3 files changed, 227 insertions, 0 deletions

diff --git a/crates/ra_assists/src/assists/add_custom_impl.rs b/crates/ra_assists/src/assists/add_custom_impl.rs
new file mode 100644
index 000000000..037306fd6
--- /dev/null
+++ b/crates/ra_assists/src/assists/add_custom_impl.rs
@@ -0,0 +1,206 @@
+//! FIXME: write short doc here
+
+use crate::{Assist, AssistCtx, AssistId};
+use hir::db::HirDatabase;
+use join_to_string::join;
+use ra_syntax::{
+    ast::{self, AstNode},
+    Direction, SmolStr,
+    SyntaxKind::{IDENT, WHITESPACE},
+    TextRange, TextUnit,
+};
+
+const DERIVE_TRAIT: &'static str = "derive";
+
+// Assist: add_custom_impl
+//
+// Adds impl block for derived trait.
+//
+// ```
+// #[derive(Deb<|>ug, Display)]
+// struct S;
+// ```
+// ->
+// ```
+// #[derive(Display)]
+// struct S;
+//
+// impl Debug for S {
+//
+// }
+// ```
+pub(crate) fn add_custom_impl(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let input = ctx.find_node_at_offset::<ast::AttrInput>()?;
+    let attr = input.syntax().parent().and_then(ast::Attr::cast)?;
+
+    let attr_name = attr
+        .syntax()
+        .descendants_with_tokens()
+        .filter(|t| t.kind() == IDENT)
+        .find_map(|i| i.into_token())
+        .filter(|t| *t.text() == DERIVE_TRAIT)?
+        .text()
+        .clone();
+
+    let trait_token =
+        ctx.token_at_offset().filter(|t| t.kind() == IDENT && *t.text() != attr_name).next()?;
+
+    let annotated = attr.syntax().siblings(Direction::Next).find_map(|s| ast::Name::cast(s))?;
+    let annotated_name = annotated.syntax().text().to_string();
+    let start_offset = annotated.syntax().parent()?.text_range().end();
+
+    ctx.add_assist(AssistId("add_custom_impl"), "add custom impl", |edit| {
+        edit.target(attr.syntax().text_range());
+
+        let new_attr_input = input
+            .syntax()
+            .descendants_with_tokens()
+            .filter(|t| t.kind() == IDENT)
+            .filter_map(|t| t.into_token().map(|t| t.text().clone()))
+            .filter(|t| t != trait_token.text())
+            .collect::<Vec<SmolStr>>();
+        let has_more_derives = new_attr_input.len() > 0;
+        let new_attr_input =
+            join(new_attr_input.iter()).separator(", ").surround_with("(", ")").to_string();
+        let new_attr_input_len = new_attr_input.len();
+
+        let mut buf = String::new();
+        buf.push_str("\n\nimpl ");
+        buf.push_str(trait_token.text().as_str());
+        buf.push_str(" for ");
+        buf.push_str(annotated_name.as_str());
+        buf.push_str(" {\n");
+
+        let cursor_delta = if has_more_derives {
+            edit.replace(input.syntax().text_range(), new_attr_input);
+            input.syntax().text_range().len() - TextUnit::from_usize(new_attr_input_len)
+        } else {
+            let attr_range = attr.syntax().text_range();
+            edit.delete(attr_range);
+
+            let line_break_range = attr
+                .syntax()
+                .next_sibling_or_token()
+                .filter(|t| t.kind() == WHITESPACE)
+                .map(|t| t.text_range())
+                .unwrap_or(TextRange::from_to(TextUnit::from(0), TextUnit::from(0)));
+            edit.delete(line_break_range);
+
+            attr_range.len() + line_break_range.len()
+        };
+
+        edit.set_cursor(start_offset + TextUnit::of_str(&buf) - cursor_delta);
+        buf.push_str("\n}");
+        edit.insert(start_offset, buf);
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn add_custom_impl_for_unique_input() {
+        check_assist(
+            add_custom_impl,
+            "
+#[derive(Debu<|>g)]
+struct Foo {
+    bar: String,
+}
+            ",
+            "
+struct Foo {
+    bar: String,
+}
+
+impl Debug for Foo {
+<|>
+}
+            ",
+        )
+    }
+
+    #[test]
+    fn add_custom_impl_for_with_visibility_modifier() {
+        check_assist(
+            add_custom_impl,
+            "
+#[derive(Debug<|>)]
+pub struct Foo {
+    bar: String,
+}
+            ",
+            "
+pub struct Foo {
+    bar: String,
+}
+
+impl Debug for Foo {
+<|>
+}
+            ",
+        )
+    }
+
+    #[test]
+    fn add_custom_impl_when_multiple_inputs() {
+        check_assist(
+            add_custom_impl,
+            "
+#[derive(Display, Debug<|>, Serialize)]
+struct Foo {}
+            ",
+            "
+#[derive(Display, Serialize)]
+struct Foo {}
+
+impl Debug for Foo {
+<|>
+}
+            ",
+        )
+    }
+
+    #[test]
+    fn test_ignore_derive_macro_without_input() {
+        check_assist_not_applicable(
+            add_custom_impl,
+            "
+#[derive(<|>)]
+struct Foo {}
+            ",
+        )
+    }
+
+    #[test]
+    fn test_ignore_if_cursor_on_param() {
+        check_assist_not_applicable(
+            add_custom_impl,
+            "
+#[derive<|>(Debug)]
+struct Foo {}
+            ",
+        );
+
+        check_assist_not_applicable(
+            add_custom_impl,
+            "
+#[derive(Debug)<|>]
+struct Foo {}
+            ",
+        )
+    }
+
+    #[test]
+    fn test_ignore_if_not_derive() {
+        check_assist_not_applicable(
+            add_custom_impl,
+            "
+#[allow(non_camel_<|>case_types)]
+struct Foo {}
+            ",
+        )
+    }
+}
diff --git a/crates/ra_assists/src/doc_tests/generated.rs b/crates/ra_assists/src/doc_tests/generated.rs
index 3c716c2d1..4586eeb59 100644
--- a/crates/ra_assists/src/doc_tests/generated.rs
+++ b/crates/ra_assists/src/doc_tests/generated.rs
@@ -3,6 +3,25 @@
 use super::check;
 
 #[test]
+fn doctest_add_custom_impl() {
+    check(
+        "add_custom_impl",
+        r#####"
+#[derive(Deb<|>ug, Display)]
+struct S;
+"#####,
+        r#####"
+#[derive(Display)]
+struct S;
+
+impl Debug for S {
+
+}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_add_derive() {
     check(
         "add_derive",
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index a372bd8b9..98fb20b22 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -95,6 +95,7 @@ mod assists {
     mod add_derive;
     mod add_explicit_type;
     mod add_impl;
+    mod add_custom_impl;
     mod add_new;
     mod apply_demorgan;
     mod invert_if;
@@ -121,6 +122,7 @@ mod assists {
             add_derive::add_derive,
             add_explicit_type::add_explicit_type,
             add_impl::add_impl,
+            add_custom_impl::add_custom_impl,
             add_new::add_new,
             apply_demorgan::apply_demorgan,
             invert_if::invert_if,