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Desugars drop and replace at MIR build
This commit desugars the drop and replace deriving from an assignment at MIR build, avoiding the construction of the DropAndReplace terminator (which will be removed in a followign PR) In order to retain the same error messages for replaces a new DesugaringKind::Replace variant is introduced.
1 parent 13471d3 commit b3a47d9

20 files changed

+268
-152
lines changed

‎compiler/rustc_borrowck/src/diagnostics/mod.rs

+14-1
Original file line numberDiff line numberDiff line change
@@ -925,7 +925,20 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
925925
return OtherUse(use_span);
926926
}
927927

928-
for stmt in &self.body[location.block].statements[location.statement_index + 1..] {
928+
// drop and replace might have moved the assignment to the next block
929+
let maybe_additional_statement =
930+
if let TerminatorKind::Drop { target: drop_target, .. } =
931+
self.body[location.block].terminator().kind
932+
{
933+
self.body[drop_target].statements.first()
934+
} else {
935+
None
936+
};
937+
938+
let statements =
939+
self.body[location.block].statements[location.statement_index + 1..].iter();
940+
941+
for stmt in statements.chain(maybe_additional_statement) {
929942
if let StatementKind::Assign(box (_, Rvalue::Aggregate(kind, places))) = &stmt.kind {
930943
let (&def_id, is_generator) = match kind {
931944
box AggregateKind::Closure(def_id, _) => (def_id, false),

‎compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

+7-1
Original file line numberDiff line numberDiff line change
@@ -828,7 +828,13 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
828828
let Some(hir::Node::Item(item)) = node else { return; };
829829
let hir::ItemKind::Fn(.., body_id) = item.kind else { return; };
830830
let body = self.infcx.tcx.hir().body(body_id);
831-
let mut v = V { assign_span: span, err, ty, suggested: false };
831+
let mut assign_span = span;
832+
// Drop desugaring is done at MIR build so it's not in the HIR
833+
if let Some(DesugaringKind::Replace) = span.desugaring_kind() {
834+
assign_span.remove_mark();
835+
}
836+
837+
let mut v = V { assign_span, err, ty, suggested: false };
832838
v.visit_body(body);
833839
if !v.suggested {
834840
err.help(&format!(

‎compiler/rustc_borrowck/src/lib.rs

+17-8
Original file line numberDiff line numberDiff line change
@@ -40,7 +40,7 @@ use rustc_middle::mir::{InlineAsmOperand, Terminator, TerminatorKind};
4040
use rustc_middle::ty::query::Providers;
4141
use rustc_middle::ty::{self, CapturedPlace, ParamEnv, RegionVid, TyCtxt};
4242
use rustc_session::lint::builtin::UNUSED_MUT;
43-
use rustc_span::{Span, Symbol};
43+
use rustc_span::{DesugaringKind, Span, Symbol};
4444

4545
use either::Either;
4646
use smallvec::SmallVec;
@@ -1184,13 +1184,22 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
11841184
this.report_conflicting_borrow(location, place_span, bk, borrow);
11851185
this.buffer_error(err);
11861186
}
1187-
WriteKind::StorageDeadOrDrop => this
1188-
.report_borrowed_value_does_not_live_long_enough(
1189-
location,
1190-
borrow,
1191-
place_span,
1192-
Some(kind),
1193-
),
1187+
WriteKind::StorageDeadOrDrop => {
1188+
if let Some(DesugaringKind::Replace) = place_span.1.desugaring_kind() {
1189+
// If this is a drop triggered by a reassignment, it's more user friendly
1190+
// to report a problem with the explicit assignment than the implicit drop.
1191+
this.report_illegal_mutation_of_borrowed(
1192+
location, place_span, borrow,
1193+
)
1194+
} else {
1195+
this.report_borrowed_value_does_not_live_long_enough(
1196+
location,
1197+
borrow,
1198+
place_span,
1199+
Some(kind),
1200+
)
1201+
}
1202+
}
11941203
WriteKind::Mutate => {
11951204
this.report_illegal_mutation_of_borrowed(location, place_span, borrow)
11961205
}

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

+1-1
Original file line numberDiff line numberDiff line change
@@ -40,7 +40,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
4040
// Generate better code for things that don't need to be
4141
// dropped.
4242
if lhs.ty.needs_drop(this.tcx, this.param_env) {
43-
let rhs = unpack!(block = this.as_local_operand(block, rhs));
43+
let rhs = unpack!(block = this.as_local_rvalue(block, rhs));
4444
let lhs = unpack!(block = this.as_place(block, lhs));
4545
unpack!(block = this.build_drop_and_replace(block, lhs_span, lhs, rhs));
4646
} else {

