Add toggle_async_sugar assist code action

4 files changed, 483 insertions, 0 deletions

diff --git a/src/tools/rust-analyzer/crates/ide-assists/src/handlers/toggle_async_sugar.rs b/src/tools/rust-analyzer/crates/ide-assists/src/handlers/toggle_async_sugar.rs
new file mode 100644
index 00000000000..ea127d65e54
--- /dev/null
+++ b/src/tools/rust-analyzer/crates/ide-assists/src/handlers/toggle_async_sugar.rs
@@ -0,0 +1,460 @@
+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: toggle_async_sugar
+//
+// Rewrites asynchronous function into `impl Future` and back.
+// This action does not touch the function body and therefore `async { 0 }`
+// block does not transform to just `0`.
+//
+// ```
+// pub async f$0n foo() -> usize {
+//     0
+// }
+// ```
+// ->
+// ```
+// pub fn foo() -> impl Future<Output = usize> {
+//     0
+// }
+// ```
+pub(crate) fn toggle_async_sugar(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    let function: ast::Fn = ctx.find_node_at_offset()?;
+    match (function.async_token(), function.ret_type()) {
+        // async function returning futures cannot be flattened
+        // const async is not yet supported
+        (None, Some(ret_type)) if function.const_token().is_none() => {
+            add_async(acc, ctx, function, ret_type)
+        }
+        (Some(async_token), ret_type) => remove_async(function, ret_type, acc, async_token),
+        _ => None,
+    }
+}
+
+fn add_async(
+    acc: &mut Assists,
+    ctx: &AssistContext<'_>,
+    function: ast::Fn,
+    ret_type: ast::RetType,
+) -> Option<()> {
+    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("toggle_async_sugar", AssistKind::RefactorRewrite),
+        "Convert `impl Future` into async",
+        function.syntax().text_range(),
+        |builder| {
+            match future_output {
+                ast::Type::TupleType(_) => {
+                    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);
+        },
+    )
+}
+
+fn remove_async(
+    function: ast::Fn,
+    ret_type: Option<ast::RetType>,
+    acc: &mut Assists,
+    async_token: SyntaxToken,
+) -> Option<()> {
+    let rparen = function.param_list()?.r_paren_token()?;
+    let return_type = match ret_type {
+        // unable to get a `ty` makes the action unapplicable
+        Some(ret_type) => Some(ret_type.ty()?),
+        // No type means `-> ()`
+        None => None,
+    };
+
+    acc.add(
+        AssistId("toggle_async_sugar", 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 Future<Output = {ret_type}>"),
+                ),
+                None => builder.insert(rparen.text_range().end(), " -> impl Future<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(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    f$0n foo() -> impl Future<Output = ()> {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    async fn foo() {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    f$0n foo() -> impl Future<Output = usize> {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    async fn foo() -> usize {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn desugar_with_use() {
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    async f$0n foo() {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    fn foo() -> impl Future<Output = ()> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    async f$0n foo() -> usize {
+        todo!()
+    }
+    "#,
+            r#"
+    use core::future::Future;
+    fn foo() -> impl Future<Output = usize> {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_without_use() {
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    f$0n foo() -> impl core::future::Future<Output = ()> {
+        todo!()
+    }
+    "#,
+            r#"
+    async fn foo() {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    f$0n foo() -> impl core::future::Future<Output = usize> {
+        todo!()
+    }
+    "#,
+            r#"
+    async fn foo() -> usize {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn desugar_without_use() {
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    async f$0n foo() {
+        todo!()
+    }
+    "#,
+            r#"
+    fn foo() -> impl Future<Output = ()> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    async f$0n foo() -> usize {
+        todo!()
+    }
+    "#,
+            r#"
