From eb80407d79a25cb9bdd98eca47764e57114d1d86 Mon Sep 17 00:00:00 2001
From: Takayuki Maeda <takoyaki0316@gmail.com>
Date: Mon, 4 Jul 2022 20:46:59 +0900
Subject: [PATCH] suggest `#[derive(Default)]` to enums with `#[default]`

---
 compiler/rustc_resolve/src/diagnostics.rs         |  9 +++++++++
 src/test/ui/enum/suggest-default-attribute.rs     |  8 ++++++++
 src/test/ui/enum/suggest-default-attribute.stderr | 14 ++++++++++++++
 3 files changed, 31 insertions(+)
 create mode 100644 src/test/ui/enum/suggest-default-attribute.rs
 create mode 100644 src/test/ui/enum/suggest-default-attribute.stderr

diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index e8b7cee573490..4fbbd9deaebb4 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -1495,6 +1495,15 @@ impl<'a> Resolver<'a> {
             err.help("have you added the `#[macro_use]` on the module/import?");
             return;
         }
+        if ident.name == kw::Default
+            && let ModuleKind::Def(DefKind::Enum, def_id, _) = parent_scope.module.kind
+            && let Some(span) = self.opt_span(def_id)
+        {
+            err.span_help(
+                self.session.source_map().guess_head_span(span),
+                "consider adding `#[derive(Default)]` to this enum",
+            );
+        }
         for ns in [Namespace::MacroNS, Namespace::TypeNS, Namespace::ValueNS] {
             if let Ok(binding) = self.early_resolve_ident_in_lexical_scope(
                 ident,
diff --git a/src/test/ui/enum/suggest-default-attribute.rs b/src/test/ui/enum/suggest-default-attribute.rs
new file mode 100644
index 0000000000000..33bd0d2408139
--- /dev/null
+++ b/src/test/ui/enum/suggest-default-attribute.rs
@@ -0,0 +1,8 @@
+pub enum Test { //~ HELP consider adding `#[derive(Default)]` to this enum
+    #[default]
+    //~^ ERROR cannot find attribute `default` in this scope
+    First,
+    Second,
+}
+
+fn main() {}
diff --git a/src/test/ui/enum/suggest-default-attribute.stderr b/src/test/ui/enum/suggest-default-attribute.stderr
new file mode 100644
index 0000000000000..791f219e8f95f
--- /dev/null
+++ b/src/test/ui/enum/suggest-default-attribute.stderr
@@ -0,0 +1,14 @@
+error: cannot find attribute `default` in this scope
+  --> $DIR/suggest-default-attribute.rs:2:7
+   |
+LL |     #[default]
+   |       ^^^^^^^
+   |
+help: consider adding `#[derive(Default)]` to this enum
+  --> $DIR/suggest-default-attribute.rs:1:1
+   |
+LL | pub enum Test {
+   | ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+