1 files changed, 62 insertions, 1 deletions

diff --git a/crates/ra_assists/src/add_missing_impl_members.rs b/crates/ra_assists/src/add_missing_impl_members.rs
index a0b656f8f..120109d4b 100644
--- a/crates/ra_assists/src/add_missing_impl_members.rs
+++ b/crates/ra_assists/src/add_missing_impl_members.rs
@@ -1,7 +1,68 @@
+use std::collections::HashSet;
+
 use crate::assist_ctx::{Assist, AssistCtx};
+
+use hir::Resolver;
 use hir::db::HirDatabase;
+use ra_syntax::{SmolStr, SyntaxKind, SyntaxNode, TreeArc};
+use ra_syntax::ast::{self, AstNode, FnDef, ImplItem, ImplItemKind, NameOwner};
+use ra_db::FilePosition;
+
+/// Given an `ast::ImplBlock`, resolves the target trait (the one being
+/// implemented) to a `ast::TraitDef`.
+pub(crate) fn resolve_target_trait_def(
+    db: &impl HirDatabase,
+    resolver: &Resolver,
+    impl_block: &ast::ImplBlock,
+) -> Option<TreeArc<ast::TraitDef>> {
+    let ast_path = impl_block.target_trait().map(AstNode::syntax).and_then(ast::PathType::cast)?;
+    let hir_path = ast_path.path().and_then(hir::Path::from_ast)?;
+
+    match resolver.resolve_path(db, &hir_path).take_types() {
+        Some(hir::Resolution::Def(hir::ModuleDef::Trait(def))) => Some(def.source(db).1),
+        _ => None,
+    }
+}
+
+pub(crate) fn add_missing_impl_members(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    use SyntaxKind::{IMPL_BLOCK, ITEM_LIST, WHITESPACE};
+
+    let node = ctx.covering_node();
+    let kinds = node.ancestors().take(3).map(SyntaxNode::kind);
+    // Only suggest this in `impl Foo for S { [Item...] <|> }` cursor position
+    if !Iterator::eq(kinds, [WHITESPACE, ITEM_LIST, IMPL_BLOCK].iter().cloned()) {
+        return None;
+    }
+
+    let impl_node = node.ancestors().find_map(ast::ImplBlock::cast)?;
+
+    let trait_def = {
+        let db = ctx.db;
+        // TODO: Can we get the position of cursor itself rather than supplied range?
+        let range = ctx.frange;
+        let position = FilePosition { file_id: range.file_id, offset: range.range.start() };
+        let resolver = hir::source_binder::resolver_for_position(db, position);
+
+        resolve_target_trait_def(db, &resolver, impl_node)?
+    };
+
+    let fn_def_opt = |kind| if let ImplItemKind::FnDef(def) = kind { Some(def) } else { None };
+    let def_name = |&def| -> Option<&SmolStr> { FnDef::name(def).map(ast::Name::text) };
+
+    let trait_items = trait_def.syntax().descendants().find_map(ast::ItemList::cast)?.impl_items();
+    let impl_items = impl_node.item_list()?.impl_items();
+
+    let trait_fns = trait_items.map(ImplItem::kind).filter_map(fn_def_opt).collect::<Vec<_>>();
+    let impl_fns = impl_items.map(ImplItem::kind).filter_map(fn_def_opt).collect::<Vec<_>>();
+
+    let trait_fn_names = trait_fns.iter().filter_map(def_name).collect::<HashSet<_>>();
+    let impl_fn_names = impl_fns.iter().filter_map(def_name).collect::<HashSet<_>>();
+
+    let missing_fn_names = trait_fn_names.difference(&impl_fn_names).collect::<HashSet<_>>();
+    let missing_fns = trait_fns
+        .iter()
+        .filter(|&t| def_name(t).map(|n| missing_fn_names.contains(&n)).unwrap_or(false));
 
-pub(crate) fn add_missing_impl_members(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
     unimplemented!()
 }