Add ide-assist: extract_to_default_generic

Extracts selected type to default generic parameter.

```rust
struct Foo(u32, $0String$0);
```
->
```rust
struct Foo<T = String>(u32, T);
```

Author: A4-Tacks

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

@@ -0,0 +1,220 @@
+use ast::Name;
+use either::Either::{self, Left, Right};
+use ide_db::{source_change::SourceChangeBuilder, syntax_helpers::suggest_name::NameGenerator};
+use syntax::{
+    ast::{self, AstNode, HasGenericParams, HasName, make},
+    syntax_editor::{Position, SyntaxEditor},
+};
+
+use crate::{AssistContext, AssistId, Assists};
+
+// Assist: extract_to_default_generic
+//
+// Extracts selected type to default generic parameter.
+//
+// ```
+// struct Foo(u32, $0String$0);
+// ```
+// ->
+// ```
+// struct Foo<T = String>(u32, T);
+// ```
+pub(crate) fn extract_to_default_generic(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    if ctx.has_empty_selection() {
+        return None;
+    }
+
+    let ty: Either<ast::Type, ast::ConstArg> = ctx.find_node_at_range()?;
+    let adt: Either<ast::Adt, Either<ast::TypeAlias, ast::Fn>> =
+        ty.syntax().ancestors().find_map(AstNode::cast)?;
+
+    extract_to_default_generic_impl(acc, ctx, adt, ty)
+}
+
+fn extract_to_default_generic_impl(
+    acc: &mut Assists,
+    ctx: &AssistContext<'_>,
+    adt: impl HasName + HasGenericParams,
+    ty: Either<ast::Type, ast::ConstArg>,
+) -> Option<()> {
+    let name = adt.name()?;
+
+    let target = ty.syntax().text_range();
+    acc.add(
+        AssistId::refactor_extract("extract_to_default_generic"),
+        "Extract type as default generic parameter",
+        target,
+        |edit| {
+            let mut editor = edit.make_editor(adt.syntax());
+            let generic_list = get_or_create_generic_param_list(&name, &adt, &mut editor, edit);
+
+            let generic_name = generic_name(&generic_list, ty.is_right());
+
+            editor.replace(ty.syntax(), generic_name.syntax());
+
+            match ty {
+                Left(ty) => {
+                    let param = make::type_default_param(generic_name, None, ty).clone_for_update();
+                    generic_list.add_generic_param(param.into());
+                }
+                Right(n) => {
+                    let param = make::const_default_param(generic_name, const_ty(ctx, &n), n)
+                        .clone_for_update();
+                    generic_list.add_generic_param(param.into());
+
+                    if let Some(ast::GenericParam::ConstParam(param)) =
+                        generic_list.generic_params().last()
+                        && let Some(ast::Type::InferType(ty)) = param.ty()
+                        && let Some(cap) = ctx.config.snippet_cap
+                    {
+                        let annotation = edit.make_placeholder_snippet(cap);
+                        editor.add_annotation(ty.syntax(), annotation);
+                    }
+                }
+            }
+
+            edit.add_file_edits(ctx.vfs_file_id(), editor);
+        },
+    )
+}
+
+fn array_index_type(n: &ast::ConstArg) -> Option<ast::Type> {
+    let kind = n.syntax().parent()?.kind();
+
+    if ast::ArrayType::can_cast(kind) || ast::ArrayExpr::can_cast(kind) {
+        Some(make::ty("usize"))
+    } else {
+        None
+    }
+}
+
+fn generic_name(generic_list: &ast::GenericParamList, is_const_param: bool) -> Name {
+    let exist_names = generic_list
+        .generic_params()
+        .filter_map(|it| match it {
+            ast::GenericParam::ConstParam(const_param) => const_param.name(),
+            ast::GenericParam::TypeParam(type_param) => type_param.name(),
+            ast::GenericParam::LifetimeParam(_) => None,
+        })
+        .map(|name| name.to_string())
+        .collect::<Vec<_>>();
+
+    let mut name_gen = NameGenerator::new_with_names(exist_names.iter().map(|name| name.as_str()));
+
+    make::name(&if is_const_param {
+        name_gen.suggest_name("N")
+    } else {
+        name_gen.suggest_name("T")
+    })
+    .clone_for_update()
+}
+
+fn const_ty(ctx: &AssistContext<'_>, n: &ast::ConstArg) -> ast::Type {
+    if let Some(expr) = n.expr()
+        && let Some(ty_info) = ctx.sema.type_of_expr(&expr)
+        && let Some(builtin) = ty_info.adjusted().as_builtin()
+    {
+        make::ty(builtin.name().as_str())
+    } else if let Some(array_index_ty) = array_index_type(n) {
+        array_index_ty
+    } else {
+        make::ty_placeholder()
+    }
+}
+
+fn get_or_create_generic_param_list(
+    name: &ast::Name,
+    adt: &impl HasGenericParams,
+    editor: &mut SyntaxEditor,
+    edit: &mut SourceChangeBuilder,
+) -> ast::GenericParamList {
+    if let Some(list) = adt.generic_param_list() {
+        edit.make_mut(list)
+    } else {
+        let generic = make::generic_param_list([]).clone_for_update();
+        editor.insert(Position::after(name.syntax()), generic.syntax());
+        generic
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::tests::check_assist;
+
+    #[test]
+    fn test_extract_to_default_generic() {
+        check_assist(
+            extract_to_default_generic,
+            r#"type X = ($0i32$0, i64);"#,
+            r#"type X<T = i32> = (T, i64);"#,
+        );
+
+        check_assist(
+            extract_to_default_generic,
+            r#"type X<T> = ($0i32$0, T);"#,
+            r#"type X<T, T1 = i32> = (T1, T);"#,
+        );
+    }
+
+    #[test]
+    fn test_extract_to_default_generic_on_adt() {
+        check_assist(
+            extract_to_default_generic,
+            r#"struct Foo($0i32$0);"#,
+            r#"struct Foo<T = i32>(T);"#,
+        );
+
+        check_assist(
+            extract_to_default_generic,
+            r#"struct Foo<T>(T, $0i32$0);"#,
+            r#"struct Foo<T, T1 = i32>(T, T1);"#,
+        );
+
+        check_assist(
+            extract_to_default_generic,
+            r#"enum Foo { A($0i32$0), B, C(i64) };"#,
+            r#"enum Foo<T = i32> { A(T), B, C(i64) };"#,
+        );
+    }
+
+    #[test]
+    fn test_extract_to_default_generic_on_fn() {
+        check_assist(
+            extract_to_default_generic,
+            r#"fn foo(x: $0i32$0) {}"#,
+            r#"fn foo<T = i32>(x: T) {}"#,
+        );
+    }
+
+    #[test]
+    fn test_extract_to_default_generic_const() {
+        check_assist(
+            extract_to_default_generic,
+            r#"type A = [i32; $08$0];"#,
+            r#"type A<const N: usize = 8> = [i32; N];"#,
+        );
+
+        check_assist(
+            extract_to_default_generic,
+            r#"type A<T> = [T; $08$0];"#,
+            r#"type A<T, const N: usize = 8> = [T; N];"#,
+        );
+    }
+
+    #[test]
+    fn test_extract_to_default_generic_const_non_array() {
+        check_assist(
+            extract_to_default_generic,
+            r#"
+                struct Foo<const N: usize>([(); N]);
+                type A = Foo<$08$0>;
+            "#,
+            r#"
+                struct Foo<const N: usize>([(); N]);
+                type A<const N: ${0:_} = 8> = Foo<N>;
+            "#,
+        );
+    }
+}

