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
24use crate::PassPolicy;
25
26pub(super) struct CleanupPostBorrowck;
27
28impl<'tcx> crate::MirPass<'tcx> for CleanupPostBorrowck {
29    fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
30        // Manually invalidate CFG caches if we actually change a terminator's edges.
31        let mut invalidate_cfg = false;
32        for basic_block in body.basic_blocks.as_mut_preserves_cfg().iter_mut() {
33            for statement in basic_block.statements.iter_mut() {
34                match statement.kind {
35                    StatementKind::AscribeUserType(..)
36                    | StatementKind::Assign((_, Rvalue::Ref(_, BorrowKind::Fake(_), _)))
37                    | StatementKind::Coverage(
38                        // These kinds of coverage statements are markers inserted during
39                        // MIR building, and are not needed after InstrumentCoverage.
40                        CoverageKind::BlockMarker { .. } | CoverageKind::SpanMarker { .. },
41                    )
42                    | StatementKind::FakeRead(..)
43                    | StatementKind::BackwardIncompatibleDropHint { .. } => {
44                        statement.make_nop(true)
45                    }
46                    StatementKind::Assign((
47                        _,
48                        Rvalue::Cast(
49                            ref mut cast_kind @ CastKind::PointerCoercion(
50                                PointerCoercion::ArrayToPointer
51                                | PointerCoercion::MutToConstPointer,
52                                _,
53                            ),
54                            ..,
55                        ),
56                    )) => {
57                        // BorrowCk needed to track whether these cases were coercions or casts,
58                        // to know whether to check lifetimes in their pointees,
59                        // but from now on that distinction doesn't matter,
60                        // so just make them ordinary pointer casts instead.
61                        *cast_kind = CastKind::PtrToPtr;
62                    }
63                    _ => (),
64                }
65            }
66
67            // If we change any terminator, we need to ensure that we invalidated the CFG cache.
68            let terminator = basic_block.terminator_mut();
69            match terminator.kind {
70                TerminatorKind::FalseEdge { real_target, .. }
71                | TerminatorKind::FalseUnwind { real_target, .. } => {
72                    invalidate_cfg = true;
73                    terminator.kind = TerminatorKind::Goto { target: real_target };
74                }
75                _ => {}
76            }
77        }
78
79        if invalidate_cfg {
80            body.basic_blocks.invalidate_cfg_cache();
81        }
82
83        body.user_type_annotations.raw.clear();
84
85        for decl in &mut body.local_decls {
86            decl.user_ty = None;
87        }
88    }
89
90    fn policy(&self, _sess: &rustc_session::Session) -> PassPolicy {
91        // Removes administrative MIR instructions that later passes must never see.
92        PassPolicy::Required
93    }
94}
rustc_mir_transform/cleanup_post_borrowck.rs

rustc_mir_transform/
cleanup_post_borrowck.rs
