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::mir::StatementKind::AscribeUserType
  15 //! [`Shallow`]: rustc::mir::BorrowKind::Shallow
  16 //! [`FakeRead`]: rustc::mir::StatementKind::FakeRead
  17 //! [`Nop`]: rustc::mir::StatementKind::Nop
  18
  19 use rustc::mir::{BasicBlock, BorrowKind, Rvalue, Location, Mir};
  20 use rustc::mir::{Statement, StatementKind};
  21 use rustc::mir::visit::MutVisitor;
  22 use rustc::ty::TyCtxt;
  23 use crate::transform::{MirPass, MirSource};
  24
  25 pub struct CleanupNonCodegenStatements;
  26
  27 pub struct DeleteNonCodegenStatements;
  28
  29 impl MirPass for CleanupNonCodegenStatements {
  30     fn run_pass<'a, 'tcx>(&self,
  31                           _tcx: TyCtxt<'a, 'tcx, 'tcx>,
  32                           _source: MirSource<'tcx>,
  33                           mir: &mut Mir<'tcx>) {
  34         let mut delete = DeleteNonCodegenStatements;
  35         delete.visit_mir(mir);
  36     }
  37 }
  38
  39 impl<'tcx> MutVisitor<'tcx> for DeleteNonCodegenStatements {
  40     fn visit_statement(&mut self,
  41                        block: BasicBlock,
  42                        statement: &mut Statement<'tcx>,
  43                        location: Location) {
  44         match statement.kind {
  45             StatementKind::AscribeUserType(..)
  46             | StatementKind::Assign(_, box Rvalue::Ref(_, BorrowKind::Shallow, _))
  47             | StatementKind::FakeRead(..) => statement.make_nop(),
  48             _ => (),
  49         }
  50         self.super_statement(block, statement, location);
  51     }
  52 }