Add UseCloned trait related code

compiler/rustc_hir/src/lang_items.rs

@@ -171,6 +171,7 @@ language_item_table! {
     Copy,                    sym::copy,                copy_trait,                 Target::Trait,          GenericRequirement::Exact(0);
     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;
     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_mir_build/src/builder/expr/into.rs

@@ -11,6 +11,7 @@ use rustc_middle::thir::*;
 use rustc_middle::ty::{CanonicalUserTypeAnnotation, Ty};
 use rustc_span::DUMMY_SP;
 use rustc_span::source_map::Spanned;
+use rustc_trait_selection::infer::InferCtxtExt;
 use tracing::{debug, instrument};
 
 use crate::builder::expr::category::{Category, RvalueFunc};
@@ -287,28 +288,46 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let place = unpack!(block = this.as_place(block, expr));
                 let ty = place.ty(&this.local_decls, this.tcx).ty;
 
-                let success = this.cfg.start_new_block();
-                let clone_trait = this.tcx.require_lang_item(LangItem::Clone, None);
-                let clone_fn = this.tcx.associated_item_def_ids(clone_trait)[0];
-                let func = Operand::function_handle(this.tcx, clone_fn, [ty.into()], expr_span);
-                let ref_ty = Ty::new_imm_ref(this.tcx, this.tcx.lifetimes.re_erased, ty);
-                let ref_place = this.temp(ref_ty, span);
-                this.cfg.push_assign(
-                    block,
-                    source_info,
-                    ref_place,
-                    Rvalue::Ref(this.tcx.lifetimes.re_erased, BorrowKind::Shared, place),
-                );
-                this.cfg.terminate(block, source_info, TerminatorKind::Call {
-                    func,
-                    args: [Spanned { node: Operand::Move(ref_place), span: DUMMY_SP }].into(),
-                    destination,
-                    target: Some(success),
-                    unwind: UnwindAction::Unreachable,
-                    call_source: CallSource::Misc,
-                    fn_span: expr_span,
-                });
-                success.unit()
+                if this.tcx.type_is_copy_modulo_regions(this.infcx.typing_env(this.param_env), ty) {
+                    this.cfg.push_assign(
+                        block,
+                        source_info,
+                        destination,
+                        Rvalue::Use(Operand::Copy(place)),
+                    );
+                    block.unit()
+                } else if this.infcx.type_is_use_cloned_modulo_regions(this.param_env, ty) {
+                    let success = this.cfg.start_new_block();
+                    let clone_trait = this.tcx.require_lang_item(LangItem::Clone, None);
+                    let clone_fn = this.tcx.associated_item_def_ids(clone_trait)[0];
+                    let func = Operand::function_handle(this.tcx, clone_fn, [ty.into()], expr_span);
+                    let ref_ty = Ty::new_imm_ref(this.tcx, this.tcx.lifetimes.re_erased, ty);
+                    let ref_place = this.temp(ref_ty, span);
+                    this.cfg.push_assign(
+                        block,
+                        source_info,
+                        ref_place,
+                        Rvalue::Ref(this.tcx.lifetimes.re_erased, BorrowKind::Shared, place),
+                    );
+                    this.cfg.terminate(block, source_info, TerminatorKind::Call {
+                        func,
+                        args: [Spanned { node: Operand::Move(ref_place), span: DUMMY_SP }].into(),
+                        destination,
+                        target: Some(success),
+                        unwind: UnwindAction::Unreachable,
+                        call_source: CallSource::Misc,
+                        fn_span: expr_span,
+                    });
+                    success.unit()
+                } else {
+                    this.cfg.push_assign(
+                        block,
+                        source_info,
+                        destination,
+                        Rvalue::Use(Operand::Move(place)),
+                    );
+                    block.unit()
+                }
             }
             ExprKind::Use { source } => this.expr_into_dest(destination, block, source),
             ExprKind::Borrow { arg, borrow_kind } => {

compiler/rustc_span/src/symbol.rs

@@ -2150,6 +2150,7 @@ symbols! {
         unwrap_binder,
         unwrap_or,
         unwrap_unsafe_binder,
+        use_cloned,
         use_extern_macros,
         use_nested_groups,
         used,

compiler/rustc_trait_selection/src/infer.rs

@@ -47,6 +47,22 @@ impl<'tcx> InferCtxt<'tcx> {
         traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, copy_def_id)
     }
 
