or_token(),
+    }?;
+    (next_token.kind() == SyntaxKind::WHITESPACE).then_some(next_token.text_range())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn sugar_with_use() {
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    fn foo() -> impl F$0uture<Output = ()> {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    async fn foo() {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    fn foo() -> impl F$0uture<Output = usize> {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    async fn foo() -> usize {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn desugar_with_use() {
+        check_assist(
+            desugar_async_into_impl_future,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    as$0ync fn foo() {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    fn foo() -> impl Future<Output = ()> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            desugar_async_into_impl_future,
+            r#"
+    //- minicore: future
+    use core::future;
+    as$0ync fn foo() {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future;
+    fn foo() -> impl future::Future<Output = ()> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            desugar_async_into_impl_future,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    as$0ync fn foo() -> usize {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    fn foo() -> impl Future<Output = usize> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            desugar_async_into_impl_future,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    as$0ync fn foo() -> impl Future<Output = usize> {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    fn foo() -> impl Future<Output = impl Future<Output = usize>> {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_without_use() {
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::F$0uture<Output = ()> {
+        todo!()
+    }
+    "#,
+            r#"
+    async fn foo() {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::F$0uture<Output = usize> {
+        todo!()
+    }
+    "#,
+            r#"
+    async fn foo() -> usize {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn desugar_without_use() {
+        check_assist(
+            desugar_async_into_impl_future,
+            r#"
+    //- minicore: future
+    as$0ync fn foo() {
+        todo!()
+    }
+    "#,
+            r#"
+    fn foo() -> impl core::future::Future<Output = ()> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            desugar_async_into_impl_future,
+            r#"
+    //- minicore: future
+    as$0ync fn foo() -> usize {
+        todo!()
+    }
+    "#,
+            r#"
+    fn foo() -> impl core::future::Future<Output = usize> {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn not_applicable() {
+        check_assist_not_applicable(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    trait Future {
+        type Output;
+    }
+    fn foo() -> impl F$0uture<Output = ()> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist_not_applicable(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    trait Future {
+        type Output;
+    }
+    fn foo() -> impl F$0uture<Output = usize> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist_not_applicable(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    f$0n foo() -> impl core::future::Future<Output = usize> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist_not_applicable(
+            desugar_async_into_impl_future,
+            r#"
+    async f$0n foo() {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_definition_with_use() {
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    fn foo() -> impl F$0uture<Output = ()>;
+    "#,
+            r#"
+    use core::future::Future;
+    async fn foo();
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    fn foo() -> impl F$0uture<Output = usize>;
+    "#,
+            r#"
+    use core::future::Future;
+    async fn foo() -> usize;
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_definition_without_use() {
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::F$0uture<Output = ()>;
+    "#,
+            r#"
+    async fn foo();
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::F$0uture<Output = usize>;
+    "#,
+            r#"
+    async fn foo() -> usize;
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_more_types() {
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::F$0uture<Output = ()> + Send + Sync;
+    "#,
+            r#"
+    async fn foo();
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::F$0uture<Output = usize> + Debug;
+    "#,
+            r#"
+    async fn foo() -> usize;
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::F$0uture<Output = (usize)> + Debug;
+    "#,
+            r#"
+    async fn foo() -> (usize);
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::F$0uture<Output = (usize, usize)> + Debug;
+    "#,
+            r#"
+    async fn foo() -> (usize, usize);
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo() -> impl core::future::Future<Output = impl core::future::F$0uture<Output = ()> + Send>;
+    "#,
+            r#"
+    async fn foo() -> impl core::future::Future<Output = ()> + Send;
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_with_modifiers() {
+        check_assist_not_applicable(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    const fn foo() -> impl core::future::F$0uture<Output = ()>;
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+            //- minicore: future
+            pub(crate) unsafe fn foo() -> impl core::future::F$0uture<Output = usize>;
+        "#,
+            r#"
+            pub(crate) async unsafe fn foo() -> usize;
+        "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    unsafe fn foo() -> impl core::future::F$0uture<Output = ()>;
+    "#,
+            r#"
+    async unsafe fn foo();
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    unsafe extern "C" fn foo() -> impl core::future::F$0uture<Output = ()>;
+    "#,
+            r#"
+    async unsafe extern "C" fn foo();
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo<T>() -> impl core::future::F$0uture<Output = T>;
+    "#,
+            r#"
+    async fn foo<T>() -> T;
+    "#,
+        );
+
+        check_assist(
+            sugar_impl_future_into_async,
+            r#"
+    //- minicore: future
+    fn foo<T>() -> impl core::future::F$0uture<Output = T>
+    where
+        T: Sized;
+    "#,
+            r#"
+    async fn foo<T>() -> T
+    where
+        T: Sized;
+    "#,
+        );
+    }
+}
diff --git a/crates/ide-assists/src/lib.rs b/crates/ide-assists/src/lib.rs
index 0df5e913a5..34ef341c44 100644
--- a/crates/ide-assists/src/lib.rs
+++ b/crates/ide-assists/src/lib.rs
@@ -209,6 +209,7 @@ mod handlers {
     mod sort_items;
     mod split_import;
     mod term_search;
+    mod toggle_async_sugar;
     mod toggle_ignore;
     mod unmerge_match_arm;
     mod unmerge_use;
@@ -238,6 +239,8 @@ mod handlers {
             change_visibility::change_visibility,
             convert_bool_then::convert_bool_then_to_if,
             convert_bool_then::convert_if_to_bool_then,
+            toggle_async_sugar::desugar_async_into_impl_future,
+            toggle_async_sugar::sugar_impl_future_into_async,
             convert_comment_block::convert_comment_block,
             convert_from_to_tryfrom::convert_from_to_tryfrom,
             convert_integer_literal::convert_integer_literal,
diff --git a/crates/ide-db/src/famous_defs.rs b/crates/ide-db/src/famous_defs.rs
index 3106772e63..e445e9fb68 100644
--- a/crates/ide-db/src/famous_defs.rs
+++ b/crates/ide-db/src/famous_defs.rs
@@ -106,6 +106,10 @@ impl FamousDefs<'_, '_> {
         self.find_trait("core:marker:Copy")
     }
 
