read_via_copy: don't prematurely optimize away the read

[RalfJung]

Always do the read to ensure consistent UB error messages in const-eval/Miri.

r? @scottmcm

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[RalfJung]

Something is broken with CI, everything fails with Error: Can't use 'tar -xzf' extract archive file: /home/runner/work/_actions/_temp_9df9ab2c-c8c6-4350-906b-1888c557ad16/584cf8e8-7843-48e0-9221-d6f46700efee.tar.gz. return code: 2..
@bors treeclosed=100

[scottmcm]

hyper-nit: extra blank lines in here

[scottmcm]

FYI @WaffleLapkin, who sortof-suggested removing the assignment in #109035 (comment)

But this PR is a good answer for why it shouldn't be removed, so the change LGTM.

@bors r+

[bors]

📌 Commit 7093903 has been approved by scottmcm

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7093903 with merge ac542f7fbb011f3811c032e3b3faa0642161a126...

[bors]

💔 Test failed - checks-actions

[ehuss]

@bors retry

GitHub checkout failed

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Prepare workflow directory
Prepare all required actions
Getting action download info
Download action repository 'actions/checkout@v3' (SHA:f43a0e5ff2bd294095638e18286ca9a3d1956744)
##[error]Can't use 'tar -xzf' extract archive file: /home/runner/work/_actions/_temp_ac181360-3d12-41f2-ada0-882bd3bf1425/81eb450d-8849-429c-b079-ddc42e08edbe.tar.gz. return code: 2.

[bors]

⌛ Testing commit 7093903 with merge c505b8b95e28731a9e5fac10c2dd2c959e6b80d3...

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Prepare workflow directory
Prepare all required actions
Getting action download info
Download action repository 'actions/checkout@v3' (SHA:f43a0e5ff2bd294095638e18286ca9a3d1956744)
##[error]Can't use 'tar -xzf' extract archive file: /home/runner/work/_actions/_temp_9ee4439e-16af-43a6-bebb-8597a7a8a9b7/d0aadeff-c388-45be-8505-7094c7881131.tar.gz. return code: 2.

[bors]

💔 Test failed - checks-actions

[ehuss]

@bors retry

GitHub checkout issue

[bors]

⌛ Testing commit 7093903 with merge 626a6ab...

[bors]

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing 626a6ab to master...

[rust-timer]

Finished benchmarking commit (626a6ab): comparison URL.

Overall result: ✅ improvements - 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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.7%	[-0.7%, -0.7%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.7%	[-0.7%, -0.7%]	5

Max RSS (memory usage)

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

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)	1.1%	[0.8%, 1.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	1.1%	[0.8%, 1.3%]	2

Binary size

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

Bootstrap: 628.296s -> 627.535s (-0.12%)
Artifact size: 316.10 MiB -> 316.30 MiB (0.06%)

[WaffleLapkin]

FYI @WaffleLapkin, who sortof-suggested removing the assignment in #109035 (comment)

Oops 😅