‎compiler/rustc_mir_build/src/build/scope.rs

+25-7
Original file line numberDiff line numberDiff line change
@@ -91,7 +91,7 @@ use rustc_middle::middle::region;
9191
use rustc_middle::mir::*;
9292
use rustc_middle::thir::{Expr, LintLevel};
9393

94-
use rustc_span::{Span, DUMMY_SP};
94+
use rustc_span::{DesugaringKind, Span, DUMMY_SP};
9595

9696
#[derive(Debug)]
9797
pub struct Scopes<'tcx> {
@@ -1118,24 +1118,35 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
11181118
}
11191119

11201120
/// Utility function for *non*-scope code to build their own drops
1121+
/// Force a drop at this point in the MIR by creating a new block.
11211122
pub(crate) fn build_drop_and_replace(
11221123
&mut self,
11231124
block: BasicBlock,
11241125
span: Span,
11251126
place: Place<'tcx>,
1126-
value: Operand<'tcx>,
1127+
value: Rvalue<'tcx>,
11271128
) -> BlockAnd<()> {
1129+
let span = self.tcx.with_stable_hashing_context(|hcx| {
1130+
span.mark_with_reason(None, DesugaringKind::Replace, self.tcx.sess.edition(), hcx)
1131+
});
11281132
let source_info = self.source_info(span);
1129-
let next_target = self.cfg.start_new_block();
1133+
1134+
// create the new block for the assignment
1135+
let assign = self.cfg.start_new_block();
1136+
self.cfg.push_assign(assign, source_info, place, value.clone());
1137+
1138+
// create the new block for the assignment in the case of unwinding
1139+
let assign_unwind = self.cfg.start_new_cleanup_block();
1140+
self.cfg.push_assign(assign_unwind, source_info, place, value.clone());
11301141

11311142
self.cfg.terminate(
11321143
block,
11331144
source_info,
1134-
TerminatorKind::DropAndReplace { place, value, target: next_target, unwind: None },
1145+
TerminatorKind::Drop { place, target: assign, unwind: Some(assign_unwind) },
11351146
);
11361147
self.diverge_from(block);
11371148

1138-
next_target.unit()
1149+
assign.unit()
11391150
}
11401151

11411152
/// Creates an `Assert` terminator and return the success block.
@@ -1413,8 +1424,15 @@ impl<'tcx> DropTreeBuilder<'tcx> for Unwind {
14131424
fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
14141425
let term = &mut cfg.block_data_mut(from).terminator_mut();
14151426
match &mut term.kind {
1416-
TerminatorKind::Drop { unwind, .. }
1417-
| TerminatorKind::DropAndReplace { unwind, .. }
1427+
TerminatorKind::Drop { unwind, .. } => {
1428+
if let Some(unwind) = *unwind {
1429+
let source_info = term.source_info;
1430+
cfg.terminate(unwind, source_info, TerminatorKind::Goto { target: to });
1431+
} else {
1432+
*unwind = Some(to);
1433+
}
1434+
}
1435+
TerminatorKind::DropAndReplace { unwind, .. }
14181436
| TerminatorKind::FalseUnwind { unwind, .. }
14191437
| TerminatorKind::Call { cleanup: unwind, .. }
14201438
| TerminatorKind::Assert { cleanup: unwind, .. }

‎compiler/rustc_mir_transform/src/elaborate_drops.rs

+14-5
Original file line numberDiff line numberDiff line change
@@ -14,7 +14,7 @@ use rustc_mir_dataflow::un_derefer::UnDerefer;
1414
use rustc_mir_dataflow::MoveDataParamEnv;
1515
use rustc_mir_dataflow::{on_all_children_bits, on_all_drop_children_bits};
1616
use rustc_mir_dataflow::{Analysis, ResultsCursor};
17-
use rustc_span::Span;
17+
use rustc_span::{DesugaringKind, Span};
1818
use rustc_target::abi::VariantIdx;
1919
use std::fmt;
2020

@@ -425,10 +425,19 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
425425
bb,
426426
),
427427
LookupResult::Parent(..) => {
428-
self.tcx.sess.delay_span_bug(
429-
terminator.source_info.span,
430-
&format!("drop of untracked value {:?}", bb),
431-
);
428+
if !matches!(
429+
terminator.source_info.span.desugaring_kind(),
430+
Some(DesugaringKind::Replace),
431+
) {
432+
self.tcx.sess.delay_span_bug(
433+
terminator.source_info.span,
434+
&format!("drop of untracked value {:?}", bb),
435+
);
436+
}
437+
// A drop and replace behind a pointer/array/whatever.
438+
// The borrow checker requires that these locations are initialized before the assignment,
439+
// so we just leave an unconditional drop.
440+
assert!(!data.is_cleanup);
432441
}
433442
}
434443
}

