regression: Copy no longer implemented

[Mark-Simulacrum]

Found by 1.56 crater - https://crater-reports.s3.amazonaws.com/beta-1.56-2/beta-2021-09-12/reg/gmorph-0.1.0/log.txt

[INFO] [stdout] error[E0204]: the trait `Copy` may not be implemented for this type
[INFO] [stdout]   --> src/enc.rs:16:17
[INFO] [stdout]    |
[INFO] [stdout] 16 | #[derive(Debug, Copy, Clone, PartialEq, Serialize, Deserialize)]
[INFO] [stdout]    |                 ^^^^
[INFO] [stdout] 17 | pub struct Enc {
[INFO] [stdout] 18 |     inner: Matrix3<Q231>,
[INFO] [stdout]    |     -------------------- this field does not implement `Copy`
[INFO] [stdout]    |
[INFO] [stdout]    = note: this error originates in the derive macro `Copy` (in Nightly builds, run with -Z macro-backtrace for more info)
[INFO] [stdout] 

Bisections seems to point at 371f3cd, cc @Aaron1011

[Mark-Simulacrum]

Potentially related:

• https://crater-reports.s3.amazonaws.com/beta-1.56-2/beta-2021-09-12/reg/mpstthree-0.0.13/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.56-2/beta-2021-09-12/reg/shaku-0.6.1/log.txt

[Aaron1011]

This is caused by a bad interaction between SelectionContext's 'intercrate' mode, the trait evaluation cache, and the evaluate_root_obligation call added in #85868

In PR #85868, we create a new SelectionContext to 'speculatively' evaluate projection sub-obligations. If they evaluate to EvaluatedToOk, we discard them from the cached results, allowing future projections of the same predicate to avoid re-processing them. However, if the original SelectionContext was in 'intercrate' mode, then we'll end up using the same InferCtxt (and therefore the same local evaluation cache) in both intercrate and non-intercrate mode. The result of evaluation differs between these modes, so this leads to bugs like this issue.

There are several different ways of fixing this - fortunately, all of them are quite straightforward, and are suitable for backporting. I've opened #88994 and #88993 to check the performance of two approaches.