diff options
| -rw-r--r-- | src/librustc_mir/transform/const_prop.rs | 44 |
1 files changed, 22 insertions, 22 deletions
diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs index 9d5dbe3564e..badca105694 100644 --- a/src/librustc_mir/transform/const_prop.rs +++ b/src/librustc_mir/transform/const_prop.rs @@ -14,7 +14,7 @@ use rustc::mir::{ SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE, }; -use rustc::traits::TraitQueryMode; +use rustc::traits; use rustc::ty::layout::{ HasDataLayout, HasTyCtxt, LayoutError, LayoutOf, Size, TargetDataLayout, TyLayout, }; @@ -90,28 +90,28 @@ impl<'tcx> MirPass<'tcx> for ConstProp { // If there are unsatisfiable where clauses, then all bets are // off, and we just give up. // - // Note that we use TraitQueryMode::Canonical here, which causes - // us to treat overflow like any other error. This is because we - // are "speculatively" evaluating this item with the default substs. - // While this usually succeeds, it may fail with tricky impls - // (e.g. the typenum crate). Const-propagation is fundamentally - // "best-effort", and does not affect correctness in any way. - // Therefore, it's perfectly fine to just "give up" if we're - // unable to check the bounds with the default substs. + // We manually filter the predicates, skipping anything that's not + // "global". We are in a potentially generic context + // (e.g. we are evaluating a function without substituging generic + // parameters, so this filtering serves two purposes: // - // False negatives (failing to run const-prop on something when we actually - // could) are fine. However, false positives (running const-prop on - // an item with unsatisfiable bounds) can lead to us generating invalid - // MIR. - if !tcx.substitute_normalize_and_test_predicates(( - source.def_id(), - InternalSubsts::identity_for_item(tcx, source.def_id()), - TraitQueryMode::Canonical, - )) { - trace!( - "ConstProp skipped for item with unsatisfiable predicates: {:?}", - source.def_id() - ); + // 1. We skip evaluating any predicates that we would + // never be able prove are unsatisfiable (e.g. `<T as Foo>` + // 2. We avoid trying to normalize predicates involving generic + // parameters (e.g. `<T as Foo>::MyItem`). This can confuse + // the normalization code (leading to cycle errors), since + // it's usually never invoked in this way. + let predicates = tcx + .predicates_of(source.def_id()) + .predicates + .iter() + .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None }) + .collect(); + if !traits::normalize_and_test_predicates( + tcx, + traits::elaborate_predicates(tcx, predicates).collect(), + ) { + trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", source.def_id()); return; } |