Rollup merge of rust-lang#101040 - danielhenrymantilla:no-bounds-for-default-annotated-derive, r=joshtriplett

Fix `#[derive(Default)]` on a generic `#[default]` enum adding unnecessary `Default` bounds

That is, given something like:

```rs
// #[default] on a generic enum does not add `Default` bounds to the type params.
#[derive(Default)]
enum MyOption<T> {
    #[default]
    None,
    Some(T),
}
```

then `MyOption<T> : Default`_as currently implemented_ only holds when `T : Default`, as reported by `@5225225` [over Zulip](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/.23.5Bderive.28Default.29.5D.20for.20enums.20with.20fields).

This is contrary to [what the accepted RFC proposes](https://rust-lang.github.io/rfcs/3107-derive-default-enum.html#generated-bounds) (_i.e._, that `T` be allowed not to be itself `Default`), and indeed seems to be a rather unnecessary limitation.

Author: matthiaskrgr

compiler/rustc_builtin_macros/src/deriving/bounds.rs

@@ -16,6 +16,7 @@ pub fn expand_deriving_copy(
     let trait_def = TraitDef {
         span,
         path: path_std!(marker::Copy),
+        skip_path_as_bound: false,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         supports_unions: true,

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -72,6 +72,7 @@ pub fn expand_deriving_clone(
     let trait_def = TraitDef {
         span,
         path: path_std!(clone::Clone),
+        skip_path_as_bound: false,
         additional_bounds: bounds,
         generics: Bounds::empty(),
         supports_unions: true,

compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs

@@ -25,6 +25,7 @@ pub fn expand_deriving_eq(
     let trait_def = TraitDef {
         span,
         path: path_std!(cmp::Eq),
+        skip_path_as_bound: false,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         supports_unions: true,

compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs

@@ -19,6 +19,7 @@ pub fn expand_deriving_ord(
     let trait_def = TraitDef {
         span,
         path: path_std!(cmp::Ord),
+        skip_path_as_bound: false,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         supports_unions: false,

compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs

@@ -83,6 +83,7 @@ pub fn expand_deriving_partial_eq(
     let trait_def = TraitDef {
         span,
         path: path_std!(cmp::PartialEq),
+        skip_path_as_bound: false,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         supports_unions: false,

compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs

@@ -37,6 +37,7 @@ pub fn expand_deriving_partial_ord(
     let trait_def = TraitDef {
         span,
         path: path_std!(cmp::PartialOrd),
+        skip_path_as_bound: false,
         additional_bounds: vec![],
         generics: Bounds::empty(),
         supports_unions: false,

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -20,6 +20,7 @@ pub fn expand_deriving_debug(
     let trait_def = TraitDef {
         span,
         path: path_std!(fmt::Debug),
+        skip_path_as_bound: false,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         supports_unions: false,

compiler/rustc_builtin_macros/src/deriving/decodable.rs

@@ -23,6 +23,7 @@ pub fn expand_deriving_rustc_decodable(
     let trait_def = TraitDef {
         span,
         path: Path::new_(vec![krate, sym::Decodable], vec![], PathKind::Global),
+        skip_path_as_bound: false,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         supports_unions: false,

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -24,6 +24,7 @@ pub fn expand_deriving_default(
     let trait_def = TraitDef {
         span,
         path: Path::new(vec![kw::Default, sym::Default]),
+        skip_path_as_bound: has_a_default_variant(item),
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         supports_unions: false,
@@ -262,3 +263,22 @@ impl<'a, 'b> rustc_ast::visit::Visitor<'a> for DetectNonVariantDefaultAttr<'a, '
         }
     }
 }
+
+fn has_a_default_variant(item: &Annotatable) -> bool {
+    struct HasDefaultAttrOnVariant {
+        found: bool,
+    }
+
+    impl<'ast> rustc_ast::visit::Visitor<'ast> for HasDefaultAttrOnVariant {
+        fn visit_variant(&mut self, v: &'ast rustc_ast::Variant) {
+            if v.attrs.iter().any(|attr| attr.has_name(kw::Default)) {
+                self.found = true;
+            }
+            // no need to subrecurse.
+        }
+    }
+
+    let mut visitor = HasDefaultAttrOnVariant { found: false };
+    item.visit_with(&mut visitor);
+    visitor.found
+}

compiler/rustc_builtin_macros/src/deriving/encodable.rs

@@ -107,6 +107,7 @@ pub fn expand_deriving_rustc_encodable(
     let trait_def = TraitDef {
         span,
         path: Path::new_(vec![krate, sym::Encodable], vec![], PathKind::Global),
+        skip_path_as_bound: false,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         supports_unions: false,