1 files changed, 142 insertions, 0 deletions

diff --git a/crates/ide_completion/src/completions/attribute/derive.rs b/crates/ide_completion/src/completions/attribute/derive.rs
new file mode 100644
index 000000000..7b0a778a2
--- /dev/null
+++ b/crates/ide_completion/src/completions/attribute/derive.rs
@@ -0,0 +1,142 @@
+//! Completion for derives
+use itertools::Itertools;
+use rustc_hash::FxHashSet;
+use syntax::ast;
+
+use crate::{
+    context::CompletionContext,
+    item::{CompletionItem, CompletionItemKind, CompletionKind},
+    Completions,
+};
+
+pub(super) fn complete_derive(
+    acc: &mut Completions,
+    ctx: &CompletionContext,
+    derive_input: ast::TokenTree,
+) {
+    if let Some(existing_derives) = super::parse_comma_sep_input(derive_input) {
+        for derive_completion in DEFAULT_DERIVE_COMPLETIONS
+            .iter()
+            .filter(|completion| !existing_derives.contains(completion.label))
+        {
+            let mut components = vec![derive_completion.label];
+            components.extend(
+                derive_completion
+                    .dependencies
+                    .iter()
+                    .filter(|&&dependency| !existing_derives.contains(dependency)),
+            );
+            let lookup = components.join(", ");
+            let label = components.iter().rev().join(", ");
+            let mut item =
+                CompletionItem::new(CompletionKind::Attribute, ctx.source_range(), label);
+            item.lookup_by(lookup).kind(CompletionItemKind::Attribute);
+            item.add_to(acc);
+        }
+
+        for custom_derive_name in get_derive_names_in_scope(ctx).difference(&existing_derives) {
+            let mut item = CompletionItem::new(
+                CompletionKind::Attribute,
+                ctx.source_range(),
+                custom_derive_name,
+            );
+            item.kind(CompletionItemKind::Attribute);
+            item.add_to(acc);
+        }
+    }
+}
+fn get_derive_names_in_scope(ctx: &CompletionContext) -> FxHashSet<String> {
+    let mut result = FxHashSet::default();
+    ctx.scope.process_all_names(&mut |name, scope_def| {
+        if let hir::ScopeDef::MacroDef(mac) = scope_def {
+            // FIXME kind() doesn't check whether proc-macro is a derive
+            if mac.kind() == hir::MacroKind::Derive || mac.kind() == hir::MacroKind::ProcMacro {
+                result.insert(name.to_string());
+            }
+        }
+    });
+    result
+}
+
+struct DeriveCompletion {
+    label: &'static str,
+    dependencies: &'static [&'static str],
+}
+
+/// Standard Rust derives and the information about their dependencies
+/// (the dependencies are needed so that the main derive don't break the compilation when added)
+const DEFAULT_DERIVE_COMPLETIONS: &[DeriveCompletion] = &[
+    DeriveCompletion { label: "Clone", dependencies: &[] },
+    DeriveCompletion { label: "Copy", dependencies: &["Clone"] },
+    DeriveCompletion { label: "Debug", dependencies: &[] },
+    DeriveCompletion { label: "Default", dependencies: &[] },
+    DeriveCompletion { label: "Hash", dependencies: &[] },
+    DeriveCompletion { label: "PartialEq", dependencies: &[] },
+    DeriveCompletion { label: "Eq", dependencies: &["PartialEq"] },
+    DeriveCompletion { label: "PartialOrd", dependencies: &["PartialEq"] },
+    DeriveCompletion { label: "Ord", dependencies: &["PartialOrd", "Eq", "PartialEq"] },
+];
+
+#[cfg(test)]
+mod tests {
+    use expect_test::{expect, Expect};
+
+    use crate::{test_utils::completion_list, CompletionKind};
+
+    fn check(ra_fixture: &str, expect: Expect) {
+        let actual = completion_list(ra_fixture, CompletionKind::Attribute);
+        expect.assert_eq(&actual);
+    }
+
+    #[test]
+    fn empty_derive_completion() {
+        check(
+            r#"
+#[derive($0)]
+struct Test {}
+        "#,
+            expect![[r#"
+                at Clone
+                at Clone, Copy
+                at Debug
+                at Default
+                at Hash
+                at PartialEq
+                at PartialEq, Eq
+                at PartialEq, PartialOrd
+                at PartialEq, Eq, PartialOrd, Ord
+            "#]],
+        );
+    }
+
+    #[test]
+    fn no_completion_for_incorrect_derive() {
+        check(
+            r#"
+#[derive{$0)]
+struct Test {}
+"#,
+            expect![[r#""#]],
+        )
+    }
+
+    #[test]
+    fn derive_with_input_completion() {
+        check(
+            r#"
+#[derive(serde::Serialize, PartialEq, $0)]
+struct Test {}
+"#,
+            expect![[r#"
+                at Clone
+                at Clone, Copy
+                at Debug
+                at Default
+                at Hash
+                at Eq
+                at PartialOrd
+                at Eq, PartialOrd, Ord
+            "#]],
+        )
+    }
+}