rustc_mir_transform/cleanup_post_borrowck.rs
1//! This module provides a pass that removes parts of MIR that are no longer relevant after
2//! analysis phase and borrowck. In particular, it removes false edges, user type annotations and
3//! replaces following statements with [`Nop`]s:
4//!
5//! - [`AscribeUserType`]
6//! - [`FakeRead`]
7//! - [`Assign`] statements with a [`Fake`] borrow
8//! - [`Coverage`] statements of kind [`BlockMarker`] or [`SpanMarker`]
9//!
10//! [`AscribeUserType`]: rustc_middle::mir::StatementKind::AscribeUserType
11//! [`Assign`]: rustc_middle::mir::StatementKind::Assign
12//! [`FakeRead`]: rustc_middle::mir::StatementKind::FakeRead
13//! [`Nop`]: rustc_middle::mir::StatementKind::Nop
14//! [`Fake`]: rustc_middle::mir::BorrowKind::Fake
15//! [`Coverage`]: rustc_middle::mir::StatementKind::Coverage
16//! [`BlockMarker`]: rustc_middle::mir::coverage::CoverageKind::BlockMarker
17//! [`SpanMarker`]: rustc_middle::mir::coverage::CoverageKind::SpanMarker
18
19use rustc_middle::mir::coverage::CoverageKind;
20use rustc_middle::mir::*;
21use rustc_middle::ty::TyCtxt;
22use rustc_middle::ty::adjustment::PointerCoercion;
23
24pub(super) struct CleanupPostBorrowck;
25
26impl<'tcx> crate::MirPass<'tcx> for CleanupPostBorrowck {
27 fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
28 // Manually invalidate CFG caches if we actually change a terminator's edges.
29 let mut invalidate_cfg = false;
30 for basic_block in body.basic_blocks.as_mut_preserves_cfg().iter_mut() {
31 for statement in basic_block.statements.iter_mut() {
32 match statement.kind {
33 StatementKind::AscribeUserType(..)
34 | StatementKind::Assign(box (_, Rvalue::Ref(_, BorrowKind::Fake(_), _)))
35 | StatementKind::Coverage(
36 // These kinds of coverage statements are markers inserted during
37 // MIR building, and are not needed after InstrumentCoverage.
38 CoverageKind::BlockMarker { .. } | CoverageKind::SpanMarker { .. },
39 )
40 | StatementKind::FakeRead(..)
41 | StatementKind::BackwardIncompatibleDropHint { .. } => {
42 statement.make_nop(true)
43 }
44 StatementKind::Assign(box (
45 _,
46 Rvalue::Cast(
47 ref mut cast_kind @ CastKind::PointerCoercion(
48 PointerCoercion::ArrayToPointer
49 | PointerCoercion::MutToConstPointer,
50 _,
51 ),
52 ..,
53 ),
54 )) => {
55 // BorrowCk needed to track whether these cases were coercions or casts,
56 // to know whether to check lifetimes in their pointees,
57 // but from now on that distinction doesn't matter,
58 // so just make them ordinary pointer casts instead.
59 *cast_kind = CastKind::PtrToPtr;
60 }
61 _ => (),
62 }
63 }
64
65 // If we change any terminator, we need to ensure that we invalidated the CFG cache.
66 let terminator = basic_block.terminator_mut();
67 match terminator.kind {
68 TerminatorKind::FalseEdge { real_target, .. }
69 | TerminatorKind::FalseUnwind { real_target, .. } => {
70 invalidate_cfg = true;
71 terminator.kind = TerminatorKind::Goto { target: real_target };
72 }
73 _ => {}
74 }
75 }
76
77 if invalidate_cfg {
78 body.basic_blocks.invalidate_cfg_cache();
79 }
80
81 body.user_type_annotations.raw.clear();
82
83 for decl in &mut body.local_decls {
84 decl.user_ty = None;
85 }
86 }
87
88 fn is_required(&self) -> bool {
89 true
90 }
91}