+    fn type_is_use_cloned_modulo_regions(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> bool {
+        let ty = self.resolve_vars_if_possible(ty);
+
+        let use_cloned_def_id = self.tcx.require_lang_item(LangItem::UseCloned, None);
+
+        // This can get called from typeck (by euv), and `moves_by_default`
+        // rightly refuses to work with inference variables, but
+        // moves_by_default has a cache, which we want to use in other
+        // cases.
+        traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, use_cloned_def_id)
+    }
+
     fn type_is_sized_modulo_regions(&self, param_env: ty::ParamEnv<'tcx>, ty: Ty<'tcx>) -> bool {
         let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
         traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, lang_item)

library/alloc/src/lib.rs

@@ -111,6 +111,7 @@
 #![feature(deprecated_suggestion)]
 #![feature(deref_pure_trait)]
 #![feature(dispatch_from_dyn)]
+#![feature(ergonomic_clones)]
 #![feature(error_generic_member_access)]
 #![feature(exact_size_is_empty)]
 #![feature(extend_one)]

library/alloc/src/rc.rs

@@ -244,7 +244,7 @@
 use core::any::Any;
 use core::cell::Cell;
 #[cfg(not(no_global_oom_handling))]
-use core::clone::CloneToUninit;
+use core::clone::{CloneToUninit, UseCloned};
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
 use core::intrinsics::abort;
@@ -2312,6 +2312,9 @@ impl<T: ?Sized, A: Allocator + Clone> Clone for Rc<T, A> {
     }
 }
 
+#[unstable(feature = "ergonomic_clones", issue = "132290")]
+impl<T: ?Sized, A: Allocator + Clone> UseCloned for Rc<T, A> {}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Rc<T> {
@@ -3484,6 +3487,9 @@ impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A> {
     }
 }
 
+#[unstable(feature = "ergonomic_clones", issue = "132290")]
+impl<T: ?Sized, A: Allocator + Clone> UseCloned for Weak<T, A> {}
+
 #[stable(feature = "rc_weak", since = "1.4.0")]
 impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

library/alloc/src/sync.rs

@@ -10,7 +10,7 @@
 
 use core::any::Any;
 #[cfg(not(no_global_oom_handling))]
-use core::clone::CloneToUninit;
+use core::clone::{CloneToUninit, UseCloned};
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
 use core::intrinsics::abort;
@@ -2172,6 +2172,9 @@ impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A> {
     }
 }
 
+#[unstable(feature = "ergonomic_clones", issue = "132290")]
+impl<T: ?Sized, A: Allocator + Clone> UseCloned for Arc<T, A> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
     type Target = T;
@@ -3143,6 +3146,9 @@ impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A> {
     }
 }
 
+#[unstable(feature = "ergonomic_clones", issue = "132290")]
+impl<T: ?Sized, A: Allocator + Clone> UseCloned for Weak<T, A> {}
+
 #[stable(feature = "downgraded_weak", since = "1.10.0")]
 impl<T> Default for Weak<T> {
     /// Constructs a new `Weak<T>`, without allocating memory.

library/core/src/clone.rs

@@ -176,6 +176,24 @@ pub trait Clone: Sized {
     }
 }
 
+/// Trait for objects whose clone impl is lightweight (e.g. reference-counted)
+///
+/// Cloning an object implementing this trait should in general:
+/// - be O(1) (constant) time regardless of the amount of data managed by the object,
+/// - not require a memory allocation,
+/// - not require copying more than roughly 64 bytes (a typical cache line size),
+/// - not block,
+/// - not have any semantic side effects (e.g. allocating a file descriptor), and
+/// - not have overhead larger than a couple of atomic operations.
+///
+/// The `Use` trait does not provide a method; instead, it indicates that
+/// `Clone::clone` is lightweight, and allows the use of the `.use` syntax.
+#[unstable(feature = "ergonomic_clones", issue = "132290")]
+#[cfg_attr(not(bootstrap), lang = "use_cloned")]
+#[rustc_diagnostic_item = "UseCloned"]
+#[rustc_trivial_field_reads]
+pub trait UseCloned: Clone {}
+
 /// Derive macro generating an impl of the trait `Clone`.
 #[rustc_builtin_macro]
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
@@ -338,6 +356,22 @@ mod impls {
         bool char
     }
 
