compiler/rustc_mir_transform/src/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 //! [`Assign`]: rustc_middle::mir::StatementKind::Assign
  18 //! [`ForMatchGuard`]: rustc_middle::mir::FakeReadCause::ForMatchGuard
  19 //! [`Nop`]: rustc_middle::mir::StatementKind::Nop
  20
  21 use crate::MirPass;
  22 use rustc_middle::mir::visit::MutVisitor;
  23 use rustc_middle::mir::{Body, BorrowKind, Location, Rvalue};
  24 use rustc_middle::mir::{Statement, StatementKind};
  25 use rustc_middle::ty::TyCtxt;
  26
  27 pub struct CleanupNonCodegenStatements;
  28
  29 pub struct DeleteNonCodegenStatements<'tcx> {
  30     tcx: TyCtxt<'tcx>,
  31 }
  32
  33 impl<'tcx> MirPass<'tcx> for CleanupNonCodegenStatements {
  34     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  35         let mut delete = DeleteNonCodegenStatements { tcx };
  36         delete.visit_body_preserves_cfg(body);
  37         body.user_type_annotations.raw.clear();
  38
  39         for decl in &mut body.local_decls {
  40             decl.user_ty = None;
  41         }
  42     }
  43 }
  44
  45 impl<'tcx> MutVisitor<'tcx> for DeleteNonCodegenStatements<'tcx> {
  46     fn tcx(&self) -> TyCtxt<'tcx> {
  47         self.tcx
  48     }
  49
  50     fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
  51         match statement.kind {
  52             StatementKind::AscribeUserType(..)
  53             | StatementKind::Assign(box (_, Rvalue::Ref(_, BorrowKind::Shallow, _)))
  54             | StatementKind::FakeRead(..) => statement.make_nop(),
  55             _ => (),
  56         }
  57         self.super_statement(statement, location);
  58     }
  59 }