Auto merge of #135634 - joboet:trivial-clone, r=Mark-Simulacrum

stop specializing on `Copy`

fixes #132442

`std` specializes on `Copy` to optimize certain library functions such as `clone_from_slice`. This is unsound, however, as the `Copy` implementation may not be always applicable because of lifetime bounds, which specialization does not take into account; the result being that values are copied even though they are not `Copy`. For instance, this code:
```rust
struct SometimesCopy<'a>(&'a Cell<bool>);

impl<'a> Clone for SometimesCopy<'a> {
    fn clone(&self) -> Self {
        self.0.set(true);
        Self(self.0)
    }
}

impl Copy for SometimesCopy<'static> {}

let clone_called = Cell::new(false);
// As SometimesCopy<'clone_called> is not 'static, this must run `clone`,
// setting the value to `true`.
let _ = [SometimesCopy(&clone_called)].clone();
assert!(clone_called.get());
```
should not panic, but does ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6be7a48cad849d8bd064491616fdb43c)).

To solve this, this PR introduces a new `unsafe` trait: `TrivialClone`. This trait may be implemented whenever the `Clone` implementation is equivalent to copying the value (so e.g. `fn clone(&self) -> Self { *self }`). Because of lifetime erasure, there is no way for the `Clone` implementation to observe lifetime bounds, meaning that even if the `TrivialClone` has stricter bounds than the `Clone` implementation, its invariant still holds. Therefore, it is sound to specialize on `TrivialClone`.

I've changed all `Copy` specializations in the standard library to specialize on `TrivialClone` instead. Unfortunately, the unsound `#[rustc_unsafe_specialization_marker]` attribute on `Copy` cannot be removed in this PR as `hashbrown` still depends on it. I'll make a PR updating `hashbrown` once this lands.

With `Copy` no longer being considered for specialization, this change alone would result in the standard library optimizations not being applied for user types unaware of `TrivialClone`. To avoid this and restore the optimizations in most cases, I have changed the expansion of `#[derive(Clone)]`: Currently, whenever both `Clone` and `Copy` are derived, the `clone` method performs a copy of the value. With this PR, the derive macro also adds a `TrivialClone` implementation to make this case observable using specialization. I anticipate that most users will use `#[derive(Clone, Copy)]` whenever both are applicable, so most users will still profit from the library optimizations.

Unfortunately, Hyrum's law applies to this PR: there are some popular crates which rely on the precise specialization behaviour of `core` to implement "specialization at home", e.g. [`libAFL`](https://github.com/AFLplusplus/LibAFL/blob/89cff637025c1652c24e8d97a30a2e3d01f187a4/libafl_bolts/src/tuples.rs#L27-L49). I have no remorse for breaking such horrible code, but perhaps we should open other, better ways to satisfy their needs – for example by dropping the `'static` bound on `TypeId::of`...

Author: bors

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 {