+    macro_rules! impl_use_cloned {
+        ($($t:ty)*) => {
+            $(
+                #[unstable(feature = "ergonomic_clones", issue = "132290")]
+                impl crate::clone::UseCloned for $t {}
+            )*
+        }
+    }
+
+    impl_use_cloned! {
+        usize u8 u16 u32 u64 u128
+        isize i8 i16 i32 i64 i128
+        f16 f32 f64 f128
+        bool char
+    }
+
     #[unstable(feature = "never_type", issue = "35121")]
     impl Clone for ! {
         #[inline]

library/core/src/num/bignum.rs

@@ -405,6 +405,8 @@ macro_rules! define_bignum {
             }
         }
 
+        impl crate::clone::UseCloned for $name {}
+
         impl crate::fmt::Debug for $name {
             fn fmt(&self, f: &mut crate::fmt::Formatter<'_>) -> crate::fmt::Result {
                 let sz = if self.size < 1 { 1 } else { self.size };

library/core/src/num/nonzero.rs

@@ -1,6 +1,7 @@
 //! Definitions of integer that is known not to equal zero.
 
 use super::{IntErrorKind, ParseIntError};
+use crate::clone::UseCloned;
 use crate::cmp::Ordering;
 use crate::hash::{Hash, Hasher};
 use crate::marker::{Freeze, StructuralPartialEq};
@@ -192,6 +193,9 @@ where
     }
 }
 
+#[unstable(feature = "ergonomic_clones", issue = "132290")]
+impl<T> UseCloned for NonZero<T> where T: ZeroablePrimitive {}
+
 #[stable(feature = "nonzero", since = "1.28.0")]
 impl<T> Copy for NonZero<T> where T: ZeroablePrimitive {}
 

library/core/src/option.rs

@@ -556,6 +556,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use crate::clone::UseCloned;
 use crate::iter::{self, FusedIterator, TrustedLen};
 use crate::ops::{self, ControlFlow, Deref, DerefMut};
 use crate::panicking::{panic, panic_display};
@@ -2050,6 +2051,9 @@ where
     }
 }
 
+#[unstable(feature = "ergonomic_clones", issue = "132290")]
+impl<T> UseCloned for Option<T> where T: UseCloned {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Option<T> {
     /// Returns [`None`][Option::None].

library/core/src/result.rs

@@ -513,6 +513,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use crate::clone::UseCloned;
 use crate::iter::{self, FusedIterator, TrustedLen};
 use crate::ops::{self, ControlFlow, Deref, DerefMut};
 use crate::{convert, fmt, hint};
@@ -1744,6 +1745,14 @@ where
     }
 }
 
+#[unstable(feature = "ergonomic_clones", issue = "132290")]
+impl<T, E> UseCloned for Result<T, E>
+where
+    T: UseCloned,
+    E: UseCloned,
+{
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, E> IntoIterator for Result<T, E> {
     type Item = T;

tests/ui/ergonomic-clones/dotuse.rs

@@ -2,11 +2,18 @@
 
 #![feature(ergonomic_clones)]
 
+use std::clone::UseCloned;
+
 fn basic_test(x: i32) -> i32 {
     x.use.use.abs()
 }
 
-fn do_not_move_test(x: String) -> String {
+#[derive(Clone)]
+struct Foo;
+
+impl UseCloned for Foo {}
+
+fn do_not_move_test(x: Foo) -> Foo {
     let s = x.use;
     x
 }

tests/ui/feature-gates/feature-gate-ergonomic-clones.rs

@@ -1,19 +1,28 @@
+use std::clone::UseCloned;
+//~^ ERROR use of unstable library feature `ergonomic_clones` [E0658]
+
 fn ergonomic_clone(x: i32) -> i32 {
     x.use
     //~^ ERROR `.use` calls are experimental [E0658]
 }
 
+#[derive(Clone)]
+struct Foo;
+
+impl UseCloned for Foo {}
+//~^ ERROR use of unstable library feature `ergonomic_clones` [E0658]
+
 fn ergonomic_closure_clone() {
-    let s1 = String::from("hi!");
+    let f1 = Foo;
 
-    let s2 = use || {
+    let f2 = use || {
         //~^ ERROR `.use` calls are experimental [E0658]
-        s1
+        f1
     };
 
-    let s3 = use || {
+    let f3 = use || {
         //~^ ERROR `.use` calls are experimental [E0658]
-        s1
+        f1
     };
 }