3 files changed, 238 insertions, 0 deletions

diff --git a/crates/assists/src/handlers/reorder_impl.rs b/crates/assists/src/handlers/reorder_impl.rs
new file mode 100644
index 000000000..309f291c8
--- /dev/null
+++ b/crates/assists/src/handlers/reorder_impl.rs
@@ -0,0 +1,201 @@
+use itertools::Itertools;
+use rustc_hash::FxHashMap;
+
+use hir::{PathResolution, Semantics};
+use ide_db::RootDatabase;
+use syntax::{
+    algo,
+    ast::{self, NameOwner},
+    AstNode,
+};
+use test_utils::mark;
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: reorder_impl
+//
+// Reorder the methods of an `impl Trait`. The methods will be ordered
+// in the same order as in the trait definition.
+//
+// ```
+// trait Foo {
+//     fn a() {}
+//     fn b() {}
+//     fn c() {}
+// }
+//
+// struct Bar;
+// $0impl Foo for Bar {
+//     fn b() {}
+//     fn c() {}
+//     fn a() {}
+// }
+// ```
+// ->
+// ```
+// trait Foo {
+//     fn a() {}
+//     fn b() {}
+//     fn c() {}
+// }
+//
+// struct Bar;
+// impl Foo for Bar {
+//     fn a() {}
+//     fn b() {}
+//     fn c() {}
+// }
+// ```
+//
+pub(crate) fn reorder_impl(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let impl_ast = ctx.find_node_at_offset::<ast::Impl>()?;
+    let items = impl_ast.assoc_item_list()?;
+    let methods = get_methods(&items);
+
+    let path = impl_ast
+        .trait_()
+        .and_then(|t| match t {
+            ast::Type::PathType(path) => Some(path),
+            _ => None,
+        })?
+        .path()?;
+
+    let ranks = compute_method_ranks(&path, ctx)?;
+    let sorted: Vec<_> = methods
+        .iter()
+        .cloned()
+        .sorted_by_key(|f| {
+            f.name().and_then(|n| ranks.get(&n.to_string()).copied()).unwrap_or(usize::max_value())
+        })
+        .collect();
+
+    // Don't edit already sorted methods:
+    if methods == sorted {
+        mark::hit!(not_applicable_if_sorted);
+        return None;
+    }
+
+    let target = items.syntax().text_range();
+    acc.add(AssistId("reorder_impl", AssistKind::RefactorRewrite), "Sort methods", target, |edit| {
+        let mut rewriter = algo::SyntaxRewriter::default();
+        for (old, new) in methods.iter().zip(&sorted) {
+            rewriter.replace(old.syntax(), new.syntax());
+        }
+        edit.rewrite(rewriter);
+    })
+}
+
+fn compute_method_ranks(path: &ast::Path, ctx: &AssistContext) -> Option<FxHashMap<String, usize>> {
+    let td = trait_definition(path, &ctx.sema)?;
+
+    Some(
+        td.items(ctx.db())
+            .iter()
+            .flat_map(|i| match i {
+                hir::AssocItem::Function(f) => Some(f),
+                _ => None,
+            })
+            .enumerate()
+            .map(|(idx, func)| ((func.name(ctx.db()).to_string(), idx)))
+            .collect(),
+    )
+}
+
+fn trait_definition(path: &ast::Path, sema: &Semantics<RootDatabase>) -> Option<hir::Trait> {
+    match sema.resolve_path(path)? {
+        PathResolution::Def(hir::ModuleDef::Trait(trait_)) => Some(trait_),
+        _ => None,
+    }
+}
+
+fn get_methods(items: &ast::AssocItemList) -> Vec<ast::Fn> {
+    items
+        .assoc_items()
+        .flat_map(|i| match i {
+            ast::AssocItem::Fn(f) => Some(f),
+            _ => None,
+        })
+        .filter(|f| f.name().is_some())
+        .collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use test_utils::mark;
+
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn not_applicable_if_sorted() {
+        mark::check!(not_applicable_if_sorted);
+        check_assist_not_applicable(
+            reorder_impl,
+            r#"
+trait Bar {
+    fn a() {}
+    fn z() {}
+    fn b() {}
+}
+struct Foo;
+$0impl Bar for Foo {
+    fn a() {}
+    fn z() {}
+    fn b() {}
+}
+        "#,
+        )
+    }
+
+    #[test]
+    fn not_applicable_if_empty() {
+        check_assist_not_applicable(
+            reorder_impl,
+            r#"
+trait Bar {};
+struct Foo;
+$0impl Bar for Foo {}
+        "#,
+        )
+    }
+
+    #[test]
+    fn reorder_impl_trait_methods() {
+        check_assist(
+            reorder_impl,
+            r#"
+trait Bar {
+    fn a() {}
+    fn c() {}
+    fn b() {}
+    fn d() {}
+}
+
+struct Foo;
+$0impl Bar for Foo {
+    fn d() {}
+    fn b() {}
+    fn c() {}
+    fn a() {}
+}
+        "#,
+            r#"
+trait Bar {
+    fn a() {}
+    fn c() {}
+    fn b() {}
+    fn d() {}
+}
+
+struct Foo;
+impl Bar for Foo {
+    fn a() {}
+    fn c() {}
+    fn b() {}
+    fn d() {}
+}
+        "#,
+        )
+    }
+}
diff --git a/crates/assists/src/lib.rs b/crates/assists/src/lib.rs
index 90009c55a..1080294ab 100644
--- a/crates/assists/src/lib.rs
+++ b/crates/assists/src/lib.rs
@@ -146,6 +146,7 @@ mod handlers {
     mod remove_mut;
     mod remove_unused_param;
     mod reorder_fields;
+    mod reorder_impl;
     mod replace_derive_with_manual_impl;
     mod replace_if_let_with_match;
     mod replace_impl_trait_with_generic;
@@ -202,6 +203,7 @@ mod handlers {
             remove_mut::remove_mut,
             remove_unused_param::remove_unused_param,
             reorder_fields::reorder_fields,
+            reorder_impl::reorder_impl,
             replace_derive_with_manual_impl::replace_derive_with_manual_impl,
             replace_if_let_with_match::replace_if_let_with_match,
             replace_if_let_with_match::replace_match_with_if_let,
diff --git a/crates/assists/src/tests/generated.rs b/crates/assists/src/tests/generated.rs
index e28837b53..217f577eb 100644
--- a/crates/assists/src/tests/generated.rs
+++ b/crates/assists/src/tests/generated.rs
@@ -896,6 +896,41 @@ const test: Foo = Foo {foo: 1, bar: 0}
 }
 
 #[test]
+fn doctest_reorder_impl() {
+    check_doc_test(
+        "reorder_impl",
+        r#####"
+trait Foo {
+    fn a() {}
+    fn b() {}
+    fn c() {}
+}
+
+struct Bar;
+$0impl Foo for Bar {
+    fn b() {}
+    fn c() {}
+    fn a() {}
+}
+"#####,
+        r#####"
+trait Foo {
+    fn a() {}
+    fn b() {}
+    fn c() {}
+}
+
+struct Bar;
+impl Foo for Bar {
+    fn a() {}
+    fn b() {}
+    fn c() {}
+}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_replace_derive_with_manual_impl() {
     check_doc_test(
         "replace_derive_with_manual_impl",