Rollup merge of #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.

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

@@ -174,6 +174,7 @@ 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};
@@ -187,6 +188,9 @@ 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>,
@@ -596,7 +600,7 @@ 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