src/librustc_mir/transform/cleanup_post_borrowck.rs

   1 //! This module provides a pass to replacing the following statements with
   2 //! [`Nop`]s
   3 //!
   4 //!   - [`AscribeUserType`]
   5 //!   - [`FakeRead`]
   6 //!   - [`Assign`] statements with a [`Shallow`] borrow
   7 //!
   8 //! The `CleanFakeReadsAndBorrows` "pass" is actually implemented as two
   9 //! traversals (aka visits) of the input MIR. The first traversal,
  10 //! [`DeleteAndRecordFakeReads`], deletes the fake reads and finds the
  11 //! temporaries read by [`ForMatchGuard`] reads, and [`DeleteFakeBorrows`]
  12 //! deletes the initialization of those temporaries.
  13 //!
  14 //! [`AscribeUserType`]: rustc_middle::mir::StatementKind::AscribeUserType
  15 //! [`Shallow`]: rustc_middle::mir::BorrowKind::Shallow
  16 //! [`FakeRead`]: rustc_middle::mir::StatementKind::FakeRead
  17 //! [`Nop`]: rustc_middle::mir::StatementKind::Nop
  18
  19 use crate::transform::{MirPass, MirSource};
  20 use rustc_middle::mir::visit::MutVisitor;
  21 use rustc_middle::mir::{BodyAndCache, BorrowKind, Location, Rvalue};
  22 use rustc_middle::mir::{Statement, StatementKind};
  23 use rustc_middle::ty::TyCtxt;
  24
  25 pub struct CleanupNonCodegenStatements;
  26
  27 pub struct DeleteNonCodegenStatements<'tcx> {
  28     tcx: TyCtxt<'tcx>,
  29 }
  30
  31 impl<'tcx> MirPass<'tcx> for CleanupNonCodegenStatements {
  32     fn run_pass(&self, tcx: TyCtxt<'tcx>, _source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
  33         let mut delete = DeleteNonCodegenStatements { tcx };
  34         delete.visit_body(body);
  35         body.user_type_annotations.raw.clear();
  36     }
  37 }
  38
  39 impl<'tcx> MutVisitor<'tcx> for DeleteNonCodegenStatements<'tcx> {
  40     fn tcx(&self) -> TyCtxt<'tcx> {
  41         self.tcx
  42     }
  43
  44     fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
  45         match statement.kind {
  46             StatementKind::AscribeUserType(..)
  47             | StatementKind::Assign(box (_, Rvalue::Ref(_, BorrowKind::Shallow, _)))
  48             | StatementKind::FakeRead(..) => statement.make_nop(),
  49             _ => (),
  50         }
  51         self.super_statement(statement, location);
  52     }
  53 }