+    fn foo() -> impl Future<Output = usize> {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_not_applicable() {
+        check_assist_not_applicable(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    trait Future {
+        type Output;
+    }
+    f$0n foo() -> impl Future<Output = ()> {
+        todo!()
+    }
+    "#,
+        );
+
+        check_assist_not_applicable(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    trait Future {
+        type Output;
+    }
+    f$0n foo() -> impl Future<Output = usize> {
+        todo!()
+    }
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_definition_with_use() {
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    f$0n foo() -> impl Future<Output = ()>;
+    "#,
+            r#"
+    use core::future::Future;
+    async fn foo();
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    use core::future::Future;
+    f$0n foo() -> impl Future<Output = usize>;
+    "#,
+            r#"
+    use core::future::Future;
+    async fn foo() -> usize;
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_definition_without_use() {
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    f$0n foo() -> impl core::future::Future<Output = ()>;
+    "#,
+            r#"
+    async fn foo();
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    f$0n foo() -> impl core::future::Future<Output = usize>;
+    "#,
+            r#"
+    async fn foo() -> usize;
+    "#,
+        );
+    }
+
+    #[test]
+    fn sugar_with_modifiers() {
+        check_assist_not_applicable(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    const f$0n foo() -> impl core::future::Future<Output = ()>;
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+            //- minicore: future
+            pub(crate) unsafe f$0n foo() -> impl core::future::Future<Output = usize>;
+        "#,
+            r#"
+            pub(crate) async unsafe fn foo() -> usize;
+        "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    unsafe f$0n foo() -> impl core::future::Future<Output = ()>;
+    "#,
+            r#"
+    async unsafe fn foo();
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    unsafe extern "C" f$0n foo() -> impl core::future::Future<Output = ()>;
+    "#,
+            r#"
+    async unsafe extern "C" fn foo();
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    f$0n foo<T>() -> impl core::future::Future<Output = T>;
+    "#,
+            r#"
+    async fn foo<T>() -> T;
+    "#,
+        );
+
+        check_assist(
+            toggle_async_sugar,
+            r#"
+    //- minicore: future
+    f$0n foo<T>() -> impl core::future::Future<Output = T>
+    where
+        T: Sized;
+    "#,
+            r#"
+    async fn foo<T>() -> T
+    where
+        T: Sized;
+    "#,
+        );
+    }
+}
diff --git a/src/tools/rust-analyzer/crates/ide-assists/src/lib.rs b/src/tools/rust-analyzer/crates/ide-assists/src/lib.rs
index 0df5e913a57..d26ac23099a 100644
--- a/src/tools/rust-analyzer/crates/ide-assists/src/lib.rs
+++ b/src/tools/rust-analyzer/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 +239,7 @@ 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::toggle_async_sugar,
             convert_comment_block::convert_comment_block,
             convert_from_to_tryfrom::convert_from_to_tryfrom,
             convert_integer_literal::convert_integer_literal,
diff --git a/src/tools/rust-analyzer/crates/ide-assists/src/tests/generated.rs b/src/tools/rust-analyzer/crates/ide-assists/src/tests/generated.rs
index 937e78f8d7d..8e0d1bd667a 100644
--- a/src/tools/rust-analyzer/crates/ide-assists/src/tests/generated.rs
+++ b/src/tools/rust-analyzer/crates/ide-assists/src/tests/generated.rs
@@ -3021,6 +3021,23 @@ use std::{collections::HashMap};
 }
 
 #[test]
+fn doctest_toggle_async_sugar() {
+    check_doc_test(
+        "toggle_async_sugar",
+        r#####"
+pub async f$0n foo() -> usize {
+    0
+}
+"#####,
+        r#####"
+pub fn foo() -> impl Future<Output = usize> {
+    0
+}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_toggle_ignore() {
     check_doc_test(
         "toggle_ignore",
diff --git a/src/tools/rust-analyzer/crates/ide-db/src/famous_defs.rs b/src/tools/rust-analyzer/crates/ide-db/src/famous_defs.rs
index 3106772e63b..e445e9fb68d 100644
--- a/src/tools/rust-analyzer/crates/ide-db/src/famous_defs.rs
+++ b/src/tools/rust-analyzer/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")
     }