core: respect precision in ByteStr Display

[joboet]

Fixes #153022.

ByteStr's Display implementation didn't respect the precision parameter. Just like Formatter::pad, this is fixed by counting off the characters in the string and truncating after the requested length – with the added complication that the ByteStr needs to be divided into chunks first. By including a fast path that avoids counting the characters when no parameters were specified this should also fix the performance regressions caused by #152865.

[rustbot]

r? @jhpratt

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The reviewer was selected based on:

• Owners of files modified in this PR: @scottmcm, libs
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[Mark-Simulacrum]

@bors try @rust-timer queue

Just to get ahead of potential perf (shouldn't block merging as a correctness fix).

[rust-bors]

☀️ Try build successful (CI)
Build commit: 2448715 (2448715f97539c847c74001978350d9b0ccc64f1, parent: b3869b94cd1ed4bfa2eb28f301535d5e9599c713)

[rust-timer]

Finished benchmarking commit (2448715): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.8%, -0.3%]	2
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	-0.5%	[-0.8%, -0.3%]	2

Max RSS (memory usage)

Results (primary 1.1%, secondary 4.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	4.8%	[4.8%, 4.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Cycles

Results (primary 7.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	7.1%	[7.1%, 7.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	7.1%	[7.1%, 7.1%]	1

Binary size

Results (primary 0.0%, secondary 0.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	4

Bootstrap: 479.41s -> 478.565s (-0.18%)
Artifact size: 395.68 MiB -> 395.73 MiB (0.01%)