Disallow setting some built-in cfg via the command-line

[Urgau]

This PR disallow users from setting some built-in cfg via set the command-line in order to prevent incoherent state, eg. windows cfg active but target is Linux based.

This implements MCP rust-lang/compiler-team#610, with the caveat that we disallow cfgs no matter if they make sense or not, since I don't think it's useful to allow users to set a cfg that will be set anyway. It also complicates the implementation.

---

The explicit_builtin_cfgs_in_flags lint detects builtin cfgs set via the --cfg flag.

(deny-by-default)

Example

rustc --cfg unix

fn main() {}

This will produce:

error: unexpected `--cfg unix` flag
  |
  = note: config `unix` is only supposed to be controlled by `--target`
  = note: manually setting a built-in cfg can and does create incoherent behaviours
  = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default

Explanation

Setting builtin cfgs can and does produce incoherent behaviour, it's better to the use the appropriate rustc flag that controls the config. For example setting the windows cfg but on Linux based target.

---

r? @petrochenkov
cc @jyn514

try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: x86_64-mingw
try-job: i686-msvc
try-job: i686-mingw
try-job: x86_64-gnu-llvm-17
try-job: dist-various-1

[Urgau]

I've changed the error from a hard-error to a deny-by-default lint as asked and included all the possible cfgs that wouldn't break the world (and added some links for the one that we can't lint on them).

@rustbot ready

[Urgau]

Addressed or replied to all the review comments.

@rustbot ready

[petrochenkov]

Ok, let's try landing this.
@bors r+

[bors]

📌 Commit 971cb13 has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Trying commit c0c57b3 with merge ac7969d...

[petrochenkov]

r=me after CI is green.
@rustbot author

[lcnr]

@bors r=petrochenkov

[bors]

📌 Commit c0c57b3 has been approved by petrochenkov

It is now in the queue for this repository.

[Urgau]

Wait, try isn't finishing yet, and bors doesn't like r+ and try at the same time.
@bors r-

[bors]

☀️ Try build successful - checks-actions
Build commit: ac7969d (ac7969db6cc18caaabd206a34b8c5034a329765d)

[Urgau]

@bors r=petrochenkov

[bors]

📌 Commit c0c57b3 has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit c0c57b3 with merge ce20e15...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing ce20e15 to master...

[rust-timer]

Finished benchmarking commit (ce20e15): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -2.6%, secondary -1.9%)

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)	3.3%	[3.3%, 3.3%]	1
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-3.6%	[-3.7%, -3.5%]	3
All ❌✅ (primary)	-2.6%	[-2.6%, -2.6%]	1

Cycles

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

Binary size

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

Bootstrap: 760.558s -> 760.891s (0.04%)
Artifact size: 336.96 MiB -> 336.96 MiB (0.00%)