make concurrency helper more pleasant to read

[tshepang]

No description provided.

[rust-highfive]

r? @dtolnay

(rust_highfive has picked a reviewer for you, use r? to override)

[pickfire]

The previous version looks a bit easier to read on this part since it specifically mention usize.

Also, I wonder why it needs to check n > 0 here?

I wonder will it be better to parse to NonZeroUsize so the guarantees are already there.

Suggested change

	match value.parse().ok() {
	Some(n) if n > 0 => n,
	_ => panic!("RUST_TEST_THREADS is `{}`, should be a positive integer.", value),
	match value.parse::<std::num::NonZeroUsize>().ok() {
	Some(n) => n.get(),
	_ => panic!("RUST_TEST_THREADS is `{}`, should be a positive integer.", value),

[tshepang]

type inference/coercion is helped by the return type, or do you feel it's too far, preferring the type spelled out close to the declaration

[dtolnay]

Looks like this failed to build. https://github.com/rust-lang/rust/pull/78065/checks?check_run_id=1270956557

    Checking test v0.0.0 (/checkout/library/test)
error[E0282]: type annotations needed
 --> library/test/src/helpers/concurrency.rs:8:24
  |
8 |             Some(n) if n.is_positive() => n,
  |                        ^ cannot infer type
  |
  = note: type must be known at this point
help: consider specifying the type argument in the method call
  |
7 |         match value.parse::<F>().ok() {
  |                          ^^^^^

error: aborting due to previous error

[pickfire]

@dtolnay How do you bold the code blocks?

[tshepang]

build failure fixed @dtolnay

[dtolnay]

@bors r+ rollup

[bors]

📌 Commit 628fb9f has been approved by dtolnay