[beta] backports and bump #38377
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[beta] backports and bump #38377
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This is an annoying gotcha with the projection cache's handling of nested obligations. Nested projection obligations enter the issue in this case: ``` DEBUG:rustc::traits::project: AssociatedTypeNormalizer: depth=3 normalized <std::iter::Map<std::ops::Range<i32>, [closure@not-a-recursion-error.rs:5:30: 5:53]> as std::iter::IntoIterator>::Item to _#7t with 12 add'l obligations ``` Here the normalization result is the result of the nested impl `<[closure@not-a-recursion-error.rs:5:30: 5:53] as FnMut(i32)>::Output`, which is an additional obligation that is a part of "add'l obligations". By itself, this is proper behaviour - the additional obligation is returned, and the RFC 447 rules ensure that it is processed before the output `#_7t` is used in any way. However, the projection cache breaks this - it caches the `<std::iter::Map<std::ops::Range<i32>,[closure@not-a-recursion-error.rs:5:30: 5:53]> as std::iter::IntoIterator>::Item = #_7t` resolution. Now everybody else that attempts to look up the projection will just get `#_7t` *without* any additional obligations. This obviously causes all sorts of trouble (here a spurious `EvaluatedToAmbig` results in specializations not being discarded [here](https://github.com/rust-lang/rust/blob/9ca50bd4d50b55456e88a8c3ad8fcc9798f57522/src/librustc/traits/select.rs#L1705)). The compiler works even with this projection cache gotcha because in most cases during "one-pass evaluation". we tend to process obligations in LIFO order - after an obligation is added to the cache, we process its nested obligations before we do anything else (and if we have a cycle, we handle it specifically) - which makes sure the inference variables are resolved before they are used. That "LIFO" order That was not done when projecting out of a closure, so let's just fix that for the time being. Fixes rust-lang#38033.
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r? @alexcrichton