Auto merge of #17138 - Kohei316:generate-function-assist-for-new, r=Veykril

feature: Make generate function assist generate a function as a constructor if the generated function has the name "new" and is an asscociated function.

close #17050
This PR makes `generate function assist` generate a function as a constructor if the generated function has the name "new" and is an asscociated function.
If the asscociate type is a record struct, it generates the constructor like this.
```rust
impl Foo {
    fn new() -> Self {
        Self { field_1: todo!(), field_2: todo!() }
    }
}
```
If the asscociate type is a tuple struct, it generates the constructor like this.
```rust
impl Foo {
    fn new() -> Self {
        Self(todo!(), todo!())
    }
}
```
If the asscociate type is a unit struct, it generates the constructor like this.
```rust
impl Foo {
    fn new() -> Self {
        Self
    }
}
```
If the asscociate type is another adt, it generates the constructor like this.
```rust
impl Foo {
    fn new() -> Self {
        todo!()
    }
}
```

crates/hir/src/lib.rs

@@ -1489,6 +1489,14 @@ impl Adt {
             .map(|arena| arena.1.clone())
     }
 
+    pub fn as_struct(&self) -> Option<Struct> {
+        if let Self::Struct(v) = self {
+            Some(*v)
+        } else {
+            None
+        }
+    }
+
     pub fn as_enum(&self) -> Option<Enum> {
         if let Self::Enum(v) = self {
             Some(*v)

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

@@ -1,6 +1,6 @@
 use hir::{
-    Adt, AsAssocItem, HasSource, HirDisplay, HirFileIdExt, Module, PathResolution, Semantics, Type,
+    Adt, AsAssocItem, HasSource, HirDisplay, HirFileIdExt, Module, PathResolution, Semantics,
-    TypeInfo,
+    StructKind, Type, TypeInfo,
 };
 use ide_db::{
     base_db::FileId,
@@ -15,8 +15,8 @@ use itertools::Itertools;
 use stdx::to_lower_snake_case;
 use syntax::{
     ast::{
-        self, edit::IndentLevel, edit_in_place::Indent, make, AstNode, CallExpr, HasArgList,
+        self, edit::IndentLevel, edit_in_place::Indent, make, AstNode, BlockExpr, CallExpr,
-        HasGenericParams, HasModuleItem, HasTypeBounds,
+        HasArgList, HasGenericParams, HasModuleItem, HasTypeBounds,
     },
     ted, SyntaxKind, SyntaxNode, TextRange, T,
 };
@@ -66,7 +66,7 @@ fn gen_fn(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
     }
 
     let fn_name = &*name_ref.text();
-    let TargetInfo { target_module, adt_name, target, file } =
+    let TargetInfo { target_module, adt_info, target, file } =
         fn_target_info(ctx, path, &call, fn_name)?;
 
     if let Some(m) = target_module {
@@ -75,15 +75,16 @@ fn gen_fn(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
         }
     }
 
-    let function_builder = FunctionBuilder::from_call(ctx, &call, fn_name, target_module, target)?;
+    let function_builder =
+        FunctionBuilder::from_call(ctx, &call, fn_name, target_module, target, &adt_info)?;
     let text_range = call.syntax().text_range();
     let label = format!("Generate {} function", function_builder.fn_name);
-    add_func_to_accumulator(acc, ctx, text_range, function_builder, file, adt_name, label)
+    add_func_to_accumulator(acc, ctx, text_range, function_builder, file, adt_info, label)
 }
 
 struct TargetInfo {
     target_module: Option<Module>,
-    adt_name: Option<hir::Name>,
+    adt_info: Option<AdtInfo>,
     target: GeneratedFunctionTarget,
     file: FileId,
 }
@@ -91,11 +92,11 @@ struct TargetInfo {
 impl TargetInfo {
     fn new(
         target_module: Option<Module>,
-        adt_name: Option<hir::Name>,
+        adt_info: Option<AdtInfo>,
         target: GeneratedFunctionTarget,
         file: FileId,
     ) -> Self {
-        Self { target_module, adt_name, target, file }
+        Self { target_module, adt_info, target, file }
     }
 }
 
