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

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,

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -172,6 +172,7 @@ use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::Span;
 use std::cell::RefCell;
 use std::iter;
+use std::ops::Not;
 use std::vec;
 use thin_vec::thin_vec;
 use ty::{Bounds, Path, Ref, Self_, Ty};
@@ -185,6 +186,9 @@ pub struct TraitDef<'a> {
  /// Path of the trait, including any type parameters
  pub path: Path,
 
+ /// Whether to skip adding the current trait as a bound to the type parameters of the type.
+ pub skip_path_as_bound: bool,
+
  /// Additional bounds required of any type parameters of the type,
  /// other than the current trait
  pub additional_bounds: Vec<Ty>,
@@ -594,7 +598,7 @@ impl<'a> TraitDef<'a> {
  cx.trait_bound(p.to_path(cx, self.span, type_ident, generics))
  }).chain(
  // require the current trait
- iter::once(cx.trait_bound(trait_path.clone()))
+ self.skip_path_as_bound.not().then(|| cx.trait_bound(trait_path.clone()))
  ).chain(
  // also add in any bounds from the declaration
  param.bounds.iter().cloned()

compiler/rustc_builtin_macros/src/deriving/hash.rs

@@ -22,6 +22,7 @@ pub fn expand_deriving_hash(
  let hash_trait_def = TraitDef {
  span,
  path,
+ skip_path_as_bound: false,
  additional_bounds: Vec::new(),
  generics: Bounds::empty(),
  supports_unions: false,

src/test/ui/deriving/deriving-default-enum.rs

@@ -12,6 +12,16 @@ enum Foo {
  Beta(NotDefault),
 }
 
+// #[default] on a generic enum does not add `Default` bounds to the type params.
+#[derive(Default)]
+enum MyOption<T> {
+ #[default]
+ None,
+ #[allow(dead_code)]
+ Some(T),
+}
+
 fn main() {
  assert_eq!(Foo::default(), Foo::Alpha);
+ assert!(matches!(MyOption::<NotDefault>::default(), MyOption::None));
 }

src/test/ui/macros/macros-nonfatal-errors.rs

@@ -116,3 +116,24 @@ fn main() {
 
  trace_macros!(invalid); //~ ERROR
 }
+
+/// Check that `#[derive(Default)]` does use a `T : Default` bound when the
+/// `#[default]` variant is `#[non_exhaustive]` (should this end up allowed).
+const _: () = {
+ #[derive(Default)]
+ enum NonExhaustiveDefaultGeneric<T> {
+ #[default]
+ #[non_exhaustive]
+ Foo, //~ ERROR default variant must be exhaustive
+ Bar(T),
+ }
+
+ fn assert_impls_default<T: Default>() {}
+
+ enum NotDefault {}
+
+ // Note: the `derive(Default)` currently bails early enough for trait-checking
+ // not to happen. Should it bail late enough, or even pass, make sure to
+ // assert that the following line fails.
+ let _ = assert_impls_default::<NonExhaustiveDefaultGeneric<NotDefault>>;
+};

src/test/ui/macros/macros-nonfatal-errors.stderr

@@ -215,11 +215,21 @@ error: trace_macros! accepts only `true` or `false`
 LL | trace_macros!(invalid);
  | ^^^^^^^^^^^^^^^^^^^^^^
 
+error: default variant must be exhaustive
+ --> $DIR/macros-nonfatal-errors.rs:127:9
+ |
+LL | #[non_exhaustive]
+ | ----------------- declared `#[non_exhaustive]` here
+LL | Foo,
+ | ^^^
+ |
+ = help: consider a manual implementation of `Default`
+
 error: cannot find macro `llvm_asm` in this scope
  --> $DIR/macros-nonfatal-errors.rs:99:5
  |
 LL | llvm_asm!(invalid);
  | ^^^^^^^^
 
-error: aborting due to 27 previous errors
+error: aborting due to 28 previous errors