Auto merge of #15152 - alibektas:14987, r=Veykril,lowr

assist : generate trait from impl

fixes #14987 . As the name suggests this assist is used to generate traits from inherent impls while adapting the original impl to fit to the newly generated trait. I made some decisions regarding when the assist should be applicable. These are surely open to discussion. I looking forward to any feedback.

![generate_trait_from_impl_v1](https://github.com/rust-lang/rust-analyzer/assets/20956650/05d4dda5-604a-4108-8b82-9b60bd45894a)

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

@@ -0,0 +1,429 @@
+use crate::assist_context::{AssistContext, Assists};
+use ide_db::assists::AssistId;
+use syntax::{
+    ast::{self, edit::IndentLevel, make, HasGenericParams, HasVisibility},
+    ted, AstNode, SyntaxKind,
+};
+
+// NOTES :
+// We generate erroneous code if a function is declared const (E0379)
+// This is left to the user to correct as our only option is to remove the
+// function completely which we should not be doing.
+
+// Assist: generate_trait_from_impl
+//
+// Generate trait for an already defined inherent impl and convert impl to a trait impl.
+//
+// ```
+// struct Foo<const N: usize>([i32; N]);
+//
+// macro_rules! const_maker {
+//     ($t:ty, $v:tt) => {
+//         const CONST: $t = $v;
+//     };
+// }
+//
+// impl<const N: usize> Fo$0o<N> {
+//     // Used as an associated constant.
+//     const CONST_ASSOC: usize = N * 4;
+//
+//     fn create() -> Option<()> {
+//         Some(())
+//     }
+//
+//     const_maker! {i32, 7}
+// }
+// ```
+// ->
+// ```
+// struct Foo<const N: usize>([i32; N]);
+//
+// macro_rules! const_maker {
+//     ($t:ty, $v:tt) => {
+//         const CONST: $t = $v;
+//     };
+// }
+//
+// trait ${0:TraitName}<const N: usize> {
+//     // Used as an associated constant.
+//     const CONST_ASSOC: usize = N * 4;
+//
+//     fn create() -> Option<()>;
+//
+//     const_maker! {i32, 7}
+// }
+//
+// impl<const N: usize> ${0:TraitName}<N> for Foo<N> {
+//     // Used as an associated constant.
+//     const CONST_ASSOC: usize = N * 4;
+//
+//     fn create() -> Option<()> {
+//         Some(())
+//     }
+//
+//     const_maker! {i32, 7}
+// }
+// ```
+pub(crate) fn generate_trait_from_impl(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    // Get AST Node
+    let impl_ast = ctx.find_node_at_offset::<ast::Impl>()?;
+
+    // Check if cursor is to the left of assoc item list's L_CURLY.
+    // if no L_CURLY then return.
+    let l_curly = impl_ast.assoc_item_list()?.l_curly_token()?;
+
+    let cursor_offset = ctx.offset();
+    let l_curly_offset = l_curly.text_range();
+    if cursor_offset >= l_curly_offset.start() {
+        return None;
+    }
+
+    // If impl is not inherent then we don't really need to go any further.
+    if impl_ast.for_token().is_some() {
+        return None;
+    }
+
+    let assoc_items = impl_ast.assoc_item_list()?;
+    let first_element = assoc_items.assoc_items().next();
+    if first_element.is_none() {
+        // No reason for an assist.
+        return None;
+    }
+
+    let impl_name = impl_ast.self_ty()?;
+
+    acc.add(
+        AssistId("generate_trait_from_impl", ide_db::assists::AssistKind::Generate),
+        "Generate trait from impl",
+        impl_ast.syntax().text_range(),
+        |builder| {
+            let trait_items = assoc_items.clone_for_update();
+            let impl_items = assoc_items.clone_for_update();
+
+            trait_items.assoc_items().for_each(|item| {
+                strip_body(&item);
+                remove_items_visibility(&item);
+            });
+
+            impl_items.assoc_items().for_each(|item| {