@@ -157,9 +158,9 @@ fn gen_method(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
         target,
     )?;
     let text_range = call.syntax().text_range();
-    let adt_name = if impl_.is_none() { Some(adt.name(ctx.sema.db)) } else { None };
+    let adt_info = AdtInfo::new(adt, impl_.is_some());
     let label = format!("Generate {} method", function_builder.fn_name);
-    add_func_to_accumulator(acc, ctx, text_range, function_builder, file, adt_name, label)
+    add_func_to_accumulator(acc, ctx, text_range, function_builder, file, Some(adt_info), label)
 }
 
 fn add_func_to_accumulator(
@@ -168,7 +169,7 @@ fn add_func_to_accumulator(
     text_range: TextRange,
     function_builder: FunctionBuilder,
     file: FileId,
-    adt_name: Option<hir::Name>,
+    adt_info: Option<AdtInfo>,
     label: String,
 ) -> Option<()> {
     acc.add(AssistId("generate_function", AssistKind::Generate), label, text_range, |edit| {
@@ -177,8 +178,14 @@ fn add_func_to_accumulator(
         let target = function_builder.target.clone();
         let func = function_builder.render(ctx.config.snippet_cap, edit);
 
-        if let Some(name) = adt_name {
+        if let Some(adt) =
-            let name = make::ty_path(make::ext::ident_path(&format!("{}", name.display(ctx.db()))));
+            adt_info
+                .and_then(|adt_info| if adt_info.impl_exists { None } else { Some(adt_info.adt) })
+        {
+            let name = make::ty_path(make::ext::ident_path(&format!(
+                "{}",
+                adt.name(ctx.db()).display(ctx.db())
+            )));
 
             // FIXME: adt may have generic params.
             let impl_ = make::impl_(None, None, name, None, None).clone_for_update();
@@ -210,6 +217,7 @@ struct FunctionBuilder {
     generic_param_list: Option<ast::GenericParamList>,
     where_clause: Option<ast::WhereClause>,
     params: ast::ParamList,
+    fn_body: BlockExpr,
     ret_type: Option<ast::RetType>,
     should_focus_return_type: bool,
     visibility: Visibility,
@@ -225,6 +233,7 @@ impl FunctionBuilder {
         fn_name: &str,
         target_module: Option<Module>,
         target: GeneratedFunctionTarget,
+        adt_info: &Option<AdtInfo>,
     ) -> Option<Self> {
         let target_module =
             target_module.or_else(|| ctx.sema.scope(target.syntax()).map(|it| it.module()))?;
@@ -243,9 +252,27 @@ impl FunctionBuilder {
         let await_expr = call.syntax().parent().and_then(ast::AwaitExpr::cast);
         let is_async = await_expr.is_some();
 
+        let ret_type;
+        let should_focus_return_type;
+        let fn_body;
+
+        // If generated function has the name "new" and is an associated function, we generate fn body
+        // as a constructor and assume a "Self" return type.
+        if let Some(body) = make_fn_body_as_new_function(ctx, &fn_name.text(), adt_info) {
+            ret_type = Some(make::ret_type(make::ty_path(make::ext::ident_path("Self"))));
+            should_focus_return_type = false;
+            fn_body = body;
+        } else {
             let expr_for_ret_ty = await_expr.map_or_else(|| call.clone().into(), |it| it.into());
-        let (ret_type, should_focus_return_type) =
+            (ret_type, should_focus_return_type) = make_return_type(
-            make_return_type(ctx, &expr_for_ret_ty, target_module, &mut necessary_generic_params);
+                ctx,
+                &expr_for_ret_ty,
+                target_module,
+                &mut necessary_generic_params,
+            );
+            let placeholder_expr = make::ext::expr_todo();
+            fn_body = make::block_expr(vec![], Some(placeholder_expr));
+        };
 
         let (generic_param_list, where_clause) =
             fn_generic_params(ctx, necessary_generic_params, &target)?;
@@ -256,6 +283,7 @@ impl FunctionBuilder {
             generic_param_list,
             where_clause,
             params,
+            fn_body,
             ret_type,
             should_focus_return_type,
             visibility,
@@ -294,12 +322,16 @@ impl FunctionBuilder {
         let (generic_param_list, where_clause) =
             fn_generic_params(ctx, necessary_generic_params, &target)?;
 
