Generate an intermediate temporary for `Drop` constants.

To limit the fallout from this, don't do this for the last (or only) operand in an rvalue.

compiler/rustc_mir_build/src/build/expr/as_operand.rs

 //! See docs in build/expr/mod.rs
 
 use crate::build::expr::category::Category;
-use crate::build::{BlockAnd, BlockAndExtension, Builder};
+use crate::build::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
 use rustc_middle::middle::region;
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
@@ -20,7 +20,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         expr: &Expr<'tcx>,
     ) -> BlockAnd<Operand<'tcx>> {
         let local_scope = self.local_scope();
-        self.as_operand(block, Some(local_scope), expr, None)
+        self.as_operand(block, Some(local_scope), expr, None, NeedsTemporary::Maybe)
     }
 
     /// Returns an operand suitable for use until the end of the current scope expression and
@@ -94,32 +94,33 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     ///
     /// Like `as_local_call_operand`, except that the argument will
     /// not be valid once `scope` ends.
+    #[instrument(level = "debug", skip(self, scope))]
     crate fn as_operand(
         &mut self,
         mut block: BasicBlock,
         scope: Option<region::Scope>,
         expr: &Expr<'tcx>,
         local_info: Option<Box<LocalInfo<'tcx>>>,
+        needs_temporary: NeedsTemporary,
     ) -> BlockAnd<Operand<'tcx>> {
-        debug!("as_operand(block={:?}, expr={:?} local_info={:?})", block, expr, local_info);
         let this = self;
 
         if let ExprKind::Scope { region_scope, lint_level, value } = expr.kind {
             let source_info = this.source_info(expr.span);
             let region_scope = (region_scope, source_info);
             return this.in_scope(region_scope, lint_level, |this| {
-                this.as_operand(block, scope, &this.thir[value], local_info)
+                this.as_operand(block, scope, &this.thir[value], local_info, needs_temporary)
             });
         }
 
         let category = Category::of(&expr.kind).unwrap();
-        debug!("as_operand: category={:?} for={:?}", category, expr.kind);
+        debug!(?category, ?expr.kind);
         match category {
-            Category::Constant => {
+            Category::Constant if let NeedsTemporary::No = needs_temporary || !expr.ty.needs_drop(this.tcx, this.param_env) => {
                 let constant = this.as_constant(expr);
                 block.and(Operand::Constant(Box::new(constant)))
             }
-            Category::Place | Category::Rvalue(..) => {
+            Category::Constant | Category::Place | Category::Rvalue(..) => {
                 let operand = unpack!(block = this.as_temp(block, scope, expr, Mutability::Mut));
                 if this.local_decls[operand].local_info.is_none() {
                     this.local_decls[operand].local_info = local_info;
@@ -176,6 +177,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
         }
 
-        this.as_operand(block, scope, expr, None)
+        this.as_operand(block, scope, expr, None, NeedsTemporary::Maybe)
     }
 }

