rand fail to compile on version 0.8.7

Common issues

Problem:

PR #1804 backported #1790 to the 0.8 branch.

After that, trying to compile v0.8.7 fails on openSUSE Tumbleweed.

Quick solution:

Change std::char::MAX to char::MAX on

https://github.com/rust-random/rand/blob/90ff85ea8c31ad2b8a1ed664f251ec376967eb64/src/distributions/uniform.rs#L605

Details:

I use a Rust program called Mago. It is a tool for PHP code analysis, and it uses rand as a dependency.

Since Mago v1.44, compiling the rand crate when updating Mago using:

cargo-install-update install-update mago

Failed with the following error:

   Compiling rand v0.8.7
error[E0433]: cannot find module or crate `std` in this scope
   --> /home/rodrigo/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rand-0.8.7/src/distributions/uniform.rs:605:24
    |
605 |     if sampler.max() > std::char::MAX as u32 - CHAR_SURROGATE_LEN {
    |                        ^^^ use of unresolved module or unlinked crate `std`
    |
    = help: if you wanted to use a crate named `std`, use `cargo add std` to add it to your `Cargo.toml`
help: consider importing this module
    |
109 + use core::char;
    |
help: if you import `char`, refer to it directly
    |
605 -     if sampler.max() > std::char::MAX as u32 - CHAR_SURROGATE_LEN {
605 +     if sampler.max() > char::MAX as u32 - CHAR_SURROGATE_LEN {
    |

I also tried compiling it on a fresh openSUSE install within a VM to rule out any custom config I could have on my main machine and got the same error.

As the original change in PR #1790 doesn't have the std:: namespace on the correspondent line:

https://github.com/rust-random/rand/blob/bb1262f703ca04e4ce56be78e1dc4e204cd6a998/src/distr/uniform_other.rs#L46

So I removed it from my local source copy at

/home/rodrigo/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rand-0.8.7/src/distributions/uniform.rs:605

Resulting in:

    if sampler.max() > char::MAX as u32 - CHAR_SURROGATE_LEN {

And then running

cargo-install-update install-update mago

Succeeds in compiling both the crate and the Mago program.

I don't have any experience with Rust. Thus, I have no idea how to test this change properly, and I am also not sure if my "fix" is a proper fix or just masquerades the problem (by reading a constant from somewhere else not intended when removing the std:: namespace).

That is why I chose to open an issue and not send a PR.

If you need any further information about my setup or anything else, I am glad to provide it.

Start at src/distributions/uniform.rs:605 and compare the corresponding change in src/distr/uniform_other.rs:46. Reproduce the rand v0.8.7 compilation failure with the cargo-install-update install-update mago command, then verify the corrected crate and Mago build compile successfully, including on the reported openSUSE environment.