+        let placeholder_expr = make::ext::expr_todo();
+        let fn_body = make::block_expr(vec![], Some(placeholder_expr));
+
         Some(Self {
             target,
             fn_name,
             generic_param_list,
             where_clause,
             params,
+            fn_body,
             ret_type,
             should_focus_return_type,
             visibility,
@@ -308,8 +340,6 @@ impl FunctionBuilder {
     }
 
     fn render(self, cap: Option<SnippetCap>, edit: &mut SourceChangeBuilder) -> ast::Fn {
-        let placeholder_expr = make::ext::expr_todo();
-        let fn_body = make::block_expr(vec![], Some(placeholder_expr));
         let visibility = match self.visibility {
             Visibility::None => None,
             Visibility::Crate => Some(make::visibility_pub_crate()),
@@ -321,7 +351,7 @@ impl FunctionBuilder {
             self.generic_param_list,
             self.where_clause,
             self.params,
-            fn_body,
+            self.fn_body,
             self.ret_type,
             self.is_async,
             false, // FIXME : const and unsafe are not handled yet.
@@ -391,6 +421,53 @@ fn make_return_type(
     (ret_type, should_focus_return_type)
 }
 
+fn make_fn_body_as_new_function(
+    ctx: &AssistContext<'_>,
+    fn_name: &str,
+    adt_info: &Option<AdtInfo>,
+) -> Option<ast::BlockExpr> {
+    if fn_name != "new" {
+        return None;
+    };
+    let adt_info = adt_info.as_ref()?;
+
+    let path_self = make::ext::ident_path("Self");
+    let placeholder_expr = make::ext::expr_todo();
+    let tail_expr = if let Some(strukt) = adt_info.adt.as_struct() {
+        match strukt.kind(ctx.db()) {
+            StructKind::Record => {
+                let fields = strukt
+                    .fields(ctx.db())
+                    .iter()
+                    .map(|field| {
+                        make::record_expr_field(
+                            make::name_ref(&format!("{}", field.name(ctx.db()).display(ctx.db()))),
+                            Some(placeholder_expr.clone()),
+                        )
+                    })
+                    .collect::<Vec<_>>();
+
+                make::record_expr(path_self, make::record_expr_field_list(fields)).into()
+            }
+            StructKind::Tuple => {
+                let args = strukt
+                    .fields(ctx.db())
+                    .iter()
+                    .map(|_| placeholder_expr.clone())
+                    .collect::<Vec<_>>();
+
+                make::expr_call(make::expr_path(path_self), make::arg_list(args))
+            }
+            StructKind::Unit => make::expr_path(path_self),
+        }
+    } else {
+        placeholder_expr
+    };
+
+    let fn_body = make::block_expr(vec![], Some(tail_expr));
+    Some(fn_body)
+}
+
 fn get_fn_target_info(
     ctx: &AssistContext<'_>,
     target_module: Option<Module>,
@@ -443,8 +520,8 @@ fn assoc_fn_target_info(
     }
     let (impl_, file) = get_adt_source(ctx, &adt, fn_name)?;
     let target = get_method_target(ctx, &impl_, &adt)?;
-    let adt_name = if impl_.is_none() { Some(adt.name(ctx.sema.db)) } else { None };
+    let adt_info = AdtInfo::new(adt, impl_.is_some());
-    Some(TargetInfo::new(target_module, adt_name, target, file))
+    Some(TargetInfo::new(target_module, Some(adt_info), target, file))
 }
 
