Add metadata to targets

[Nilstrieb]

follow up to #121905 and #122157

This adds four pieces of metadata to every target:

• description
• tier
• host tools
• std

This information is currently scattered across target docs and both

• not machine readable, making validation harder
• sometimes subtly encoding by the table it's in, causing mistakes and making it harder to review changes to the properties

By putting it in the compiler, we improve this. Later, we will use this canonical information to generate target documentation from it.

I used find-replace for all the description: None.

One thing I'm not sure about is the behavior for the JSON. It doesn't really make sense that custom targets supply this information, especially the tier. But for the roundtrip tests, we do need to print and parse it. Maybe emit a warning when a custom target provides the metadata key? Either way, I don't think that's important right now, this PR should get merged ASAP or it will conflict all over the place.

r? davidtwco

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[Nilstrieb]

@bors p=1

[davidtwco]

@bors r+

[bors]

📌 Commit 5bcb66c has been approved by davidtwco

It is now in the queue for this repository.

[bors]

⌛ Testing commit 5bcb66c with merge d255c6a...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing d255c6a to master...

[rust-timer]

Finished benchmarking commit (d255c6a): 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)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.6%, 0.6%]	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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-3.8%, -3.8%]	1
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.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.7%	[3.6%, 6.2%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 647.279s -> 647.596s (0.05%)
Artifact size: 310.00 MiB -> 310.01 MiB (0.01%)