Remove unnecessary tmp variable in default_read_exact

[Wilfred]

This tmp variable has existed since the original implementation (added in ff81920), but it's not necessary (maybe non-lexical lifetimes helped?).

It's common to read std source code to understand how things actually work, and this tripped me up on my first read.

[rustbot]

r? @joshtriplett

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[workingjubilee]

Sure.
@bors r+

[bors]

📌 Commit dca90f7 has been approved by workingjubilee

It is now in the queue for this repository.

[bors]

⌛ Testing commit dca90f7 with merge f562931...

[bors]

☀️ Test successful - checks-actions
Approved by: workingjubilee
Pushing f562931 to master...

[rust-timer]

Finished benchmarking commit (f562931): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

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)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[0.7%, 0.7%]	1

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)	3.5%	[2.4%, 5.5%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.3%	[-5.3%, -1.4%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-5.3%, 5.5%]	9

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)	-	-	0
Improvements ✅ (primary)	-1.1%	[-1.4%, -0.8%]	2
Improvements ✅ (secondary)	-2.0%	[-2.3%, -1.8%]	4
All ❌✅ (primary)	-1.1%	[-1.4%, -0.8%]	2

Binary size

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)	0.1%	[0.0%, 0.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.0%, 0.1%]	3

Bootstrap: 629.226s -> 628.721s (-0.08%)
Artifact size: 271.32 MiB -> 271.30 MiB (-0.01%)