‎compiler/rustc_span/src/hygiene.rs

+2
Original file line numberDiff line numberDiff line change
@@ -1151,6 +1151,7 @@ pub enum DesugaringKind {
11511151
Await,
11521152
ForLoop,
11531153
WhileLoop,
1154+
Replace,
11541155
}
11551156

11561157
impl DesugaringKind {
@@ -1166,6 +1167,7 @@ impl DesugaringKind {
11661167
DesugaringKind::OpaqueTy => "`impl Trait`",
11671168
DesugaringKind::ForLoop => "`for` loop",
11681169
DesugaringKind::WhileLoop => "`while` loop",
1170+
DesugaringKind::Replace => "drop and replace",
11691171
}
11701172
}
11711173
}

‎tests/mir-opt/basic_assignment.main.ElaborateDrops.diff

+85
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,85 @@
1+
- // MIR for `main` before ElaborateDrops
2+
+ // MIR for `main` after ElaborateDrops
3+
4+
fn main() -> () {
5+
let mut _0: (); // return place in scope 0 at $DIR/basic_assignment.rs:+0:11: +0:11
6+
let _1: bool; // in scope 0 at $DIR/basic_assignment.rs:+1:9: +1:17
7+
let mut _3: bool; // in scope 0 at $DIR/basic_assignment.rs:+6:16: +6:24
8+
let mut _6: std::option::Option<std::boxed::Box<u32>>; // in scope 0 at $DIR/basic_assignment.rs:+13:14: +13:20
9+
scope 1 {
10+
debug nodrop_x => _1; // in scope 1 at $DIR/basic_assignment.rs:+1:9: +1:17
11+
let _2: bool; // in scope 1 at $DIR/basic_assignment.rs:+2:9: +2:17
12+
scope 2 {
13+
debug nodrop_y => _2; // in scope 2 at $DIR/basic_assignment.rs:+2:9: +2:17
14+
let _4: std::option::Option<std::boxed::Box<u32>>; // in scope 2 at $DIR/basic_assignment.rs:+8:9: +8:15
15+
scope 3 {
16+
debug drop_x => _4; // in scope 3 at $DIR/basic_assignment.rs:+8:9: +8:15
17+
let _5: std::option::Option<std::boxed::Box<u32>>; // in scope 3 at $DIR/basic_assignment.rs:+9:9: +9:15
18+
scope 4 {
19+
debug drop_y => _5; // in scope 4 at $DIR/basic_assignment.rs:+9:9: +9:15
20+
}
21+
}
22+
}
23+
}
24+
25+
bb0: {
26+
StorageLive(_1); // scope 0 at $DIR/basic_assignment.rs:+1:9: +1:17
27+
_1 = const false; // scope 0 at $DIR/basic_assignment.rs:+1:20: +1:25
28+
StorageLive(_2); // scope 1 at $DIR/basic_assignment.rs:+2:9: +2:17
29+
StorageLive(_3); // scope 2 at $DIR/basic_assignment.rs:+6:16: +6:24
30+
_3 = _1; // scope 2 at $DIR/basic_assignment.rs:+6:16: +6:24
31+
_2 = move _3; // scope 2 at $DIR/basic_assignment.rs:+6:5: +6:24
32+
StorageDead(_3); // scope 2 at $DIR/basic_assignment.rs:+6:23: +6:24
33+
StorageLive(_4); // scope 2 at $DIR/basic_assignment.rs:+8:9: +8:15
34+
_4 = Option::<Box<u32>>::None; // scope 2 at $DIR/basic_assignment.rs:+8:36: +8:40
35+
StorageLive(_5); // scope 3 at $DIR/basic_assignment.rs:+9:9: +9:15
36+
StorageLive(_6); // scope 4 at $DIR/basic_assignment.rs:+13:14: +13:20
37+
_6 = move _4; // scope 4 at $DIR/basic_assignment.rs:+13:14: +13:20
38+
- drop(_5) -> [return: bb1, unwind: bb2]; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
39+
+ goto -> bb1; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
40+
}
41+
42+
bb1: {
43+
_5 = move _6; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
44+
- drop(_6) -> [return: bb3, unwind: bb6]; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
45+
+ goto -> bb3; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
46+
}
47+
48+
bb2 (cleanup): {
49+
_5 = move _6; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
50+
drop(_6) -> bb6; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
51+
}
52+
53+
bb3: {
54+
StorageDead(_6); // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
55+
_0 = const (); // scope 0 at $DIR/basic_assignment.rs:+0:11: +14:2
56+
drop(_5) -> [return: bb4, unwind: bb7]; // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
57+
}
58+
59+
bb4: {
60+
StorageDead(_5); // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
61+
- drop(_4) -> bb5; // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
62+
+ goto -> bb5; // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
63+
}
64+
65+
bb5: {
66+
StorageDead(_4); // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
67+
StorageDead(_2); // scope 1 at $DIR/basic_assignment.rs:+14:1: +14:2
68+
StorageDead(_1); // scope 0 at $DIR/basic_assignment.rs:+14:1: +14:2
69+
return; // scope 0 at $DIR/basic_assignment.rs:+14:2: +14:2
70+
}
71+
72+
bb6 (cleanup): {
73+
drop(_5) -> bb7; // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
74+
}
75+
76+
bb7 (cleanup): {
77+
- drop(_4) -> bb8; // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
78+
+ goto -> bb8; // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
79+
}
80+
81+
bb8 (cleanup): {
82+
resume; // scope 0 at $DIR/basic_assignment.rs:+0:1: +14:2
83+
}
84+
}
85+

