Ergonomic ref counting: optimize away clones when possible

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -3,7 +3,7 @@ use std::iter;
 use rustc_index::IndexVec;
 use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
-use rustc_middle::mir::{Local, UnwindTerminateReason, traversal};
+use rustc_middle::mir::{Body, Local, UnwindTerminateReason, traversal};
 use rustc_middle::ty::layout::{FnAbiOf, HasTyCtxt, HasTypingEnv, TyAndLayout};
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
 use rustc_middle::{bug, mir, span_bug};
@@ -170,19 +170,29 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 ) {
     assert!(!instance.args.has_infer());
 
+    let tcx = cx.tcx();
     let llfn = cx.get_fn(instance);
 
-    let mir = cx.tcx().instance_mir(instance.def);
+    let mut mir = tcx.instance_mir(instance.def);
 
     let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
     debug!("fn_abi: {:?}", fn_abi);
 
-    if cx.tcx().codegen_fn_attrs(instance.def_id()).flags.contains(CodegenFnAttrFlags::NAKED) {
+    if tcx.codegen_fn_attrs(instance.def_id()).flags.contains(CodegenFnAttrFlags::NAKED) {
         crate::mir::naked_asm::codegen_naked_asm::<Bx>(cx, &mir, instance);
         return;
     }
 
-    let debug_context = cx.create_function_debug_context(instance, fn_abi, llfn, mir);
+    if tcx.features().ergonomic_clones() {
+        let monomorphized_mir = instance.instantiate_mir_and_normalize_erasing_regions(
+            tcx,
+            ty::TypingEnv::fully_monomorphized(),
+            ty::EarlyBinder::bind(mir.clone()),
+        );
+        mir = tcx.arena.alloc(optimize_use_clone::<Bx>(cx, monomorphized_mir));
+    }
+
+    let debug_context = cx.create_function_debug_context(instance, fn_abi, llfn, &mir);
 
     let start_llbb = Bx::append_block(cx, llfn, "start");
     let mut start_bx = Bx::build(cx, start_llbb);
@@ -194,7 +204,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 
     let cleanup_kinds =
-        base::wants_new_eh_instructions(cx.tcx().sess).then(|| analyze::cleanup_kinds(mir));
+        base::wants_new_eh_instructions(tcx.sess).then(|| analyze::cleanup_kinds(&mir));
 
     let cached_llbbs: IndexVec<mir::BasicBlock, CachedLlbb<Bx::BasicBlock>> =
         mir.basic_blocks
@@ -217,7 +227,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         cleanup_kinds,
         landing_pads: IndexVec::from_elem(None, &mir.basic_blocks),
         funclets: IndexVec::from_fn_n(|_| None, mir.basic_blocks.len()),
-        cold_blocks: find_cold_blocks(cx.tcx(), mir),
+        cold_blocks: find_cold_blocks(tcx, mir),
         locals: locals::Locals::empty(),
         debug_context,
         per_local_var_debug_info: None,
@@ -233,7 +243,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         fx.compute_per_local_var_debug_info(&mut start_bx).unzip();
     fx.per_local_var_debug_info = per_local_var_debug_info;
 
-    let traversal_order = traversal::mono_reachable_reverse_postorder(mir, cx.tcx(), instance);
+    let traversal_order = traversal::mono_reachable_reverse_postorder(mir, tcx, instance);
     let memory_locals = analyze::non_ssa_locals(&fx, &traversal_order);
 
     // Allocate variable and temp allocas
@@ -310,6 +320,61 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 }
 
+// FIXME: Move this function to mir::transform when post-mono MIR passes land.
+fn optimize_use_clone<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
+    cx: &'a Bx::CodegenCx,
+    mut mir: Body<'tcx>,
+) -> Body<'tcx> {
+    let tcx = cx.tcx();
+
+    if tcx.features().ergonomic_clones() {
+        for bb in mir.basic_blocks.as_mut() {
+            let mir::TerminatorKind::Call {
+                args,
+                destination,
+                target,
+                call_source: mir::CallSource::Use,
+                ..
+            } = &bb.terminator().kind
+            else {
+                continue;
+            };
+
+            // CallSource::Use calls always use 1 argument.
+            assert_eq!(args.len(), 1);
+            let arg = &args[0];
+
+            // These types are easily available from locals, so check that before
+            // doing DefId lookups to figure out what we're actually calling.
+            let arg_ty = arg.node.ty(&mir.local_decls, tcx);
+
+            let ty::Ref(_region, inner_ty, mir::Mutability::Not) = *arg_ty.kind() else { continue };
+
+            if !tcx.type_is_copy_modulo_regions(cx.typing_env(), inner_ty) {
+                continue;
+            }
+
+            let Some(arg_place) = arg.node.place() else { continue };
+
+            let destination_block = target.unwrap();
+
+            bb.statements.push(mir::Statement {
+                source_info: bb.terminator().source_info,
+                kind: mir::StatementKind::Assign(Box::new((
+                    *destination,
+                    mir::Rvalue::Use(mir::Operand::Copy(
+                        arg_place.project_deeper(&[mir::ProjectionElem::Deref], tcx),
+                    )),
+                ))),
+            });
+
+            bb.terminator_mut().kind = mir::TerminatorKind::Goto { target: destination_block };
+        }
+    }
+
+    mir
+}
+
 /// Produces, for each argument, a `Value` pointing at the
 /// argument's value. As arguments are places, these are always
 /// indirect.

