stop specializing on Copy

compiler/rustc_builtin_macros/src/deriving/bounds.rs

@@ -1,4 +1,4 @@
-use rustc_ast::MetaItem;
+use rustc_ast::{MetaItem, Safety};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::Span;
 
@@ -24,6 +24,8 @@ pub(crate) fn expand_deriving_copy(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
 
     trait_def.expand(cx, mitem, item, push);
@@ -48,6 +50,8 @@ pub(crate) fn expand_deriving_const_param_ty(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
 
     trait_def.expand(cx, mitem, item, push);

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -1,7 +1,7 @@
-use rustc_ast::{self as ast, Generics, ItemKind, MetaItem, VariantData};
+use rustc_ast::{self as ast, Generics, ItemKind, MetaItem, Safety, VariantData};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_expand::base::{Annotatable, ExtCtxt};
-use rustc_span::{Ident, Span, kw, sym};
+use rustc_span::{DUMMY_SP, Ident, Span, kw, sym};
 use thin_vec::{ThinVec, thin_vec};
 
 use crate::deriving::generic::ty::*;
@@ -68,6 +68,29 @@ pub(crate) fn expand_deriving_clone(
         _ => cx.dcx().span_bug(span, "`#[derive(Clone)]` on trait item or impl item"),
     }
 
+    // If the clone method is just copying the value, also mark the type as
+    // `TrivialClone` to allow some library optimizations.
+    if is_simple {
+        let trivial_def = TraitDef {
+            span,
+            path: path_std!(clone::TrivialClone),
+            skip_path_as_bound: false,
+            needs_copy_as_bound_if_packed: true,
+            additional_bounds: bounds.clone(),
+            supports_unions: true,
+            methods: Vec::new(),
+            associated_types: Vec::new(),
+            is_const,
+            is_staged_api_crate: cx.ecfg.features.staged_api(),
+            safety: Safety::Unsafe(DUMMY_SP),
+            // `TrivialClone` is not part of an API guarantee, so it shouldn't
+            // appear in rustdoc output.
+            document: false,
+        };
+
+        trivial_def.expand_ext(cx, mitem, item, push, true);
+    }
+
     let trait_def = TraitDef {
         span,
         path: path_std!(clone::Clone),
@@ -88,6 +111,8 @@ pub(crate) fn expand_deriving_clone(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
 
     trait_def.expand_ext(cx, mitem, item, push, is_simple)

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

@@ -1,4 +1,4 @@
-use rustc_ast::{self as ast, MetaItem};
+use rustc_ast::{self as ast, MetaItem, Safety};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::{Span, sym};
@@ -44,6 +44,8 @@ pub(crate) fn expand_deriving_eq(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
     trait_def.expand_ext(cx, mitem, item, push, true)
 }

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

@@ -1,4 +1,4 @@
-use rustc_ast::MetaItem;
+use rustc_ast::{MetaItem, Safety};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::{Ident, Span, sym};
 use thin_vec::thin_vec;
@@ -35,6 +35,8 @@ pub(crate) fn expand_deriving_ord(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
 
     trait_def.expand(cx, mitem, item, push)

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

@@ -1,4 +1,4 @@
-use rustc_ast::{BinOpKind, BorrowKind, Expr, ExprKind, MetaItem, Mutability};
+use rustc_ast::{BinOpKind, BorrowKind, Expr, ExprKind, MetaItem, Mutability, Safety};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::{Span, sym};
 use thin_vec::thin_vec;
@@ -30,6 +30,8 @@ pub(crate) fn expand_deriving_partial_eq(
         associated_types: Vec::new(),
         is_const: false,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
     structural_trait_def.expand(cx, mitem, item, push);
 
@@ -59,6 +61,8 @@ pub(crate) fn expand_deriving_partial_eq(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
     trait_def.expand(cx, mitem, item, push)
 }

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

@@ -1,4 +1,4 @@
-use rustc_ast::{ExprKind, ItemKind, MetaItem, PatKind};
+use rustc_ast::{ExprKind, ItemKind, MetaItem, PatKind, Safety};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::{Ident, Span, sym};
 use thin_vec::thin_vec;
@@ -65,6 +65,8 @@ pub(crate) fn expand_deriving_partial_ord(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
     trait_def.expand(cx, mitem, item, push)
 }

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -1,4 +1,4 @@
-use rustc_ast::{self as ast, EnumDef, MetaItem};
+use rustc_ast::{self as ast, EnumDef, MetaItem, Safety};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_session::config::FmtDebug;
 use rustc_span::{Ident, Span, Symbol, sym};
@@ -42,6 +42,8 @@ pub(crate) fn expand_deriving_debug(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
     trait_def.expand(cx, mitem, item, push)
 }

