Improve hover message for inert attributes

crates/ide/src/hover/render.rs

@@ -2,7 +2,7 @@
 use std::fmt::Display;
 
 use either::Either;
-use hir::{AsAssocItem, HasAttrs, HasSource, HirDisplay, Semantics, TypeInfo};
+use hir::{AsAssocItem, AttributeTemplate, HasAttrs, HasSource, HirDisplay, Semantics, TypeInfo};
 use ide_db::{
     base_db::SourceDatabase,
     defs::Definition,
@@ -370,13 +370,31 @@ pub(super) fn definition(
         Definition::GenericParam(it) => label_and_docs(db, it),
         Definition::Label(it) => return Some(Markup::fenced_block(&it.name(db))),
         // FIXME: We should be able to show more info about these
-        Definition::BuiltinAttr(it) => return Some(Markup::fenced_block(&it.name(db))),
+        Definition::BuiltinAttr(it) => return render_builtin_attr(db, it),
         Definition::ToolModule(it) => return Some(Markup::fenced_block(&it.name(db))),
     };
 
     markup(docs.filter(|_| config.documentation.is_some()).map(Into::into), label, mod_path)
 }
 
+fn render_builtin_attr(db: &RootDatabase, attr: hir::BuiltinAttr) -> Option<Markup> {
+    let name = attr.name(db);
+    let desc = format!("#[{}]", name);
+
+    let AttributeTemplate { word, list, name_value_str } = attr.template(db);
+    let mut docs = "Valid forms are:".to_owned();
+    if word {
+        format_to!(docs, "\n - #\\[{}]", name);
+    }
+    if let Some(list) = list {
+        format_to!(docs, "\n - #\\[{}({})]", name, list);
+    }
+    if let Some(name_value_str) = name_value_str {
+        format_to!(docs, "\n - #\\[{} = {}]", name, name_value_str);
+    }
+    markup(Some(docs.replace('*', "\\*")), desc, None)
+}
+
 fn label_and_docs<D>(db: &RootDatabase, def: D) -> (String, Option<hir::Documentation>)
 where
     D: HasAttrs + HirDisplay,

crates/ide/src/hover/tests.rs

@@ -4277,3 +4277,46 @@ pub struct Foo;
         "#]],
     );
 }
+
+#[test]
+fn hover_inert_attr() {
+    check(
+        r#"
+#[doc$0 = ""]
+pub struct Foo;
+"#,
+        expect![[r##"
+            *doc*
+
+            ```rust
+            #[doc]
+            ```
+
+            ---
+
+            Valid forms are:
+
+            * \#\[doc(hidden|inline|...)\]
+            * \#\[doc = string\]
+        "##]],
+    );
+    check(
+        r#"
+#[allow$0()]
+pub struct Foo;
+"#,
+        expect![[r##"
+            *allow*
+
+            ```rust
+            #[allow]
+            ```
+
+            ---
+
+            Valid forms are:
+
+            * \#\[allow(lint1, lint2, ..., /\*opt\*/ reason = "...")\]
+        "##]],
+    );
+}