+                remove_items_visibility(&item);
+            });
+
+            let trait_ast = make::trait_(
+                false,
+                "NewTrait",
+                impl_ast.generic_param_list(),
+                impl_ast.where_clause(),
+                trait_items,
+            );
+
+            // Change `impl Foo` to `impl NewTrait for Foo`
+            let arg_list = if let Some(genpars) = impl_ast.generic_param_list() {
+                genpars.to_generic_args().to_string()
+            } else {
+                "".to_string()
+            };
+
+            if let Some(snippet_cap) = ctx.config.snippet_cap {
+                builder.replace_snippet(
+                    snippet_cap,
+                    impl_name.syntax().text_range(),
+                    format!("${{0:TraitName}}{} for {}", arg_list, impl_name.to_string()),
+                );
+
+                // Insert trait before TraitImpl
+                builder.insert_snippet(
+                    snippet_cap,
+                    impl_ast.syntax().text_range().start(),
+                    format!(
+                        "{}\n\n{}",
+                        trait_ast.to_string().replace("NewTrait", "${0:TraitName}"),
+                        IndentLevel::from_node(impl_ast.syntax())
+                    ),
+                );
+            } else {
+                builder.replace(
+                    impl_name.syntax().text_range(),
+                    format!("NewTrait{} for {}", arg_list, impl_name.to_string()),
+                );
+
+                // Insert trait before TraitImpl
+                builder.insert(
+                    impl_ast.syntax().text_range().start(),
+                    format!(
+                        "{}\n\n{}",
+                        trait_ast.to_string(),
+                        IndentLevel::from_node(impl_ast.syntax())
+                    ),
+                );
+            }
+
+            builder.replace(assoc_items.syntax().text_range(), impl_items.to_string());
+        },
+    );
+
+    Some(())
+}
+
+/// `E0449` Trait items always share the visibility of their trait
+fn remove_items_visibility(item: &ast::AssocItem) {
+    match item {
+        ast::AssocItem::Const(c) => {
+            if let Some(vis) = c.visibility() {
+                ted::remove(vis.syntax());
+            }
+        }
+        ast::AssocItem::Fn(f) => {
+            if let Some(vis) = f.visibility() {
+                ted::remove(vis.syntax());
+            }
+        }
+        ast::AssocItem::TypeAlias(t) => {
+            if let Some(vis) = t.visibility() {
+                ted::remove(vis.syntax());
+            }
+        }
+        _ => (),
+    }
+}
+
+fn strip_body(item: &ast::AssocItem) {
+    match item {
+        ast::AssocItem::Fn(f) => {
+            if let Some(body) = f.body() {
+                // In constrast to function bodies, we want to see no ws before a semicolon.
+                // So let's remove them if we see any.
+                if let Some(prev) = body.syntax().prev_sibling_or_token() {
+                    if prev.kind() == SyntaxKind::WHITESPACE {
+                        ted::remove(prev);
+                    }
+                }
+
+                ted::replace(body.syntax(), make::tokens::semicolon());
+            }
+        }
+        _ => (),
+    };
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::tests::{check_assist, check_assist_no_snippet_cap, check_assist_not_applicable};
+
+    #[test]
+    fn test_trigger_when_cursor_on_header() {
+        check_assist_not_applicable(
+            generate_trait_from_impl,
+            r#"
+struct Foo(f64);
+
+impl Foo { $0
+    fn add(&mut self, x: f64) {
