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Invalid (unstable, ill-formed, ...) const code is evaluated even after emitting error #76064
Description
Nightly-only ICEs are exposed on stable because array lengths are evaluated even when they are invalid:
struct Bug([u8; panic!(1)]);This first emits an error that a feature flag is missing, but then const-evaluates the array length anyway and later leads to an ICE. This is not a stability hole (there's an error, the code will not compile), but it's an ICE on stable, so it's a bug.
While the ICE should also be fixed, the underlying problem is that we should not const-evaluate code that failed stability checking (and there are possibly other kinds of checks to which this applies as well)
@oli-obk proposed some solutions:
- poison the mir Body if const checks fail (by adding a field like https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TypeckResults.html#structfield.tainted_by_errors or by just nuking the body by replacing it with
_0 = ConstKind::Error; return;) and thus make allow const evaluation to check if the mir Body failed the checks and bail out before doing anything- make the https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.ConstQualifs.html have an
errorfield similar to https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TypeckResults.html#structfield.tainted_by_errors
Cc @rust-lang/wg-const-eval
Activity
ecstatic-morse commentedon Aug 30, 2020
Putting this on
ConstQualifsseems reasonable. It seems better than adding more cruft tomir::Bodyat first glance. Currently each field ofConstQualifsis tied to an implementer of theQualiftrait but this isn't a hard requirement.RalfJung commentedon Nov 2, 2020
Once this is fixed, we can hopefully also get rid of the
TransmuteSizeDiffhack. I am not sure if we carry any other similar hacks that we still carry around.RalfJung commentedon Nov 7, 2020
@oli-obk the solution at #78809, if I understand correctly, does not fix
TransmuteSizeDiff... should that become a separate issue once this one is closed, or is there some reason thatTransmuteSizeDiffcannot be fixed by adding a similarErrorReportedto the transmute size check?oli-obk commentedon Nov 9, 2020
It does indeed not fix
TransmuteSizeDiff. We'll need to investigate what actually causes a transmute size check failure to not affecttainted_by_errors. We can likely fix this in a similar manner. Though we also have other bugs like{ let x; &x }getting all the way to interning when it fails due to a dangling pointer. We'll be collecting more and more error flags to keep track of. I'm not sure how to address this properly. Maybe we'll need moreResult<T, ErrorReported>on queries that don't return an enum which has an error variant (likety::ErrorConstKind::Err)add error_occured field to ConstQualifs, fix rust-lang#76064
add error_occured field to ConstQualifs, fix rust-lang#76064
RalfJung commentedon Nov 14, 2020
I opened a new issue for the
transmuteproblem: #79047.Auto merge of rust-lang#78809 - vn-ki:fix-issue-76064, r=oli-obk
Auto merge of rust-lang#78809 - vn-ki:fix-issue-76064, r=oli-obk