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Explain the Reveal::UserFacing deduplication trick

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Oliver Schneider 2018-09-03 17:09:24 +02:00 • committed by Oliver Scherer
parent b4ee38ede3
commit d736197bc2
1 changed files with 7 additions and 0 deletions
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7
src/librustc_mir/const_eval.rs
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@ -577,6 +577,7 @@ pub fn const_eval_provider<'a, 'tcx>(
tcx: TyCtxt<'a, 'tcx, 'tcx>,
key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
// see comment in const_eval_provider for what we're doing here
if key.param_env.reveal == Reveal::All {
let mut key = key.clone();
key.param_env.reveal = Reveal::UserFacing;
@ -596,6 +597,12 @@ pub fn const_eval_raw_provider<'a, 'tcx>(
tcx: TyCtxt<'a, 'tcx, 'tcx>,
key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
// so we do a small trick here. We check whether we can evaluate the constant in the more
// restrictive `Reveal::UserFacing`, which most likely already was computed. In a large
// percentage of constants that will already have succeeded. Only associated constants of
// generic functions will fail due to not enough monomorphization information being available
// in case we fail in the `UserFacing` variant, we just do the real computation.
if key.param_env.reveal == Reveal::All {
let mut key = key.clone();
key.param_env.reveal = Reveal::UserFacing;