Add ide-assist: convert_attr_cfg_to_if

Convert `#[cfg(...)] {}` to `if cfg!(...) {}`.

```rust
fn foo() {
    $0#[cfg(feature = "foo")]
    {
        let _x = 2;
    }
}
```
->
```rust
fn foo() {
    if cfg!(feature = "foo") {
        let _x = 2;
    }
}
```

Author: A4-Tacks

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

@@ -0,0 +1,134 @@
+use ide_db::assists::AssistId;
+use syntax::ast::edit_in_place::Indent;
+use syntax::syntax_editor::SyntaxEditor;
+use syntax::{
+    AstNode, SyntaxKind,
+    ast::{self, make},
+};
+
+use crate::assist_context::{AssistContext, Assists};
+
+// Assist: convert_attr_cfg_to_if
+//
+// Convert `#[cfg(...)] {}` to `if cfg!(...) {}`.
+//
+// ```
+// fn foo() {
+//     $0#[cfg(feature = "foo")]
+//     {
+//         let _x = 2;
+//     }
+// }
+// ```
+// ->
+// ```
+// fn foo() {
+//     if cfg!(feature = "foo") {
+//         let _x = 2;
+//     }
+// }
+// ```
+pub(crate) fn convert_attr_cfg_to_if(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    let cfg = ctx.find_node_at_offset::<ast::Attr>()?;
+    let stmt_list = find_stmt_list(&cfg)?;
+
+    if cfg.path()?.as_single_name_ref()?.text() != "cfg" {
+        return None;
+    }
+
+    let cfg_expr = make::expr_macro(cfg.path()?, cfg.token_tree()?).into();
+
+    acc.add(
+        AssistId::refactor_rewrite("convert_attr_cfg_to_if"),
+        "Convert `#[cfg()]` to `if cfg!()`",
+        cfg.syntax().text_range(),
+        |builder| {
+            let mut edit = builder.make_editor(stmt_list.syntax());
+
+            remove_cfg(&cfg, &mut edit);
+
+            let block = make::block_expr(stmt_list.statements(), None);
+            let if_expr = make::expr_if(cfg_expr, block, None).clone_for_update();
+            if_expr.indent(stmt_list.indent_level());
+            edit.replace(stmt_list.syntax(), if_expr.syntax());
+
+            builder.add_file_edits(ctx.vfs_file_id(), edit);
+        },
+    )
+}
+
+fn find_stmt_list(attr: &ast::Attr) -> Option<ast::StmtList> {
+    let mut node = attr.syntax().clone();
+
+    while node.kind().is_trivia() || node.kind() == SyntaxKind::ATTR {
+        node = node.next_sibling()?;
+    }
+
+    AstNode::cast(node)
+}
+
+fn remove_cfg(cfg: &ast::Attr, edit: &mut SyntaxEditor) {
+    if let Some(next) = cfg.syntax().last_token().and_then(|it| it.next_token())
+        && next.kind() == SyntaxKind::WHITESPACE
+    {
+        edit.delete(next);
+    }
+    edit.delete(cfg.syntax());
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::check_assist;
+
+    use super::*;
+
+    #[test]
+    fn test_stmt_list() {
+        check_assist(
+            convert_attr_cfg_to_if,
+            r#"
+fn foo() {
+    $0#[cfg(feature = "foo")]
+    {
+        let x = 2;
+        let _ = x+1;
+    }
+}
+            "#,
+            r#"
+fn foo() {
+    if cfg!(feature = "foo") {
+        let x = 2;
+        let _ = x+1;
+    }
+}
+            "#,
+        );
+    }
+
+    #[test]
+    fn test_other_attr() {
+        check_assist(
+            convert_attr_cfg_to_if,
+            r#"
+fn foo() {
+    $0#[cfg(feature = "foo")]
+    #[allow(unused)]
+    {
+        let x = 2;
+        let _ = x+1;
+    }
+}
+            "#,
+            r#"
+fn foo() {
+    #[allow(unused)]
+    if cfg!(feature = "foo") {
+        let x = 2;
+        let _ = x+1;
+    }
+}
+            "#,
+        );
+    }
+}

crates/ide-assists/src/lib.rs

@@ -117,6 +117,7 @@ mod handlers {
     mod auto_import;
     mod bind_unused_param;
     mod change_visibility;
+    mod convert_attr_cfg_to_if;
     mod convert_bool_then;
     mod convert_bool_to_enum;
     mod convert_closure_to_fn;
@@ -254,6 +255,7 @@ mod handlers {
             convert_bool_then::convert_if_to_bool_then,
             convert_bool_to_enum::convert_bool_to_enum,
             convert_closure_to_fn::convert_closure_to_fn,
+            convert_attr_cfg_to_if::convert_attr_cfg_to_if,
             convert_comment_block::convert_comment_block,
             convert_comment_from_or_to_doc::convert_comment_from_or_to_doc,
             convert_for_to_while_let::convert_for_loop_to_while_let,

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

@@ -355,6 +355,28 @@ fn doctest_comment_to_doc() {
     )
 }
 
+#[test]
+fn doctest_convert_attr_cfg_to_if() {
+    check_doc_test(
+        "convert_attr_cfg_to_if",
+        r#####"
+fn foo() {
+    $0#[cfg(feature = "foo")]
+    {
+        let _x = 2;
+    }
+}
+"#####,
+        r#####"
+fn foo() {
+    if cfg!(feature = "foo") {
+        let _x = 2;
+    }
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_convert_bool_then_to_if() {
     check_doc_test(