diff --git a/crates/ra_assists/src/add_missing_impl_members.rs b/crates/ra_assists/src/add_missing_impl_members.rs
new file mode 100644
index 000000000000..4435c4b5d4df
--- /dev/null
+++ b/crates/ra_assists/src/add_missing_impl_members.rs
@@ -0,0 +1,283 @@
+use crate::{Assist, AssistId, AssistCtx};
+
+use hir::Resolver;
+use hir::db::HirDatabase;
+use ra_syntax::{SmolStr, SyntaxKind, TextRange, TextUnit, TreeArc};
+use ra_syntax::ast::{self, AstNode, FnDef, ImplItem, ImplItemKind, NameOwner};
+use ra_db::FilePosition;
+use ra_fmt::{leading_indent, reindent};
+
+use itertools::Itertools;
+
+pub(crate) fn add_missing_impl_members(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let impl_node = ctx.node_at_offset::<ast::ImplBlock>()?;
+    let impl_item_list = impl_node.item_list()?;
+
+    let trait_def = {
+        let file_id = ctx.frange.file_id;
+        let position = FilePosition { file_id, offset: impl_node.syntax().range().start() };
+        let resolver = hir::source_binder::resolver_for_position(ctx.db, position);
+
+        resolve_target_trait_def(ctx.db, &resolver, impl_node)?
+    };
+
+    let missing_fns: Vec<_> = {
+        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_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<_>>();
+
+        trait_fns
+            .into_iter()
+            .filter(|t| def_name(t).is_some())
+            .filter(|t| impl_fns.iter().all(|i| def_name(i) != def_name(t)))
+            .collect()
+    };
+    if missing_fns.is_empty() {
+        return None;
+    }
+
+    ctx.add_action(AssistId("add_impl_missing_members"), "add missing impl members", |edit| {
+        let (parent_indent, indent) = {
+            // FIXME: Find a way to get the indent already used in the file.
+            // Now, we copy the indent of first item or indent with 4 spaces relative to impl block
+            const DEFAULT_INDENT: &str = "    ";
+            let first_item = impl_item_list.impl_items().next();
+            let first_item_indent =
+                first_item.and_then(|i| leading_indent(i.syntax())).map(ToOwned::to_owned);
+            let impl_block_indent = leading_indent(impl_node.syntax()).unwrap_or_default();
+
+            (
+                impl_block_indent.to_owned(),
+                first_item_indent.unwrap_or_else(|| impl_block_indent.to_owned() + DEFAULT_INDENT),
+            )
+        };
+
+        let changed_range = {
+            let children = impl_item_list.syntax().children();
+            let last_whitespace = children.filter_map(ast::Whitespace::cast).last();
+
+            last_whitespace.map(|w| w.syntax().range()).unwrap_or_else(|| {
+                let in_brackets = impl_item_list.syntax().range().end() - TextUnit::of_str("}");
+                TextRange::from_to(in_brackets, in_brackets)
+            })
+        };
+
+        let func_bodies = format!("\n{}", missing_fns.into_iter().map(build_func_body).join("\n"));
+        let trailing_whitespace = format!("\n{}", parent_indent);
+        let func_bodies = reindent(&func_bodies, &indent) + &trailing_whitespace;
+
+        let replaced_text_range = TextUnit::of_str(&func_bodies);
+
+        edit.replace(changed_range, func_bodies);
+        edit.set_cursor(
+            changed_range.start() + replaced_text_range - TextUnit::of_str(&trailing_whitespace),
+        );
+    });
+
+    ctx.build()
+}
+
+/// Given an `ast::ImplBlock`, resolves the target trait (the one being
+/// implemented) to a `ast::TraitDef`.
