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e2be5f568d
make `ConstEvaluatable` more strict relevant zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/.60ConstEvaluatable.60.20generic.20functions/near/204125452 Let's see how much this impacts. Depending on how this goes this should probably be a future compat warning. Short explanation: we currently forbid anonymous constants which depend on generic types, e.g. `[0; std::mem::size_of::<T>]` currently errors. We previously checked this by evaluating the constant and returned an error if that failed. This however allows things like ```rust const fn foo<T>() -> usize { if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T std::mem::size_of::<T>() } else { 8 } } fn test<T>() { let _ = [0; foo::<T>()]; } ``` which is a backwards compatibility hazard. This also has worrying interactions with mir optimizations (https://github.com/rust-lang/rust/pull/74491#issuecomment-661890421) and intrinsics (#74538). r? `@oli-obk` `@eddyb` |
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