compute temporary scopes when building MIR, not THIR

Author: dianne

compiler/rustc_middle/src/middle/region.rs

@@ -234,6 +234,17 @@ pub struct ScopeTree {
     pub backwards_incompatible_scope: UnordMap<hir::ItemLocalId, Scope>,
 }
 
+/// Temporary lifetime information for expressions, used when lowering to MIR.
+#[derive(Clone, Copy, Debug, HashStable)]
+pub struct TempLifetime {
+    /// The scope in which a temporary should be dropped. If `None`, no drop is scheduled; this is
+    /// the case for lifetime-extended temporaries extended by a const/static item or const block.
+    pub temp_lifetime: Option<Scope>,
+    /// If `Some(lt)`, indicates that the lifetime of this temporary will change to `lt` in a future edition.
+    /// If `None`, then no changes are expected, or lints are disabled.
+    pub backwards_incompatible: Option<Scope>,
+}
+
 impl ScopeTree {
     pub fn record_scope_parent(&mut self, child: Scope, parent: Option<Scope>) {
         debug!("{:?}.parent = {:?}", child, parent);
@@ -332,16 +343,16 @@ impl ScopeTree {
     /// Returns the scope when the temp created by `expr_id` will be cleaned up.
     /// It also emits a lint on potential backwards incompatible change to the temporary scope
     /// which is *for now* always shortening.
-    pub fn temporary_scope(&self, expr_id: hir::ItemLocalId) -> (Option<Scope>, Option<Scope>) {
+    pub fn temporary_scope(&self, expr_id: hir::ItemLocalId) -> TempLifetime {
         // Check for a designated extended temporary scope.
         if let Some(&s) = self.extended_temp_scopes.get(&expr_id) {
             debug!("temporary_scope({expr_id:?}) = {s:?} [custom]");
-            return (s, None);
+            return TempLifetime { temp_lifetime: s, backwards_incompatible: None };
         }
 
         // Otherwise, locate the innermost terminating scope.
-        let (scope, backward_incompatible) =
+        let (scope, backwards_incompatible) =
             self.default_temporary_scope(Scope { local_id: expr_id, data: ScopeData::Node });
-        (Some(scope), backward_incompatible)
+        TempLifetime { temp_lifetime: Some(scope), backwards_incompatible }
     }
 }

compiler/rustc_middle/src/thir.rs

@@ -256,25 +256,15 @@ pub struct Expr<'tcx> {
     /// The type of this expression
     pub ty: Ty<'tcx>,
 
-    /// The lifetime of this expression if it should be spilled into a
-    /// temporary
-    pub temp_lifetime: TempLifetime,
+    /// The id of the HIR expression whose [temporary scope] should be used for this expression.
+    ///
+    /// [temporary scope]: https://doc.rust-lang.org/reference/destructors.html#temporary-scopes
+    pub temp_scope_id: hir::ItemLocalId,
 
     /// span of the expression in the source
     pub span: Span,
 }
 
-/// Temporary lifetime information for THIR expressions
-#[derive(Clone, Copy, Debug, HashStable)]
-pub struct TempLifetime {
-    /// Lifetime for temporaries as expected.
-    /// This should be `None` in a constant context.
-    pub temp_lifetime: Option<region::Scope>,
-    /// If `Some(lt)`, indicates that the lifetime of this temporary will change to `lt` in a future edition.
-    /// If `None`, then no changes are expected, or lints are disabled.
-    pub backwards_incompatible: Option<region::Scope>,
-}
-
 #[derive(Clone, Debug, HashStable)]
 pub enum ExprKind<'tcx> {
     /// `Scope`s are used to explicitly mark destruction scopes,
@@ -1127,7 +1117,7 @@ mod size_asserts {
     use super::*;
     // tidy-alphabetical-start
     static_assert_size!(Block, 48);
-    static_assert_size!(Expr<'_>, 72);
+    static_assert_size!(Expr<'_>, 64);
     static_assert_size!(ExprKind<'_>, 40);
     static_assert_size!(Pat<'_>, 64);
     static_assert_size!(PatKind<'_>, 48);

compiler/rustc_middle/src/thir/visit.rs

@@ -45,7 +45,7 @@ pub fn walk_expr<'thir, 'tcx: 'thir, V: Visitor<'thir, 'tcx>>(
     expr: &'thir Expr<'tcx>,
 ) {
     use ExprKind::*;
-    let Expr { kind, ty: _, temp_lifetime: _, span: _ } = expr;
+    let Expr { kind, ty: _, temp_scope_id: _, span: _ } = expr;
     match *kind {
         Scope { value, region_scope: _, lint_level: _ } => {
             visitor.visit_expr(&visitor.thir()[value])