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -4,7 +4,7 @@
 
 use crate::build::expr::as_place::PlaceBase;
 use crate::build::expr::category::{Category, RvalueFunc};
-use crate::build::{BlockAnd, BlockAndExtension, Builder};
+use crate::build::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
 use rustc_hir::lang_items::LangItem;
 use rustc_middle::middle::region;
 use rustc_middle::mir::AssertKind;
@@ -52,17 +52,28 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 })
             }
             ExprKind::Repeat { value, count } => {
-                let value_operand =
-                    unpack!(block = this.as_operand(block, scope, &this.thir[value], None));
+                let value_operand = unpack!(
+                    block =
+                        this.as_operand(block, scope, &this.thir[value], None, NeedsTemporary::No)
+                );
                 block.and(Rvalue::Repeat(value_operand, count))
             }
             ExprKind::Binary { op, lhs, rhs } => {
-                let lhs = unpack!(block = this.as_operand(block, scope, &this.thir[lhs], None));
-                let rhs = unpack!(block = this.as_operand(block, scope, &this.thir[rhs], None));
+                let lhs = unpack!(
+                    block =
+                        this.as_operand(block, scope, &this.thir[lhs], None, NeedsTemporary::Maybe)
+                );
+                let rhs = unpack!(
+                    block =
+                        this.as_operand(block, scope, &this.thir[rhs], None, NeedsTemporary::No)
+                );
                 this.build_binary_op(block, op, expr_span, expr.ty, lhs, rhs)
             }
             ExprKind::Unary { op, arg } => {
-                let arg = unpack!(block = this.as_operand(block, scope, &this.thir[arg], None));
+                let arg = unpack!(
+                    block =
+                        this.as_operand(block, scope, &this.thir[arg], None, NeedsTemporary::No)
+                );
                 // Check for -MIN on signed integers
                 if this.check_overflow && op == UnOp::Neg && expr.ty.is_signed() {
                     let bool_ty = this.tcx.types.bool;
@@ -167,13 +178,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.and(Rvalue::Use(Operand::Move(Place::from(result))))
             }
             ExprKind::Cast { source } => {
-                let source =
-                    unpack!(block = this.as_operand(block, scope, &this.thir[source], None));
+                let source = unpack!(
+                    block =
+                        this.as_operand(block, scope, &this.thir[source], None, NeedsTemporary::No)
+                );
                 block.and(Rvalue::Cast(CastKind::Misc, source, expr.ty))
             }
             ExprKind::Pointer { cast, source } => {
-                let source =
-                    unpack!(block = this.as_operand(block, scope, &this.thir[source], None));
+                let source = unpack!(
+                    block =
+                        this.as_operand(block, scope, &this.thir[source], None, NeedsTemporary::No)
+                );
                 block.and(Rvalue::Cast(CastKind::Pointer(cast), source, expr.ty))
             }
             ExprKind::Array { ref fields } => {
@@ -208,7 +223,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let fields: Vec<_> = fields
                     .into_iter()
                     .copied()
-                    .map(|f| unpack!(block = this.as_operand(block, scope, &this.thir[f], None)))
+                    .map(|f| {
+                        unpack!(
+                            block = this.as_operand(
+                                block,
+                                scope,
+                                &this.thir[f],
+                                None,
+                                NeedsTemporary::Maybe
+                            )
+                        )
+                    })
                     .collect();
 
                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Array(el_ty)), fields))
@@ -219,7 +244,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let fields: Vec<_> = fields
                     .into_iter()
                     .copied()
-                    .map(|f| unpack!(block = this.as_operand(block, scope, &this.thir[f], None)))
+                    .map(|f| {
+                        unpack!(
+                            block = this.as_operand(
+                                block,
+                                scope,
+                                &this.thir[f],
+                                None,
+                                NeedsTemporary::Maybe
+                            )
+                        )
+                    })
                     .collect();
 
                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Tuple), fields))
@@ -296,7 +331,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                         )
                                     ),
                                     _ => {
-                                        unpack!(block = this.as_operand(block, scope, upvar, None))
+                                        unpack!(
+                                            block = this.as_operand(
+                                                block,
+                                                scope,
+                                                upvar,
+                                                None,
+                                                NeedsTemporary::Maybe
+                                            )
+                                        )
                                     }
                                 }
                             }
@@ -325,13 +368,18 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     literal: ConstantKind::zero_sized(this.tcx.types.unit),
                 }))))
             }
-            ExprKind::Yield { .. }
-            | ExprKind::Literal { .. }
+
+            ExprKind::Literal { .. }
             | ExprKind::NamedConst { .. }
             | ExprKind::NonHirLiteral { .. }
             | ExprKind::ConstParam { .. }
             | ExprKind::ConstBlock { .. }
-            | ExprKind::StaticRef { .. }
+            | ExprKind::StaticRef { .. } => {
+                let constant = this.as_constant(expr);
+                block.and(Rvalue::Use(Operand::Constant(Box::new(constant))))
+            }
+
+            ExprKind::Yield { .. }
             | ExprKind::Block { .. }
             | ExprKind::Match { .. }
             | ExprKind::If { .. }
@@ -359,9 +407,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // so make an operand and then return that
                 debug_assert!(!matches!(
                     Category::of(&expr.kind),
-                    Some(Category::Rvalue(RvalueFunc::AsRvalue))
+                    Some(Category::Rvalue(RvalueFunc::AsRvalue) | Category::Constant)
                 ));
-                let operand = unpack!(block = this.as_operand(block, scope, expr, None));
+                let operand =
+                    unpack!(block = this.as_operand(block, scope, expr, None, NeedsTemporary::No));
                 block.and(Rvalue::Use(operand))
             }
         }

