Miscellaneous tracing fixes and improvements#4860
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Fixes RefCell being borrowed mutably twice, see rust-lang#4563 (comment) . There are some places in rustc where the implementation of an argument passed to a tracing span makes a nested tracing call, e.g. [here](https://github.com/rust-lang/rust/blob/f6092f224d2b1774b31033f12d0bee626943b02f/compiler/rustc_const_eval/src/interpret/step.rs#L82-L89)->[here](https://github.com/rust-lang/rust/blob/f6092f224d2b1774b31033f12d0bee626943b02f/compiler/rustc_public/src/ty.rs#L265)->[here](https://github.com/rust-lang/rust/blob/f6092f224d2b1774b31033f12d0bee626943b02f/compiler/rustc_span/src/lib.rs#L2302). This is what caused the crash. Co-authored-by: Ralf Jung <post@ralfj.de>
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This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed. Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers. |
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Fixes
RefCellbeing borrowed mutably twice, in case of nested tracing calls while formatting an argument of a tracing call, see #4563 (comment). There are some places in rustc where that happens, e.g. here->here->here.After applying the fix in 3367573 (which avoids keeping the
RefCellborrowed while formatting arguments of tracing calls), I was surprised that the code was still crashing on the same tracing call, this time with a different backtrace that didn't contain any mention of ourtracing-chromelayer. So I investigated a bit and it turned out to be a bug intracing-tree, for which I could create a minimum reproducible example that has nothing to do withtracing-chrome: davidbarsky/tracing-tree#93.This PR also closes #4563 by removing unnecessary RDTSC code. This could possibly go in a separate PR but it touches the same code as the fix above, so I preferred to just make it a separate commit.
This PR finally updates the documentation for tracing, to explain how to ensure the system uses
tscas a timer, and to point out the two blockers that prevent tracing from actually being used at the moment.Note: this PR uses f6466ce as a base, instead of the latest commit on
master, because that's the last Miri commit that is not affected by #4752, and wheretracingcould still be tested. Before merging I will rebase ontomaster.