Merge #6477

6477: Add infer_function_return_type assist r=matklad a=Veykril

This adds an assist to insert a functions return type if it hasn't been specified yet by inferring it from the functions tail expression. This assist only becomes active if the cursor is on the tail expression. See https://github.com/rust-analyzer/rust-analyzer/issues/6303#issuecomment-714657326

Co-authored-by: Lukas Wirth <lukastw97@gmail.com>

crates/assists/src/handlers/infer_function_return_type.rs

@@ -0,0 +1,337 @@
+use hir::HirDisplay;
+use syntax::{ast, AstNode, TextRange, TextSize};
+use test_utils::mark;
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: infer_function_return_type
+//
+// Adds the return type to a function or closure inferred from its tail expression if it doesn't have a return
+// type specified. This assists is useable in a functions or closures tail expression or return type position.
+//
+// ```
+// fn foo() { 4<|>2i32 }
+// ```
+// ->
+// ```
+// fn foo() -> i32 { 42i32 }
+// ```
+pub(crate) fn infer_function_return_type(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let (tail_expr, builder_edit_pos, wrap_expr) = extract_tail(ctx)?;
+    let module = ctx.sema.scope(tail_expr.syntax()).module()?;
+    let ty = ctx.sema.type_of_expr(&tail_expr)?;
+    if ty.is_unit() {
+        return None;
+    }
+    let ty = ty.display_source_code(ctx.db(), module.into()).ok()?;
+
+    acc.add(
+        AssistId("infer_function_return_type", AssistKind::RefactorRewrite),
+        "Add this function's return type",
+        tail_expr.syntax().text_range(),
+        |builder| {
+            match builder_edit_pos {
+                InsertOrReplace::Insert(insert_pos) => {
+                    builder.insert(insert_pos, &format!("-> {} ", ty))
+                }
+                InsertOrReplace::Replace(text_range) => {
+                    builder.replace(text_range, &format!("-> {}", ty))
+                }
+            }
+            if wrap_expr {
+                mark::hit!(wrap_closure_non_block_expr);
+                // `|x| x` becomes `|x| -> T x` which is invalid, so wrap it in a block
+                builder.replace(tail_expr.syntax().text_range(), &format!("{{{}}}", tail_expr));
+            }
+        },
+    )
+}
+
+enum InsertOrReplace {
+    Insert(TextSize),
+    Replace(TextRange),
+}
+
+/// Check the potentially already specified return type and reject it or turn it into a builder command
+/// if allowed.
+fn ret_ty_to_action(ret_ty: Option<ast::RetType>, insert_pos: TextSize) -> Option<InsertOrReplace> {
+    match ret_ty {
+        Some(ret_ty) => match ret_ty.ty() {
+            Some(ast::Type::InferType(_)) | None => {
+                mark::hit!(existing_infer_ret_type);
+                mark::hit!(existing_infer_ret_type_closure);
+                Some(InsertOrReplace::Replace(ret_ty.syntax().text_range()))
+            }
+            _ => {
+                mark::hit!(existing_ret_type);
+                mark::hit!(existing_ret_type_closure);
+                None
+            }
+        },
+        None => Some(InsertOrReplace::Insert(insert_pos + TextSize::from(1))),
+    }
+}
+
+fn extract_tail(ctx: &AssistContext) -> Option<(ast::Expr, InsertOrReplace, bool)> {
+    let (tail_expr, return_type_range, action, wrap_expr) =
+        if let Some(closure) = ctx.find_node_at_offset::<ast::ClosureExpr>() {
+            let rpipe_pos = closure.param_list()?.syntax().last_token()?.text_range().end();
+            let action = ret_ty_to_action(closure.ret_type(), rpipe_pos)?;
+
+            let body = closure.body()?;
+            let body_start = body.syntax().first_token()?.text_range().start();
+            let (tail_expr, wrap_expr) = match body {
+                ast::Expr::BlockExpr(block) => (block.expr()?, false),
+                body => (body, true),
+            };
+
+            let ret_range = TextRange::new(rpipe_pos, body_start);
+            (tail_expr, ret_range, action, wrap_expr)
+        } else {
+            let func = ctx.find_node_at_offset::<ast::Fn>()?;
+            let rparen_pos = func.param_list()?.r_paren_token()?.text_range().end();
+            let action = ret_ty_to_action(func.ret_type(), rparen_pos)?;
+
+            let body = func.body()?;
+            let tail_expr = body.expr()?;
+
+            let ret_range_end = body.l_curly_token()?.text_range().start();
+            let ret_range = TextRange::new(rparen_pos, ret_range_end);
+            (tail_expr, ret_range, action, false)
+        };
+    let frange = ctx.frange.range;
+    if return_type_range.contains_range(frange) {
+        mark::hit!(cursor_in_ret_position);
+        mark::hit!(cursor_in_ret_position_closure);
+    } else if tail_expr.syntax().text_range().contains_range(frange) {
+        mark::hit!(cursor_on_tail);
+        mark::hit!(cursor_on_tail_closure);
+    } else {
+        return None;
+    }