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -1,8 +1,7 @@
 use core::ops::ControlFlow;
 
-use rustc_ast as ast;
 use rustc_ast::visit::visit_opt;
-use rustc_ast::{EnumDef, VariantData, attr};
+use rustc_ast::{self as ast, EnumDef, Safety, VariantData, attr};
 use rustc_expand::base::{Annotatable, DummyResult, ExtCtxt};
 use rustc_span::{ErrorGuaranteed, Ident, Span, kw, sym};
 use smallvec::SmallVec;
@@ -52,6 +51,8 @@ pub(crate) fn expand_deriving_default(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
     trait_def.expand(cx, mitem, item, push)
 }

compiler/rustc_builtin_macros/src/deriving/from.rs

@@ -1,5 +1,5 @@
 use rustc_ast as ast;
-use rustc_ast::{ItemKind, VariantData};
+use rustc_ast::{ItemKind, Safety, VariantData};
 use rustc_errors::MultiSpan;
 use rustc_expand::base::{Annotatable, DummyResult, ExtCtxt};
 use rustc_span::{Ident, Span, kw, sym};
@@ -127,6 +127,8 @@ pub(crate) fn expand_deriving_from(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
 
     from_trait_def.expand(cx, mitem, annotatable, push);

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

@@ -225,6 +225,12 @@ pub(crate) struct TraitDef<'a> {
     pub is_const: bool,
 
     pub is_staged_api_crate: bool,
+
+    /// The safety of the `impl`.
+    pub safety: Safety,
+
+    /// Whether the added `impl` should appear in rustdoc output.
+    pub document: bool,
 }
 
 pub(crate) struct MethodDef<'a> {
@@ -826,13 +832,17 @@ impl<'a> TraitDef<'a> {
             )
         }
 
+        if !self.document {
+            attrs.push(cx.attr_nested_word(sym::doc, sym::hidden, self.span));
+        }
+
         cx.item(
             self.span,
             attrs,
             ast::ItemKind::Impl(ast::Impl {
                 generics: trait_generics,
                 of_trait: Some(Box::new(ast::TraitImplHeader {
-                    safety: ast::Safety::Default,
+                    safety: self.safety,
                     polarity: ast::ImplPolarity::Positive,
                     defaultness: ast::Defaultness::Final,
                     constness: if self.is_const {

compiler/rustc_builtin_macros/src/deriving/hash.rs

@@ -1,4 +1,4 @@
-use rustc_ast::{MetaItem, Mutability};
+use rustc_ast::{MetaItem, Mutability, Safety};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::{Span, sym};
 use thin_vec::thin_vec;
@@ -42,6 +42,8 @@ pub(crate) fn expand_deriving_hash(
         associated_types: Vec::new(),
         is_const,
         is_staged_api_crate: cx.ecfg.features.staged_api(),
+        safety: Safety::Default,
+        document: true,
     };
 
     hash_trait_def.expand(cx, mitem, item, push);

compiler/rustc_hir/src/lang_items.rs

@@ -176,6 +176,7 @@ language_item_table! {
     Clone,                   sym::clone,               clone_trait,                Target::Trait,          GenericRequirement::None;
     CloneFn,                 sym::clone_fn,            clone_fn,                   Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     UseCloned,               sym::use_cloned,          use_cloned_trait,           Target::Trait,          GenericRequirement::None;
+    TrivialClone,            sym::trivial_clone,       trivial_clone_trait,        Target::Trait,          GenericRequirement::None;
     Sync,                    sym::sync,                sync_trait,                 Target::Trait,          GenericRequirement::Exact(0);
     DiscriminantKind,        sym::discriminant_kind,   discriminant_kind_trait,    Target::Trait,          GenericRequirement::None;
     /// The associated item of the `DiscriminantKind` trait.

compiler/rustc_middle/src/ty/context.rs

@@ -856,6 +856,7 @@ bidirectional_lang_item_map! {
     PointeeTrait,
     Sized,
     TransmuteTrait,
+    TrivialClone,
     Tuple,
     Unpin,
     Unsize,

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

@@ -547,9 +547,11 @@ where
                 Some(SolverTraitLangItem::PointeeSized) => {
                     unreachable!("`PointeeSized` is removed during lowering");
                 }
-                Some(SolverTraitLangItem::Copy | SolverTraitLangItem::Clone) => {
-                    G::consider_builtin_copy_clone_candidate(self, goal)
-                }
+                Some(
+                    SolverTraitLangItem::Copy
+                    | SolverTraitLangItem::Clone
+                    | SolverTraitLangItem::TrivialClone,
+                ) => G::consider_builtin_copy_clone_candidate(self, goal),
                 Some(SolverTraitLangItem::Fn) => {
                     G::consider_builtin_fn_trait_candidates(self, goal, ty::ClosureKind::Fn)
                 }

compiler/rustc_span/src/symbol.rs

@@ -372,6 +372,7 @@ symbols! {
         ToString,
         TokenStream,
         Trait,
+        TrivialClone,
         Try,
         TryCaptureGeneric,
         TryCapturePrintable,
@@ -2270,6 +2271,7 @@ symbols! {
         transparent_enums,
         transparent_unions,
         trivial_bounds,
+        trivial_clone,
         truncf16,
         truncf32,
         truncf64,

