Clarify the guarantees of Vec::as_ptr and Vec::as_mut_ptr when there's no allocation

[SkiFire13]

Currently the documentation says they return a pointer to the vector's buffer, which has the implied precondition that the vector allocated some memory. However Vec's documentation also specifies that it won't always allocate, so it's unclear whether the pointer returned is valid in that case. Of course you won't be able to read/write actual bytes to/from it since the capacity is 0, but there's an exception: zero sized read/writes. They are still valid as long as the pointer is not null and the memory it points to wasn't deallocated, but Vec::as_ptr and Vec::as_mut_ptr don't specify that's not the case. This PR thus specifies they are actually valid for zero sized reads since Vec is implemented to hold a dangling pointer in those cases, which is neither null nor was deallocated.

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

r? @kennytm

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[Dylan-DPC]

@bors r+ rollup

[bors]

📌 Commit 8ef2dd7 has been approved by Dylan-DPC

[bors]

⌛ Testing commit 8ef2dd7 with merge 16a0d03...

[bors]

☀️ Test successful - checks-actions
Approved by: Dylan-DPC
Pushing 16a0d03 to master...

[rust-timer]

Finished benchmarking commit (16a0d03): comparison url.

Instruction count

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	0.1%	0.1%	1
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-1.1%	-1.1%	3
All 😿🎉 (primary)	N/A	N/A	0

Max RSS (memory usage)

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: 🎉 relevant improvement found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	N/A	N/A	0
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-3.9%	-3.9%	1
All 😿🎉 (primary)	N/A	N/A	0

Cycles

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: 🎉 relevant improvement found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	N/A	N/A	0
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-2.6%	-2.6%	1
All 😿🎉 (primary)	N/A	N/A	0

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³