Use Cow in {D,Subd}iagnosticMessage.

[nnethercote]

Each of {D,Subd}iagnosticMessage::{Str,Eager} has a comment:

// FIXME(davidtwco): can a `Cow<'static, str>` be used here?

This commit answers that question in the affirmative. It's not the most compelling change ever, but it might be worth merging.

This requires changing the impl<'a> From<&'a str> impls to impl From<&'static str>, which involves a bunch of knock-on changes that require/result in call sites being a little more precise about exactly what kind of string they use to create errors, and not just &str. This will result in fewer unnecessary allocations, though this will not have any notable perf effects given that these are error paths.

Note that I was lazy within Clippy, using to_string in a few places to preserve the existing string imprecision. I could have used impl Into<{D,Subd}iagnosticMessage> in various places as is done in the compiler, but that would have required changes to many call sites (mostly changing &format("...") to format!("...")) which didn't seem worthwhile.

r? @WaffleLapkin

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

The Miri subtree was changed

cc @rust-lang/miri

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @JohnTitor, @TaKO8Ki

[WaffleLapkin]

Do I understand it right, that since we need to store diagnostics, having &'not_static str and String are equivalent (because we convert former into latter)?

(i.e. I want to make sure we don't exclude anything by requiring &'static str)

[nnethercote]

Do I understand it right, that since we need to store diagnostics, having &'not_static str and String are equivalent (because we convert former into latter)?

I guess you could say that. I.e. currently we convert any &str into a String in the From impl by allocating. This PR means we avoid doing that most of the time when the error message is a &'static str.

[bors]

☔ The latest upstream changes (presumably #111778) made this pull request unmergeable. Please resolve the merge conflicts.

[nnethercote]

I have rebased and addressed @RalfJung's comment.

Resolved

[nnethercote]

@WaffleLapkin: what's the status here, are we good to proceed?

[WaffleLapkin]

I'm not entirely sure this is a good idea, since some call cites got marginally more complex, but it's probably fine.

r=me with the nit

[nnethercote]

@bors r=WaffleLapkin

[bors]

📌 Commit 781111e has been approved by WaffleLapkin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 781111e with merge 70e04bd...

[bors]

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing 70e04bd to master...

[rust-timer]

Finished benchmarking commit (70e04bd): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.7%	[0.6%, 0.8%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.8%, -0.3%]	10
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.1%	[2.9%, 6.6%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 642.999s -> 643.87s (0.14%)

[nnethercote]

Perf changes are all for secondary benchmarks. The small wins and small losses balance out.

@rustbot label: +perf-regression-triaged