+    Some((tail_expr, action, wrap_expr))
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn infer_return_type_specified_inferred() {
+        mark::check!(existing_infer_ret_type);
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() -> <|>_ {
+    45
+}"#,
+            r#"fn foo() -> i32 {
+    45
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_specified_inferred_closure() {
+        mark::check!(existing_infer_ret_type_closure);
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+    || -> _ {<|>45};
+}"#,
+            r#"fn foo() {
+    || -> i32 {45};
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_cursor_at_return_type_pos() {
+        mark::check!(cursor_in_ret_position);
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() <|>{
+    45
+}"#,
+            r#"fn foo() -> i32 {
+    45
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_cursor_at_return_type_pos_closure() {
+        mark::check!(cursor_in_ret_position_closure);
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+    || <|>45
+}"#,
+            r#"fn foo() {
+    || -> i32 {45}
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type() {
+        mark::check!(cursor_on_tail);
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+    45<|>
+}"#,
+            r#"fn foo() -> i32 {
+    45
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_nested() {
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+    if true {
+        3<|>
+    } else {
+        5
+    }
+}"#,
+            r#"fn foo() -> i32 {
+    if true {
+        3
+    } else {
+        5
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_ret_type_specified() {
+        mark::check!(existing_ret_type);
+        check_assist_not_applicable(
+            infer_function_return_type,
+            r#"fn foo() -> i32 {
+    ( 45<|> + 32 ) * 123
+}"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_non_tail_expr() {
+        check_assist_not_applicable(
+            infer_function_return_type,
+            r#"fn foo() {
+    let x = <|>3;
+    ( 45 + 32 ) * 123
+}"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_unit_return_type() {
+        check_assist_not_applicable(
+            infer_function_return_type,
+            r#"fn foo() {
+    (<|>)
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_closure_block() {
+        mark::check!(cursor_on_tail_closure);
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+    |x: i32| {
+        x<|>
+    };
+}"#,
+            r#"fn foo() {
+    |x: i32| -> i32 {
+        x
+    };
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_closure() {
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+    |x: i32| { x<|> };
+}"#,
+            r#"fn foo() {
+    |x: i32| -> i32 { x };
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_closure_wrap() {
+        mark::check!(wrap_closure_non_block_expr);
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+    |x: i32| x<|>;
+}"#,
+            r#"fn foo() {
+    |x: i32| -> i32 {x};
+}"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_nested_closure() {
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+    || {
+        if true {
+            3<|>
+        } else {
+            5
+        }
+    }
+}"#,
+            r#"fn foo() {
+    || -> i32 {
+        if true {
+            3
+        } else {
+            5
+        }
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_ret_type_specified_closure() {
+        mark::check!(existing_ret_type_closure);
+        check_assist_not_applicable(
+            infer_function_return_type,
+            r#"fn foo() {
+    || -> i32 { 3<|> }
+}"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_non_tail_expr_closure() {
+        check_assist_not_applicable(
+            infer_function_return_type,
+            r#"fn foo() {
+    || -> i32 {
+        let x = 3<|>;
+        6
+    }
+}"#,
+        );
+    }
+}

crates/assists/src/lib.rs

@@ -143,6 +143,7 @@ mod handlers {
     mod generate_function;
     mod generate_impl;
     mod generate_new;
+    mod infer_function_return_type;
     mod inline_local_variable;
     mod introduce_named_lifetime;
     mod invert_if;
@@ -190,6 +191,7 @@ mod handlers {
             generate_function::generate_function,
             generate_impl::generate_impl,
             generate_new::generate_new,
+            infer_function_return_type::infer_function_return_type,
             inline_local_variable::inline_local_variable,
             introduce_named_lifetime::introduce_named_lifetime,
             invert_if::invert_if,

crates/assists/src/tests/generated.rs

@@ -505,6 +505,19 @@ impl<T: Clone> Ctx<T> {
     )
 }
 
+#[test]
+fn doctest_infer_function_return_type() {
+    check_doc_test(
+        "infer_function_return_type",
+        r#####"
+fn foo() { 4<|>2i32 }
+"#####,
+        r#####"
+fn foo() -> i32 { 42i32 }
+"#####,
+    )
+}
+
 #[test]
 fn doctest_inline_local_variable() {
     check_doc_test(