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

 //! See docs in build/expr/mod.rs
 
 use crate::build::expr::category::{Category, RvalueFunc};
-use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
+use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder, NeedsTemporary};
 use rustc_ast::InlineAsmOptions;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
@@ -329,7 +329,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                     block,
                                     Some(scope),
                                     &this.thir[f.expr],
-                                    Some(local_info)
+                                    Some(local_info),
+                                    NeedsTemporary::Maybe,
                                 )
                             ),
                         )
@@ -516,8 +517,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
             ExprKind::Yield { value } => {
                 let scope = this.local_scope();
-                let value =
-                    unpack!(block = this.as_operand(block, Some(scope), &this.thir[value], None));
+                let value = unpack!(
+                    block = this.as_operand(
+                        block,
+                        Some(scope),
+                        &this.thir[value],
+                        None,
+                        NeedsTemporary::No
+                    )
+                );
                 let resume = this.cfg.start_new_block();
                 this.cfg.terminate(
                     block,

compiler/rustc_mir_build/src/build/mod.rs

@@ -549,6 +549,18 @@ struct CFG<'tcx> {
     struct ScopeId { .. }
 }
 
+#[derive(Debug)]
+enum NeedsTemporary {
+    /// Use this variant when whatever you are converting with `as_operand`
+    /// is the last thing you are converting. This means that if we introduced
+    /// an intermediate temporary, we'd only read it immediately after, so we can
+    /// also avoid it.
+    No,
+    /// For all cases where you aren't sure or that are too expensive to compute
+    /// for now. It is always safe to fall back to this.
+    Maybe,
+}
+
 ///////////////////////////////////////////////////////////////////////////
 /// The `BlockAnd` "monad" packages up the new basic block along with a
 /// produced value (sometimes just unit, of course). The `unpack!`

compiler/rustc_mir_build/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(crate_visibility_modifier)]
+#![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]

src/test/mir-opt/inline/inline_into_box_place.main.Inline.32bit.diff

@@ -16,6 +16,7 @@
       scope 2 {
       }
 +     scope 3 (inlined Vec::<u32>::new) {  // at $DIR/inline-into-box-place.rs:8:33: 8:43
++         let mut _8: alloc::raw_vec::RawVec<u32>; // in scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 +     }
   
       bb0: {
@@ -34,8 +35,8 @@
 -         (*_5) = Vec::<u32>::new() -> [return: bb2, unwind: bb5]; // scope 0 at $DIR/inline-into-box-place.rs:8:33: 8:43
 +         StorageLive(_7);                 // scope 0 at $DIR/inline-into-box-place.rs:8:33: 8:43
 +         _7 = &mut (*_5);                 // scope 0 at $DIR/inline-into-box-place.rs:8:33: 8:43
-+         Deinit((*_7));                   // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
-+         ((*_7).0: alloc::raw_vec::RawVec<u32>) = const alloc::raw_vec::RawVec::<u32> { ptr: Unique::<u32> { pointer: NonNull::<u32> { pointer: {0x4 as *const u32} }, _marker: PhantomData::<u32> }, cap: 0_usize, alloc: std::alloc::Global }; // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
++         StorageLive(_8);                 // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
++         _8 = const alloc::raw_vec::RawVec::<u32> { ptr: Unique::<u32> { pointer: NonNull::<u32> { pointer: {0x4 as *const u32} }, _marker: PhantomData::<u32> }, cap: 0_usize, alloc: std::alloc::Global }; // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
                                            // mir::Constant
 -                                          // + span: $DIR/inline-into-box-place.rs:8:33: 8:41
 -                                          // + user_ty: UserType(1)
@@ -46,7 +47,10 @@
 +                                          // + span: $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 +                                          // + user_ty: UserType(0)
 +                                          // + literal: Const { ty: alloc::raw_vec::RawVec<u32>, val: Value(ByRef { alloc: Allocation { bytes: [4, 0, 0, 0, 0, 0, 0, 0], relocations: Relocations(SortedMap { data: [] }), init_mask: InitMask { blocks: [255], len: Size { raw: 8 } }, align: Align { pow2: 2 }, mutability: Not, extra: () }, offset: Size { raw: 0 } }) }
++         Deinit((*_7));                   // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
++         ((*_7).0: alloc::raw_vec::RawVec<u32>) = move _8; // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 +         ((*_7).1: usize) = const 0_usize; // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
++         StorageDead(_8);                 // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 +         StorageDead(_7);                 // scope 0 at $DIR/inline-into-box-place.rs:8:33: 8:43
           _1 = move _5;                    // scope 0 at $DIR/inline-into-box-place.rs:8:29: 8:43
           StorageDead(_5);                 // scope 0 at $DIR/inline-into-box-place.rs:8:42: 8:43