+    pub fn core_future_Future(&self) -> Option<Trait> {
+        self.find_trait("core:future:Future")
+    }
+
     pub fn core_macros_builtin_derive(&self) -> Option<Macro> {
         self.find_macro("core:macros:builtin:derive")
     }


Test patch:
diff --git a/crates/ide-assists/src/tests/generated.rs b/crates/ide-assists/src/tests/generated.rs
index 937e78f8d7..94d9e5edef 100644
--- a/crates/ide-assists/src/tests/generated.rs
+++ b/crates/ide-assists/src/tests/generated.rs
@@ -800,6 +800,24 @@ fn main() {
     )
 }
 
+#[test]
+fn doctest_desugar_async_into_impl_future() {
+    check_doc_test(
+        "desugar_async_into_impl_future",
+        r#####"
+//- minicore: future
+pub as$0ync fn foo() -> usize {
+    0
+}
+"#####,
+        r#####"
+pub fn foo() -> impl core::future::Future<Output = usize> {
+    0
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_desugar_doc_comment() {
     check_doc_test(
@@ -3020,6 +3038,24 @@ use std::{collections::HashMap};
     )
 }
 
+#[test]
+fn doctest_sugar_impl_future_into_async() {
+    check_doc_test(
+        "sugar_impl_future_into_async",
+        r#####"
+//- minicore: future
+pub fn foo() -> impl core::future::F$0uture<Output = usize> {
+    async { 0 }
+}
+"#####,
+        r#####"
+pub async fn foo() -> usize {
+    async { 0 }
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_toggle_ignore() {
     check_doc_test(