crates/ide-assists/src/lib.rs

@@ -146,6 +146,7 @@ mod handlers {
     mod extract_function;
     mod extract_module;
     mod extract_struct_from_enum_variant;
+    mod extract_to_default_generic;
     mod extract_type_alias;
     mod extract_variable;
     mod fix_visibility;
@@ -281,6 +282,7 @@ mod handlers {
             extract_expressions_from_format_string::extract_expressions_from_format_string,
             extract_struct_from_enum_variant::extract_struct_from_enum_variant,
             extract_type_alias::extract_type_alias,
+            extract_to_default_generic::extract_to_default_generic,
             fix_visibility::fix_visibility,
             flip_binexpr::flip_binexpr,
             flip_comma::flip_comma,

crates/ide-assists/src/tests/generated.rs

@@ -1187,6 +1187,19 @@ enum A { One(One) }
     )
 }
 
+#[test]
+fn doctest_extract_to_default_generic() {
+    check_doc_test(
+        "extract_to_default_generic",
+        r#####"
+struct Foo(u32, $0String$0);
+"#####,
+        r#####"
+struct Foo<T = String>(u32, T);
+"#####,
+    )
+}
+
 #[test]
 fn doctest_extract_type_alias() {
     check_doc_test(

crates/syntax/src/ast/make.rs

@@ -1069,10 +1069,27 @@ pub fn type_param(name: ast::Name, bounds: Option<ast::TypeBoundList>) -> ast::T
     ast_from_text(&format!("fn f<{name}{bounds}>() {{ }}"))
 }
 
+pub fn type_default_param(
+    name: ast::Name,
+    bounds: Option<ast::TypeBoundList>,
+    default: ast::Type,
+) -> ast::TypeParam {
+    let bounds = bounds.map_or_else(String::new, |it| format!(": {it}"));
+    ast_from_text(&format!("fn f<{name}{bounds} = {default}>() {{ }}"))
+}
+
 pub fn const_param(name: ast::Name, ty: ast::Type) -> ast::ConstParam {
     ast_from_text(&format!("fn f<const {name}: {ty}>() {{ }}"))
 }
 
+pub fn const_default_param(
+    name: ast::Name,
+    ty: ast::Type,
+    default: ast::ConstArg,
+) -> ast::ConstParam {
+    ast_from_text(&format!("fn f<const {name}: {ty} = {default}>() {{ }}"))
+}
+
 pub fn lifetime_param(lifetime: ast::Lifetime) -> ast::LifetimeParam {
     ast_from_text(&format!("fn f<{lifetime}>() {{ }}"))
 }

crates/syntax/src/ast/traits.rs

@@ -161,3 +161,4 @@ impl Iterator for AttrDocCommentIter {
 }
 
 impl<A: HasName, B: HasName> HasName for Either<A, B> {}
+impl<A: HasGenericParams, B: HasGenericParams> HasGenericParams for Either<A, B> {}