 #[derive(Clone)]
@@ -560,6 +637,17 @@ impl GeneratedFunctionTarget {
     }
 }
 
+struct AdtInfo {
+    adt: hir::Adt,
+    impl_exists: bool,
+}
+
+impl AdtInfo {
+    fn new(adt: Adt, impl_exists: bool) -> Self {
+        Self { adt, impl_exists }
+    }
+}
+
 /// Computes parameter list for the generated function.
 fn fn_args(
     ctx: &AssistContext<'_>,
@@ -2758,18 +2846,18 @@ fn main() {
             r"
 enum Foo {}
 fn main() {
-    Foo::new$0();
+    Foo::bar$0();
 }
 ",
             r"
 enum Foo {}
 impl Foo {
-    fn new() ${0:-> _} {
+    fn bar() ${0:-> _} {
         todo!()
     }
 }
 fn main() {
-    Foo::new();
+    Foo::bar();
 }
 ",
         )
@@ -2849,4 +2937,152 @@ fn main() {
 ",
         );
     }
+
+    #[test]
+    fn new_function_assume_self_type() {
+        check_assist(
+            generate_function,
+            r"
+pub struct Foo {
+    field_1: usize,
+    field_2: String,
+}
+
+fn main() {
+    let foo = Foo::new$0();
+}
+        ",
+            r"
+pub struct Foo {
+    field_1: usize,
+    field_2: String,
+}
+impl Foo {
+    fn new() -> Self {
+        ${0:Self { field_1: todo!(), field_2: todo!() }}
+    }
+}
+
+fn main() {
+    let foo = Foo::new();
+}
+        ",
+        )
+    }
+
+    #[test]
+    fn new_function_assume_self_type_for_tuple_struct() {
+        check_assist(
+            generate_function,
+            r"
+pub struct Foo (usize, String);
+
+fn main() {
+    let foo = Foo::new$0();
+}
+        ",
+            r"
+pub struct Foo (usize, String);
+impl Foo {
+    fn new() -> Self {
+        ${0:Self(todo!(), todo!())}
+    }
+}
+
+fn main() {
+    let foo = Foo::new();
+}
+        ",
+        )
+    }
+
+    #[test]
+    fn new_function_assume_self_type_for_unit_struct() {
+        check_assist(
+            generate_function,
+            r"
+pub struct Foo;
+
+fn main() {
+    let foo = Foo::new$0();
+}
+        ",
+            r"
+pub struct Foo;
+impl Foo {
+    fn new() -> Self {
+        ${0:Self}
+    }
+}
+
+fn main() {
+    let foo = Foo::new();
+}
+        ",
+        )
+    }
+
+    #[test]
+    fn new_function_assume_self_type_for_enum() {
+        check_assist(
+            generate_function,
+            r"
+pub enum Foo {}
+
+fn main() {
+    let foo = Foo::new$0();
+}
+        ",
+            r"
+pub enum Foo {}
+impl Foo {
+    fn new() -> Self {
+        ${0:todo!()}
+    }
+}
+
+fn main() {
+    let foo = Foo::new();
+}
+        ",
+        )
+    }
+
+    #[test]
+    fn new_function_assume_self_type_with_args() {
+        check_assist(
+            generate_function,
+            r#"
+pub struct Foo {
+    field_1: usize,
+    field_2: String,
+}
+
+struct Baz;
+fn baz() -> Baz { Baz }
+
+fn main() {
+    let foo = Foo::new$0(baz(), baz(), "foo", "bar");
+}
+        "#,
+            r#"
+pub struct Foo {
+    field_1: usize,
+    field_2: String,
+}
+impl Foo {
+    fn new(baz_1: Baz, baz_2: Baz, arg_1: &str, arg_2: &str) -> Self {
+        ${0:Self { field_1: todo!(), field_2: todo!() }}
+    }
+}
+
+struct Baz;
+fn baz() -> Baz { Baz }
+
+fn main() {
+    let foo = Foo::new(baz(), baz(), "foo", "bar");
+}
+        "#,
+        )
+    }
 }