compiler/rustc_middle/src/mir/syntax.rs

@@ -652,6 +652,8 @@ pub enum CallSource {
     /// Other types of desugaring that did not come from the HIR, but we don't care about
     /// for diagnostics (yet).
     Misc,
+    /// Use of value, generating a clone function call
+    Use,
     /// Normal function call, no special source
     Normal,
 }

compiler/rustc_mir_build/src/builder/expr/into.rs

@@ -328,7 +328,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             destination,
                             target: Some(success),
                             unwind: UnwindAction::Unreachable,
-                            call_source: CallSource::Misc,
+                            call_source: CallSource::Use,
                             fn_span: expr_span,
                         },
                     );

rustfmt.toml

@@ -53,4 +53,8 @@ ignore = [
     # Code automatically generated and included.
     "compiler/rustc_codegen_gcc/src/intrinsic/archs.rs",
     "compiler/rustc_codegen_gcc/example",
+
+    # Rustfmt doesn't support use closures yet
+    "tests/mir-opt/ergonomic-clones/closure.rs",
+    "tests/codegen/ergonomic-clones/closure.rs",
 ]

tests/codegen/ergonomic-clones/closure.rs

@@ -0,0 +1,55 @@
+//@ compile-flags: -C no-prepopulate-passes -Copt-level=0 -Zmir-opt-level=0
+
+#![crate_type = "lib"]
+
+#![feature(ergonomic_clones)]
+#![allow(incomplete_features)]
+
+use std::clone::UseCloned;
+
+pub fn ergonomic_clone_closure_move() -> String {
+    let s = String::from("hi");
+
+    // CHECK-NOT: ; call core::clone::impls::<impl core::clone::Clone for String>::clone
+    let cl = use || s;
+    cl()
+}
+
+#[derive(Clone)]
+struct Foo;
+
+impl UseCloned for Foo {}
+
+pub fn ergonomic_clone_closure_use_cloned() -> Foo {
+    let f = Foo;
+
+    // CHECK: ; call <closure::Foo as core::clone::Clone>::clone
+    let f1 = use || f;
+
+    // CHECK: ; call <closure::Foo as core::clone::Clone>::clone
+    let f2 = use || f;
+
+    f
+}
+
+pub fn ergonomic_clone_closure_copy() -> i32 {
+    let i = 1;
+
+    // CHECK-NOT: ; call core::clone::impls::<impl core::clone::Clone for i32>::clone
+    let i1 = use || i;
+
+    // CHECK-NOT: ; call core::clone::impls::<impl core::clone::Clone for i32>::clone
+    let i2 = use || i;
+
+    i
+}
+
+pub fn ergonomic_clone_closure_use_cloned_generics<T: UseCloned>(f: T) -> T {
+    // CHECK-NOT: ; call core::clone::impls::<impl core::clone::Clone for i32>::clone
+    let f1 = use || f;
+
+    // CHECK-NOT: ; call core::clone::impls::<impl core::clone::Clone for i32>::clone
+    let f2 = use || f;
+
+    f
+}

tests/mir-opt/ergonomic-clones/closure.rs

@@ -0,0 +1,55 @@
+#![crate_type = "lib"]
+#![feature(ergonomic_clones)]
+#![allow(incomplete_features)]
+
+use std::clone::UseCloned;
+
+pub fn ergonomic_clone_closure_move() -> String {
+    // CHECK-LABEL: fn ergonomic_clone_closure_move(
+    // CHECK: _0 = move (_1.0: std::string::String);
+    // CHECK-NOT: <String as Clone>::clone
+    let s = String::from("hi");
+
+    let cl = use || s;
+    cl()
+}
+
+#[derive(Clone)]
+struct Foo;
+
+impl UseCloned for Foo {}
+
+pub fn ergonomic_clone_closure_use_cloned() -> Foo {
+    // CHECK-LABEL: fn ergonomic_clone_closure_use_cloned(
+    // CHECK: <Foo as Clone>::clone
+    let f = Foo;
+
+    let f1 = use || f;
+
+    let f2 = use || f;
+
+    f
+}
+
+pub fn ergonomic_clone_closure_copy() -> i32 {
+    // CHECK-LABEL: fn ergonomic_clone_closure_copy(
+    // CHECK: _0 = copy ((*_1).0: i32);
+    // CHECK-NOT: <i32 as Clone>::clone
+    let i = 1;
+
+    let i1 = use || i;
+
+    let i2 = use || i;
+
+    i
+}
+
+pub fn ergonomic_clone_closure_use_cloned_generics<T: UseCloned>(f: T) -> T {
+    // CHECK-LABEL: fn ergonomic_clone_closure_use_cloned_generics(
+    // CHECK: <T as Clone>::clone
+    let f1 = use || f;
+
+    let f2 = use || f;
+
+    f
+}