src/test/mir-opt/inline/inline_into_box_place.main.Inline.64bit.diff

@@ -16,6 +16,7 @@
       scope 2 {
       }
 +     scope 3 (inlined Vec::<u32>::new) {  // at $DIR/inline-into-box-place.rs:8:33: 8:43
++         let mut _8: alloc::raw_vec::RawVec<u32>; // in scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 +     }
   
       bb0: {
@@ -34,8 +35,8 @@
 -         (*_5) = Vec::<u32>::new() -> [return: bb2, unwind: bb5]; // scope 0 at $DIR/inline-into-box-place.rs:8:33: 8:43
 +         StorageLive(_7);                 // scope 0 at $DIR/inline-into-box-place.rs:8:33: 8:43
 +         _7 = &mut (*_5);                 // scope 0 at $DIR/inline-into-box-place.rs:8:33: 8:43
-+         Deinit((*_7));                   // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
-+         ((*_7).0: alloc::raw_vec::RawVec<u32>) = const alloc::raw_vec::RawVec::<u32> { ptr: Unique::<u32> { pointer: NonNull::<u32> { pointer: {0x4 as *const u32} }, _marker: PhantomData::<u32> }, cap: 0_usize, alloc: std::alloc::Global }; // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
++         StorageLive(_8);                 // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
++         _8 = const alloc::raw_vec::RawVec::<u32> { ptr: Unique::<u32> { pointer: NonNull::<u32> { pointer: {0x4 as *const u32} }, _marker: PhantomData::<u32> }, cap: 0_usize, alloc: std::alloc::Global }; // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
                                            // mir::Constant
 -                                          // + span: $DIR/inline-into-box-place.rs:8:33: 8:41
 -                                          // + user_ty: UserType(1)
@@ -46,7 +47,10 @@
 +                                          // + span: $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 +                                          // + user_ty: UserType(0)
 +                                          // + literal: Const { ty: alloc::raw_vec::RawVec<u32>, val: Value(ByRef { alloc: Allocation { bytes: [4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], relocations: Relocations(SortedMap { data: [] }), init_mask: InitMask { blocks: [65535], len: Size { raw: 16 } }, align: Align { pow2: 3 }, mutability: Not, extra: () }, offset: Size { raw: 0 } }) }
++         Deinit((*_7));                   // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
++         ((*_7).0: alloc::raw_vec::RawVec<u32>) = move _8; // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 +         ((*_7).1: usize) = const 0_usize; // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
++         StorageDead(_8);                 // scope 3 at $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 +         StorageDead(_7);                 // scope 0 at $DIR/inline-into-box-place.rs:8:33: 8:43
           _1 = move _5;                    // scope 0 at $DIR/inline-into-box-place.rs:8:29: 8:43
           StorageDead(_5);                 // scope 0 at $DIR/inline-into-box-place.rs:8:42: 8:43

src/test/ui/consts/issue-90762.rs

 // run-pass
 #![allow(unreachable_code)]
 
-use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::atomic::{AtomicBool, Ordering, AtomicUsize};
 
 struct Print(usize);
 
 impl Drop for Print {
     fn drop(&mut self) {
+        println!("{}", self.0);
         FOO[self.0].store(true, Ordering::Relaxed);
+        assert_eq!(BAR.fetch_sub(1, Ordering::Relaxed), self.0);
     }
 }
 
 const A: Print = Print(0);
 const B: Print = Print(1);
 
-static FOO: [AtomicBool; 3] = [AtomicBool::new(false), AtomicBool::new(false), AtomicBool::new(false)];
+static FOO: [AtomicBool; 3] =
+    [AtomicBool::new(false), AtomicBool::new(false), AtomicBool::new(false)];
+static BAR: AtomicUsize = AtomicUsize::new(2);
 
 fn main() {
     loop {
-        std::mem::forget(({A}, B, Print(2), break));
+        std::mem::forget(({ A }, B, Print(2), break));
     }
     for (i, b) in FOO.iter().enumerate() {
         assert!(b.load(Ordering::Relaxed), "{} not set", i);
     }
-}
\ No newline at end of file
+    assert_eq!(BAR.fetch_add(1, Ordering::Relaxed), usize::max_value());
+}