+        self.0 += x;
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn test_assoc_item_fn() {
+        check_assist_no_snippet_cap(
+            generate_trait_from_impl,
+            r#"
+struct Foo(f64);
+
+impl F$0oo {
+    fn add(&mut self, x: f64) {
+        self.0 += x;
+    }
+}"#,
+            r#"
+struct Foo(f64);
+
+trait NewTrait {
+    fn add(&mut self, x: f64);
+}
+
+impl NewTrait for Foo {
+    fn add(&mut self, x: f64) {
+        self.0 += x;
+    }
+}"#,
+        )
+    }
+
+    #[test]
+    fn test_assoc_item_macro() {
+        check_assist_no_snippet_cap(
+            generate_trait_from_impl,
+            r#"
+struct Foo;
+
+macro_rules! const_maker {
+    ($t:ty, $v:tt) => {
+        const CONST: $t = $v;
+    };
+}
+
+impl F$0oo {
+    const_maker! {i32, 7}
+}"#,
+            r#"
+struct Foo;
+
+macro_rules! const_maker {
+    ($t:ty, $v:tt) => {
+        const CONST: $t = $v;
+    };
+}
+
+trait NewTrait {
+    const_maker! {i32, 7}
+}
+
+impl NewTrait for Foo {
+    const_maker! {i32, 7}
+}"#,
+        )
+    }
+
+    #[test]
+    fn test_assoc_item_const() {
+        check_assist_no_snippet_cap(
+            generate_trait_from_impl,
+            r#"
+struct Foo;
+
+impl F$0oo {
+    const ABC: i32 = 3;
+}"#,
+            r#"
+struct Foo;
+
+trait NewTrait {
+    const ABC: i32 = 3;
+}
+
+impl NewTrait for Foo {
+    const ABC: i32 = 3;
+}"#,
+        )
+    }
+
+    #[test]
+    fn test_impl_with_generics() {
+        check_assist_no_snippet_cap(
+            generate_trait_from_impl,
+            r#"
+struct Foo<const N: usize>([i32; N]);
+
+impl<const N: usize> F$0oo<N> {
+    // Used as an associated constant.
+    const CONST: usize = N * 4;
+}
+            "#,
+            r#"
+struct Foo<const N: usize>([i32; N]);
+
+trait NewTrait<const N: usize> {
+    // Used as an associated constant.
+    const CONST: usize = N * 4;
+}
+
+impl<const N: usize> NewTrait<N> for Foo<N> {
+    // Used as an associated constant.
+    const CONST: usize = N * 4;
+}
+            "#,
+        )
+    }
+
+    #[test]
+    fn test_trait_items_should_not_have_vis() {
+        check_assist_no_snippet_cap(
+            generate_trait_from_impl,
+            r#"
+struct Foo;
+
+impl F$0oo {
+    pub fn a_func() -> Option<()> {
+        Some(())
+    }
+}"#,
+            r#"
+struct Foo;
+
+trait NewTrait {
+     fn a_func() -> Option<()>;
+}
+
+impl NewTrait for Foo {
+     fn a_func() -> Option<()> {
+        Some(())
+    }
+}"#,
+        )
+    }
+
+    #[test]
+    fn test_empty_inherent_impl() {
+        check_assist_not_applicable(
+            generate_trait_from_impl,
+            r#"
+impl Emp$0tyImpl{}
+"#,
+        )
+    }
+
+    #[test]
+    fn test_not_top_level_impl() {
+        check_assist_no_snippet_cap(
+            generate_trait_from_impl,
+            r#"
+mod a {
+    impl S$0 {
+        fn foo() {}
+    }
+}"#,
+            r#"
+mod a {
+    trait NewTrait {
+        fn foo();
+    }
+
+    impl NewTrait for S {
+        fn foo() {}
+    }
+}"#,
+        )
+    }
+
+    #[test]
+    fn test_snippet_cap_is_some() {
+        check_assist(
+            generate_trait_from_impl,
+            r#"
+struct Foo<const N: usize>([i32; N]);
+
+impl<const N: usize> F$0oo<N> {
+    // Used as an associated constant.
+    const CONST: usize = N * 4;
+}
+            "#,
+            r#"
+struct Foo<const N: usize>([i32; N]);
+
+trait ${0:TraitName}<const N: usize> {
+    // Used as an associated constant.
+    const CONST: usize = N * 4;
+}
+
+impl<const N: usize> ${0:TraitName}<N> for Foo<N> {
+    // Used as an associated constant.
+    const CONST: usize = N * 4;
+}
+            "#,
+        )
+    }
+}

