put fake reads for guards' fake borrows on the guard's failure path

[dianne]

Conceptually, I think it makes sense to have fake reads be on guard failure, since it's the failure path where we continue matching (and thus it's soundness-critical to prevent mutation). Practically, this fixes #161578.

This is technically a breaking change, so it'll need a crater run and FCP.

[dianne]

@bors try

[dianne]

I'm guessing a types fcp would make the most sense since this is kind of a borrow-checking change?

r? @oli-obk maybe since you're familiar with both MIR building and T-types procedure, but feel free to reassign ^^

[rustbot]

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

[rustbot]

Some changes occurred in coverage tests.

cc @Zalathar

[rust-bors]

☀️ Try build successful (CI)
Build commit: 8511135 (85111357388f7cbb75083e86a3f3a1cc76d512fb)
Base parent: baf1f7c (baf1f7cc4faa79b938148ad52c53e96c0102cbc4)

[dianne]

@craterbot check

[craterbot]

👌 Experiment pr-161581 created and queued.
🤖 Automatically detected try build 8511135
⚠️ Try build based on commit 6579371, but latest commit is 80441e9. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

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[dianne]

just noticed oli is on vacation; oops! @rustbot reroll

[Zalathar]

You will also need to bless the coverage-run tests with ./x test coverage --bless --set=build.profiler=true.

(You can also [build] profiler = true in your bootstrap.toml to make running/blessing the coverage-run tests work by default.)

I'm not sure whether the resulting changes to coverage output are a problem or not, but my hope is that #161517 will make them vanish anyway.

[dianne]

the coverage-run tests should be blessed now. I'm not familiar with how coverage uses spans (or how to read coverage tests), so if there is a problem I'll need some guidance. though I imagine #161517 should help if it means coverage won't scrape spans from fake reads anymore