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -67,7 +67,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
             let lang_item = tcx.as_lang_item(def_id);
             match lang_item {
-                Some(LangItem::Copy | LangItem::Clone) => {
+                Some(LangItem::Copy | LangItem::Clone | LangItem::TrivialClone) => {
                     debug!(obligation_self_ty = ?obligation.predicate.skip_binder().self_ty());
 
                     // User-defined copy impls are permitted, but only for

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -249,7 +249,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             Some(LangItem::PointeeSized) => {
                 bug!("`PointeeSized` is removing during lowering");
             }
-            Some(LangItem::Copy | LangItem::Clone) => self.copy_clone_conditions(self_ty),
+            Some(LangItem::Copy | LangItem::Clone | LangItem::TrivialClone) => {
+                self.copy_clone_conditions(self_ty)
+            }
             Some(LangItem::FusedIterator) => {
                 if self.coroutine_is_gen(self_ty) {
                     ty::Binder::dummy(vec![])

compiler/rustc_type_ir/src/lang_items.rs

@@ -48,6 +48,7 @@ pub enum SolverTraitLangItem {
     PointeeTrait,
     Sized,
     TransmuteTrait,
+    TrivialClone,
     Tuple,
     Unpin,
     Unsize,

library/alloc/src/boxed/convert.rs

@@ -1,4 +1,6 @@
 use core::any::Any;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::TrivialClone;
 use core::error::Error;
 use core::mem;
 use core::pin::Pin;
@@ -75,11 +77,13 @@ impl<T: Clone> BoxFromSlice<T> for Box<[T]> {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy> BoxFromSlice<T> for Box<[T]> {
+impl<T: TrivialClone> BoxFromSlice<T> for Box<[T]> {
     #[inline]
     fn from_slice(slice: &[T]) -> Self {
         let len = slice.len();
         let buf = RawVec::with_capacity(len);
+        // SAFETY: since `T` implements `TrivialClone`, this is sound and
+        // equivalent to the above.
         unsafe {
             ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
             buf.into_box(slice.len()).assume_init()

library/alloc/src/collections/vec_deque/mod.rs

@@ -7,6 +7,8 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+#[cfg(not(no_global_oom_handling))]
+use core::clone::TrivialClone;
 use core::cmp::{self, Ordering};
 use core::hash::{Hash, Hasher};
 use core::iter::{ByRefSized, repeat_n, repeat_with};
@@ -3419,7 +3421,7 @@ impl<T: Clone, A: Allocator> SpecExtendFromWithin for VecDeque<T, A> {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy, A: Allocator> SpecExtendFromWithin for VecDeque<T, A> {
+impl<T: TrivialClone, A: Allocator> SpecExtendFromWithin for VecDeque<T, A> {
     unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
         let dst = self.len();
         let count = src.end - src.start;

library/alloc/src/lib.rs

@@ -146,6 +146,7 @@
 #![feature(std_internals)]
 #![feature(str_internals)]
 #![feature(temporary_niche_types)]
+#![feature(trivial_clone)]
 #![feature(trusted_fused)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]

library/alloc/src/rc.rs

@@ -243,9 +243,9 @@
 
 use core::any::Any;
 use core::cell::{Cell, CloneFromCell};
-#[cfg(not(no_global_oom_handling))]
-use core::clone::CloneToUninit;
 use core::clone::UseCloned;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::{CloneToUninit, TrivialClone};
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
 use core::intrinsics::abort;
@@ -2224,7 +2224,8 @@ impl<T> Rc<[T]> {
 
     /// Copy elements from slice into newly allocated `Rc<[T]>`
     ///
-    /// Unsafe because the caller must either take ownership or bind `T: Copy`
+    /// Unsafe because the caller must either take ownership, bind `T: Copy` or
+    /// bind `T: TrivialClone`.
     #[cfg(not(no_global_oom_handling))]
     unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
         unsafe {
@@ -2314,9 +2315,11 @@ impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
+impl<T: TrivialClone> RcFromSlice<T> for Rc<[T]> {
     #[inline]
     fn from_slice(v: &[T]) -> Self {
+        // SAFETY: `T` implements `TrivialClone`, so this is sound and equivalent
+        // to the above.
         unsafe { Rc::copy_from_slice(v) }
     }
 }