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

@@ -21,7 +21,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub(crate) fn as_constant(&mut self, expr: &Expr<'tcx>) -> ConstOperand<'tcx> {
         let this = self;
         let tcx = this.tcx;
-        let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
+        let Expr { ty, temp_scope_id: _, span, ref kind } = *expr;
         match kind {
             ExprKind::Scope { region_scope: _, lint_level: _, value } => {
                 this.as_constant(&this.thir[*value])
@@ -46,7 +46,7 @@ pub(crate) fn as_constant_inner<'tcx>(
     push_cuta: impl FnMut(&Box<CanonicalUserType<'tcx>>) -> Option<UserTypeAnnotationIndex>,
     tcx: TyCtxt<'tcx>,
 ) -> ConstOperand<'tcx> {
-    let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
+    let Expr { ty, temp_scope_id: _, span, ref kind } = *expr;
     match *kind {
         ExprKind::Literal { lit, neg } => {
             let const_ = lit_to_mir_constant(tcx, LitToConstInput { lit: lit.node, ty, neg });

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

@@ -1,5 +1,6 @@
 //! See docs in build/expr/mod.rs
 
+use rustc_middle::middle::region::TempLifetime;
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use tracing::{debug, instrument};

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

@@ -503,10 +503,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.and(place_builder)
             }
             ExprKind::ValueTypeAscription { source, ref user_ty, user_ty_span } => {
-                let source_expr = &this.thir[source];
-                let temp = unpack!(
-                    block = this.as_temp(block, source_expr.temp_lifetime, source, mutability)
-                );
+                let temp_lifetime =
+                    this.region_scope_tree.temporary_scope(this.thir[source].temp_scope_id);
+                let temp = unpack!(block = this.as_temp(block, temp_lifetime, source, mutability));
                 if let Some(user_ty) = user_ty {
                     let ty_source_info = this.source_info(user_ty_span);
                     let annotation_index =
@@ -539,10 +538,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.and(place_builder.project(PlaceElem::UnwrapUnsafeBinder(expr.ty)))
             }
             ExprKind::ValueUnwrapUnsafeBinder { source } => {
-                let source_expr = &this.thir[source];
-                let temp = unpack!(
-                    block = this.as_temp(block, source_expr.temp_lifetime, source, mutability)
-                );
+                let temp_lifetime =
+                    this.region_scope_tree.temporary_scope(this.thir[source].temp_scope_id);
+                let temp = unpack!(block = this.as_temp(block, temp_lifetime, source, mutability));
                 block.and(PlaceBuilder::from(temp).project(PlaceElem::UnwrapUnsafeBinder(expr.ty)))
             }
 
@@ -590,8 +588,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::WrapUnsafeBinder { .. } => {
                 // these are not places, so we need to make a temporary.
                 debug_assert!(!matches!(Category::of(&expr.kind), Some(Category::Place)));
-                let temp =
-                    unpack!(block = this.as_temp(block, expr.temp_lifetime, expr_id, mutability));
+                let temp_lifetime = this.region_scope_tree.temporary_scope(expr.temp_scope_id);
+                let temp = unpack!(block = this.as_temp(block, temp_lifetime, expr_id, mutability));
                 block.and(PlaceBuilder::from(temp))
             }
         }
@@ -637,7 +635,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // Making this a *fresh* temporary means we do not have to worry about
         // the index changing later: Nothing will ever change this temporary.
         // The "retagging" transformation (for Stacked Borrows) relies on this.
-        let index_lifetime = self.thir[index].temp_lifetime;
+        // Using the enclosing temporary scope for the index ensures it will live past where this
+        // place is used. This lifetime may be larger than strictly necessary but it means we don't
+        // need to pass a scope for operands to `as_place`.
+        let index_lifetime = self.region_scope_tree.temporary_scope(self.thir[index].temp_scope_id);
         let idx = unpack!(block = self.as_temp(block, index_lifetime, index, Mutability::Not));
 
         block = self.bounds_check(block, &base_place, idx, expr_span, source_info);

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

@@ -4,7 +4,7 @@ use rustc_abi::FieldIdx;
 use rustc_hir::lang_items::LangItem;
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::bug;
-use rustc_middle::middle::region;
+use rustc_middle::middle::region::{self, TempLifetime};
 use rustc_middle::mir::interpret::Scalar;
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;

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

@@ -2,7 +2,7 @@
 
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::HirId;
-use rustc_middle::middle::region::{Scope, ScopeData};
+use rustc_middle::middle::region::{Scope, ScopeData, TempLifetime};
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use tracing::{debug, instrument};

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

@@ -1,4 +1,4 @@
-use rustc_middle::middle::region;
+use rustc_middle::middle::region::{self, TempLifetime};
 use rustc_middle::mir::*;
 use rustc_middle::span_bug;
 use rustc_middle::thir::*;

compiler/rustc_mir_build/src/builder/matches/mod.rs

@@ -15,7 +15,7 @@ use rustc_abi::{FIRST_VARIANT, FieldIdx, VariantIdx};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::{BindingMode, ByRef, LangItem, LetStmt, LocalSource, Node, Pinnedness};
-use rustc_middle::middle::region;
+use rustc_middle::middle::region::{self, TempLifetime};
 use rustc_middle::mir::*;
 use rustc_middle::thir::{self, *};
 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty, ValTree, ValTreeKind};