+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,
+    }
+}
+
+fn build_func_body(def: &ast::FnDef) -> String {
+    let mut buf = String::new();
+
+    for child in def.syntax().children() {
+        if child.kind() == SyntaxKind::SEMI {
+            buf.push_str(" { unimplemented!() }")
+        } else {
+            child.text().push_to(&mut buf);
+        }
+    }
+
+    buf.trim_end().to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn test_add_missing_impl_members() {
+        check_assist(
+            add_missing_impl_members,
+            "
+trait Foo {
+    fn foo(&self);
+    fn bar(&self);
+    fn baz(&self);
+}
+
+struct S;
+
+impl Foo for S {
+    fn bar(&self) {}
+    <|>
+}",
+            "
+trait Foo {
+    fn foo(&self);
+    fn bar(&self);
+    fn baz(&self);
+}
+
+struct S;
+
+impl Foo for S {
+    fn bar(&self) {}
+    fn foo(&self) { unimplemented!() }
+    fn baz(&self) { unimplemented!() }<|>
+}",
+        );
+    }
+
+    #[test]
+    fn test_copied_overriden_members() {
+        check_assist(
+            add_missing_impl_members,
+            "
+trait Foo {
+    fn foo(&self);
+    fn bar(&self) -> bool { true }
+    fn baz(&self) -> u32 { 42 }
+}
+
+struct S;
+
+impl Foo for S {
+    fn bar(&self) {}
+    <|>
+}",
+            "
+trait Foo {
+    fn foo(&self);
+    fn bar(&self) -> bool { true }
+    fn baz(&self) -> u32 { 42 }
+}
+
+struct S;
+
+impl Foo for S {
+    fn bar(&self) {}
+    fn foo(&self) { unimplemented!() }
+    fn baz(&self) -> u32 { 42 }<|>
+}",
+        );
+    }
+
+    #[test]
+    fn test_empty_impl_block() {
+        check_assist(
+            add_missing_impl_members,
+            "
+trait Foo { fn foo(&self); }
+struct S;
+impl Foo for S {<|>}",
+            "
+trait Foo { fn foo(&self); }
+struct S;
+impl Foo for S {
+    fn foo(&self) { unimplemented!() }<|>
+}",
+        );
+    }
+
+    #[test]
+    fn test_cursor_after_empty_impl_block() {
+        check_assist(
+            add_missing_impl_members,
+            "
+trait Foo { fn foo(&self); }
+struct S;
+impl Foo for S {}<|>",
+            "
+trait Foo { fn foo(&self); }
+struct S;
+impl Foo for S {
+    fn foo(&self) { unimplemented!() }<|>
+}",
+        )
+    }
+
+    #[test]
+    fn test_empty_trait() {
+        check_assist_not_applicable(
+            add_missing_impl_members,
+            "
+trait Foo;
+struct S;
+impl Foo for S { <|> }",
+        )
+    }
+
+    #[test]
+    fn test_ignore_unnamed_trait_members() {
+        check_assist(
+            add_missing_impl_members,
+            "
+trait Foo {
+    fn (arg: u32);
+    fn valid(some: u32) -> bool { false }
+}
+struct S;
+impl Foo for S { <|> }",
+            "
+trait Foo {
+    fn (arg: u32);
+    fn valid(some: u32) -> bool { false }
+}
+struct S;
+impl Foo for S {
+    fn valid(some: u32) -> bool { false }<|>
+}",
+        )
+    }
+
+    #[test]
+    fn test_indented_impl_block() {
+        check_assist(
+            add_missing_impl_members,
+            "
+trait Foo {
+    fn valid(some: u32) -> bool { false }
+}
+struct S;
+
+mod my_mod {
+    impl crate::Foo for S { <|> }
+}",
+            "
+trait Foo {
+    fn valid(some: u32) -> bool { false }
+}
+struct S;
+
+mod my_mod {
+    impl crate::Foo for S {
+        fn valid(some: u32) -> bool { false }<|>
+    }
+}",
+        )
+    }
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index 6c3d75d798f2..0c4abb4506c4 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -95,6 +95,7 @@ mod replace_if_let_with_match;
 mod split_import;
 mod remove_dbg;
 mod auto_import;
+mod add_missing_impl_members;
 
 fn all_assists<DB: HirDatabase>() -> &'static [fn(AssistCtx<DB>) -> Option<Assist>] {
     &[
@@ -108,6 +109,7 @@ fn all_assists<DB: HirDatabase>() -> &'static [fn(AssistCtx<DB>) -> Option<Assis
         split_import::split_import,
         remove_dbg::remove_dbg,
         auto_import::auto_import,
+        add_missing_impl_members::add_missing_impl_members,
     ]
 }