‎tests/mir-opt/basic_assignment.main.SimplifyCfg-initial.after.mir

+15-13
Original file line numberDiff line numberDiff line change
@@ -1,8 +1,8 @@
11
// MIR for `main` after SimplifyCfg-initial
22

33
| User Type Annotations
4-
| 0: user_ty: Canonical { max_universe: U0, variables: [], value: Ty(std::option::Option<std::boxed::Box<u32>>) }, span: $DIR/basic_assignment.rs:18:17: 18:33, inferred_ty: std::option::Option<std::boxed::Box<u32>>
5-
| 1: user_ty: Canonical { max_universe: U0, variables: [], value: Ty(std::option::Option<std::boxed::Box<u32>>) }, span: $DIR/basic_assignment.rs:18:17: 18:33, inferred_ty: std::option::Option<std::boxed::Box<u32>>
4+
| 0: user_ty: Canonical { max_universe: U0, variables: [], value: Ty(std::option::Option<std::boxed::Box<u32>>) }, span: $DIR/basic_assignment.rs:19:17: 19:33, inferred_ty: std::option::Option<std::boxed::Box<u32>>
5+
| 1: user_ty: Canonical { max_universe: U0, variables: [], value: Ty(std::option::Option<std::boxed::Box<u32>>) }, span: $DIR/basic_assignment.rs:19:17: 19:33, inferred_ty: std::option::Option<std::boxed::Box<u32>>
66
|
77
fn main() -> () {
88
let mut _0: (); // return place in scope 0 at $DIR/basic_assignment.rs:+0:11: +0:11
@@ -41,35 +41,37 @@ fn main() -> () {
4141
StorageLive(_5); // scope 3 at $DIR/basic_assignment.rs:+9:9: +9:15
4242
StorageLive(_6); // scope 4 at $DIR/basic_assignment.rs:+13:14: +13:20
4343
_6 = move _4; // scope 4 at $DIR/basic_assignment.rs:+13:14: +13:20
44-
replace(_5 <- move _6) -> [return: bb1, unwind: bb5]; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
44+
drop(_5) -> [return: bb1, unwind: bb2]; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
4545
}
4646

4747
bb1: {
48-
drop(_6) -> [return: bb2, unwind: bb6]; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
48+
_5 = move _6; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
49+
drop(_6) -> [return: bb3, unwind: bb6]; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
4950
}
5051

51-
bb2: {
52+
bb2 (cleanup): {
53+
_5 = move _6; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
54+
drop(_6) -> bb6; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
55+
}
56+
57+
bb3: {
5258
StorageDead(_6); // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
5359
_0 = const (); // scope 0 at $DIR/basic_assignment.rs:+0:11: +14:2
54-
drop(_5) -> [return: bb3, unwind: bb7]; // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
60+
drop(_5) -> [return: bb4, unwind: bb7]; // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
5561
}
5662

57-
bb3: {
63+
bb4: {
5864
StorageDead(_5); // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
59-
drop(_4) -> [return: bb4, unwind: bb8]; // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
65+
drop(_4) -> [return: bb5, unwind: bb8]; // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
6066
}
6167

62-
bb4: {
68+
bb5: {
6369
StorageDead(_4); // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
6470
StorageDead(_2); // scope 1 at $DIR/basic_assignment.rs:+14:1: +14:2
6571
StorageDead(_1); // scope 0 at $DIR/basic_assignment.rs:+14:1: +14:2
6672
return; // scope 0 at $DIR/basic_assignment.rs:+14:2: +14:2
6773
}
6874

69-
bb5 (cleanup): {
70-
drop(_6) -> bb6; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
71-
}
72-
7375
bb6 (cleanup): {
7476
drop(_5) -> bb7; // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
7577
}

‎tests/mir-opt/basic_assignment.rs

+1
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,6 @@
11
// this tests move up progration, which is not yet implemented
22

3+
// EMIT_MIR basic_assignment.main.ElaborateDrops.diff
34
// EMIT_MIR basic_assignment.main.SimplifyCfg-initial.after.mir
45

56
// Check codegen for assignments (`a = b`) where the left-hand-side is

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