Add Box<[T; N]>: TryFrom<Vec<T>>

[scottmcm]

We have [T; N]: TryFrom<Vec<T>> (#76310) and Box<[T; N]>: TryFrom<Box<[T]>>, but not this combination.

vec.into_boxed_slice().try_into() isn't quite a replacement for this, as that'll reallocate unnecessarily in the error case.

Insta-stable, so needs an FCP

(I tried to make this work with , A, but that's disallowed because of #[fundamental] #29635 (comment))

[scottmcm]

@rustbot label +T-libs-api -T-libs

And a coin flip between the two -api members on the review rotation says…
r? @m-ou-se

[m-ou-se]

@rfcbot merge

[rfcbot]

Team member @m-ou-se has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @yaahc

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[m-ou-se]

@bors r+

[bors]

📌 Commit 4d3a31c has been approved by m-ou-se

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4d3a31c with merge 06f049a...

[bors]

☀️ Test successful - checks-actions
Approved by: m-ou-se
Pushing 06f049a to master...

[rust-timer]

Finished benchmarking commit (06f049a): 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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.2%, 2.9%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Footnotes

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

2. number of relevant changes ↩ ↩² ↩³

[jplatte]

Is it really a good idea to stabilize this when that likely makes it impossible to generalize it even if fundamental gets "downgraded" to "fundamental-in-T" for Box down the road?