crates/ide-assists/src/lib.rs

@@ -160,6 +160,7 @@ mod handlers {
     mod generate_new;
     mod generate_setter;
     mod generate_delegate_methods;
+    mod generate_trait_from_impl;
     mod add_return_type;
     mod inline_call;
     mod inline_const_as_literal;
@@ -266,6 +267,7 @@ mod handlers {
             generate_impl::generate_trait_impl,
             generate_is_empty_from_len::generate_is_empty_from_len,
             generate_new::generate_new,
+            generate_trait_from_impl::generate_trait_from_impl,
             inline_call::inline_call,
             inline_call::inline_into_callers,
             inline_const_as_literal::inline_const_as_literal,

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

@@ -1500,6 +1500,62 @@ impl Person {
     )
 }
 
+#[test]
+fn doctest_generate_trait_from_impl() {
+    check_doc_test(
+        "generate_trait_from_impl",
+        r#####"
+struct Foo<const N: usize>([i32; N]);
+
+macro_rules! const_maker {
+    ($t:ty, $v:tt) => {
+        const CONST: $t = $v;
+    };
+}
+
+impl<const N: usize> Fo$0o<N> {
+    // Used as an associated constant.
+    const CONST_ASSOC: usize = N * 4;
+
+    fn create() -> Option<()> {
+        Some(())
+    }
+
+    const_maker! {i32, 7}
+}
+"#####,
+        r#####"
+struct Foo<const N: usize>([i32; N]);
+
+macro_rules! const_maker {
+    ($t:ty, $v:tt) => {
+        const CONST: $t = $v;
+    };
+}
+
+trait ${0:TraitName}<const N: usize> {
+    // Used as an associated constant.
+    const CONST_ASSOC: usize = N * 4;
+
+    fn create() -> Option<()>;
+
+    const_maker! {i32, 7}
+}
+
+impl<const N: usize> ${0:TraitName}<N> for Foo<N> {
+    // Used as an associated constant.
+    const CONST_ASSOC: usize = N * 4;
+
+    fn create() -> Option<()> {
+        Some(())
+    }
+
+    const_maker! {i32, 7}
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_generate_trait_impl() {
     check_doc_test(

crates/syntax/src/ast/make.rs

@@ -863,6 +863,36 @@ pub fn param_list(
     ast_from_text(&list)
 }
 
+pub fn trait_(
+    is_unsafe: bool,
+    ident: &str,
+    gen_params: Option<ast::GenericParamList>,
+    where_clause: Option<ast::WhereClause>,
+    assoc_items: ast::AssocItemList,
+) -> ast::Trait {
+    let mut text = String::new();
+
+    if is_unsafe {
+        format_to!(text, "unsafe ");
+    }
+
+    format_to!(text, "trait {ident}");
+
+    if let Some(gen_params) = gen_params {
+        format_to!(text, "{} ", gen_params.to_string());
+    } else {
+        text.push(' ');
+    }
+
+    if let Some(where_clause) = where_clause {
+        format_to!(text, "{} ", where_clause.to_string());
+    }
+
+    format_to!(text, "{}", assoc_items.to_string());
+
+    ast_from_text(&text)
+}
+
 pub fn type_bound(bound: &str) -> ast::TypeBound {
     ast_from_text(&format!("fn f<T: {bound}>() {{ }}"))
 }
@@ -1037,6 +1067,17 @@ pub mod tokens {
         )
     });
 
+    pub fn semicolon() -> SyntaxToken {
+        SOURCE_FILE
+            .tree()
+            .syntax()
+            .clone_for_update()
+            .descendants_with_tokens()
+            .filter_map(|it| it.into_token())
+            .find(|it| it.kind() == SEMICOLON)
+            .unwrap()
+    }
+
     pub fn single_space() -> SyntaxToken {
         SOURCE_FILE
             .tree()