Ensure using using "getrandom_backend" in our build.rs doesn't override a "getrandom_backend" set by the user.

[josephlr]

In #724, we changed getrandom_windows_legacy to getrandom_backend="windows_legacy", which is a good and reasonable change.

However, if a user is already setting --cfg=getrandom_backend, things can get confusing.

For example::

RUSTFLAGS='--cfg=getrandom_backend="rdrand"' cargo +1.77 -v check --target=x86_64-pc-windows-gnu

works (using rdrand as expected), but:

RUSTFLAGS='--cfg=getrandom_backend="unsupported"' cargo +1.77 -v check --target=x86_64-pc-windows-gnu

does not work (it uses windows_legacy instead).

This is because of the order backends appear in our backends.rs file:

getrandom/src/backends.rs

Lines 11 to 49 in 3983e0f

	if #[cfg(getrandom_backend = "custom")] {
	mod custom;
	pub use custom::*;
	} else if #[cfg(getrandom_backend = "linux_getrandom")] {
	mod getrandom;
	mod sanitizer;
	pub use getrandom::*;
	} else if #[cfg(getrandom_backend = "linux_raw")] {
	mod linux_raw;
	mod sanitizer;
	pub use linux_raw::*;
	} else if #[cfg(getrandom_backend = "rdrand")] {
	mod rdrand;
	pub use rdrand::*;
	} else if #[cfg(getrandom_backend = "rndr")] {
	mod rndr;
	pub use rndr::*;
	} else if #[cfg(getrandom_backend = "efi_rng")] {
	mod efi_rng;
	pub use efi_rng::*;
	} else if #[cfg(getrandom_backend = "windows_legacy")] {
	mod windows_legacy;
	pub use windows_legacy::*;
	} else if #[cfg(all(getrandom_backend = "wasm_js"))] {
	cfg_if! {
	if #[cfg(feature = "wasm_js")] {
	mod wasm_js;
	pub use wasm_js::*;
	} else {
	compile_error!(concat!(
	"The \"wasm_js\" backend requires the `wasm_js` feature \
	for `getrandom`. For more information see: \
	https://docs.rs/getrandom/", env!("CARGO_PKG_VERSION"), "/#webassembly-support"
	));
	}
	}
	} else if #[cfg(getrandom_backend = "unsupported")] {
	mod unsupported;
	pub use unsupported::*;

When providing multiple backends (either explicitly or implicitly) with --cfg=getrandom_backend, all instances of #[cfg(getrandom_backend = "foo")] will match. This means we should move our else if #[cfg(getrandom_backend = "windows_legacy")] to be last in the explicit backends list.

When fixing this, we should be sure to document this in the build.rs and backends.rs. We should also add a test.

[newpavlov]

Can we check in build.rs whether user has provided getrandom_backend or not? If we can , then a better solution would be to disable the compiler version detection in such cases.

[josephlr]

Can we check in build.rs whether user has provided getrandom_backend or not? If we can , then a better solution would be to disable the compiler version detection in such cases.

I'll look into seeing how possible this is. Checking for a specified getrandom_backend could also allow us to detect (and raise an error) I multiple getrandom_backends are specified. Right now, using --cfg=getrandom_backend twice silently succeeds.