Disable `std` feature of `rand_core` by default?

[dhardy]

If we do this,

• direct users of rand_core might have issues (if they don't request the std feature and try to use missing functionality)
• indirect users of rand_core (i.e. users of rand etc.) shouldn't notice any differences, so long as the direct dependencies use the feature correctly
• this should help avoid problems like in Having rand 0.5 and rand 0.6 with default features turned off in the crate graph results in a compliation error #645, where the build happens to require std in rand_core (likely accidentally) but not in rand

There isn't a lot of functionality in rand_core which is enabled by the std feature, but there is a little:

• rand_core::Error::take_error is defined
• rand_core::Error implements std::error::Error
• std::io::Error implements From<rand_core::Error>
• RngCore implements std::io::Read

Of course, this is potentially a breaking change, so we should bump the version number to 0.4.0 and have a shim 0.3 → 0.4 which still enables std by default — except, since the change is unlikely to cause much breakage and we are already breaking some builds, I wonder if it is worth it?

[dhardy]

Note: all our rand* crates which depend on rand_core disable its default features, and only rand itself optionally enables its std feature. Interestingly, rand_jitter has optional std support implementing std::error::Error for its error type, yet works fine without requiring rand_core's std feature.

In other words: most users of rand_core probably don't need std features, and users of rand get them anyway (unless disabling its default features), so IMO this is the right way to go (and also used to be the case, to workaround a Cargo issue).

[newpavlov]

Isn't it disabled by default already? AFAIK all crates enable rand_core/std only as part of their own std feature. So I don't see any problems with the current setup.

[dhardy]

Sorry, I just merged #703. Before that, std was enabled by default. Judging by the above links, this caused several problems.

[newpavlov]

Ah, then it's indeed was a wrong behavior.