Use Rust 2018 and remove build.rs

[vks]

This updates the remaining code to use Rust 2018 and removes all build.rs, which are no longer necessary, because we support only Rust 1.32 and higher.

[dhardy]

Thanks @vks!

Shall we add a new policy? All PRs should add a note to the changelog (unless very minor). This would save us the job of going back through PRs for the next release. (@newpavlov may like to comment too since he is also a long-term contributor.)

Edit: I see someone suggested the same thing on Reddit. Lets start that trend from here on, unless you have objections.

[vks]

I don't understand why the examples fail to build.

[dhardy]

Me neither (I can reproduce the issue). Not much time to look into it now.

[dhardy]

It's caused by adding the following code in 294d16c.

#![cfg(all(feature="alloc", not(feature="std")))]
extern crate alloc;

[dhardy]

With the nightly compiler, we can (a) remove feature(alloc) (rust-lang/rust#59675), and (b) use just extern crate alloc; (without the cfg above). This appears to solve our issues.

Unfortunately (b) is incompatible with current stable compilers, leaving us stuck.

@SimonSapin landed the alloc crate stabilisation; do you have any idea why this PR fails with such an odd message? I think it must be a Rustc bug. (Just checkout the PR and build with cargo test; both stable and nightly do the same thing.)

[tesuji]

Could we force to use edition idioms by adding #![deny(rust_2018_idioms)] in crate root?

[vks]

@dhardy Could it also be a cargo bug related to examples?

Currently, it seems like we have to wait for feature(alloc) to become stable and increase the MSRV accordingly, before we can use Rust 2018 without regressions.

[dhardy]

I don't believe it is a Cargo bug. I can extract the rustc commands from cargo test --verbose and reproduce, and I don't see anything odd about these commands. @alexcrichton any ideas?

~/projects/rand/rand$ rustc --edition=2018 --crate-name rand src/lib.rs --color always --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="alloc"' --cfg 'feature="default"' --cfg 'feature="getrandom"' --cfg 'feature="getrandom_package"' --cfg 'feature="rand_core"' --cfg 'feature="std"' -C metadata=1b726c70baf9c429 -C extra-filename=-1b726c70baf9c429 --out-dir /home/dhardy/projects/rand/rand/target/debug/deps -L dependency=/home/dhardy/projects/rand/rand/target/debug/deps --extern getrandom_package=/home/dhardy/projects/rand/rand/target/debug/deps/libgetrandom-3257ec60faaf3040.rlib --extern libc=/home/dhardy/projects/rand/rand/target/debug/deps/liblibc-bdbdc84b2556a7c9.rlib --extern rand_chacha=/home/dhardy/projects/rand/rand/target/debug/deps/librand_chacha-6decec9409e8722c.rlib --extern rand_core=/home/dhardy/projects/rand/rand/target/debug/deps/librand_core-6c84f106de869675.rlib
~/projects/rand/rand$ rustc --edition=2018 --crate-name monte_carlo examples/monte-carlo.rs --color always --crate-type bin --emit=dep-info,link -C debuginfo=2 --cfg 'feature="alloc"' --cfg 'feature="default"' --cfg 'feature="getrandom"' --cfg 'feature="getrandom_package"' --cfg 'feature="rand_core"' --cfg 'feature="std"' -C metadata=0d912ac65a32d291 -C extra-filename=-0d912ac65a32d291 --out-dir /home/dhardy/projects/rand/rand/target/debug/examples -L dependency=/home/dhardy/projects/rand/rand/target/debug/deps --extern getrandom_package=/home/dhardy/projects/rand/rand/target/debug/deps/libgetrandom-3257ec60faaf3040.rlib --extern libc=/home/dhardy/projects/rand/rand/target/debug/deps/liblibc-bdbdc84b2556a7c9.rlib --extern rand=/home/dhardy/projects/rand/rand/target/debug/deps/librand-1b726c70baf9c429.rlib --extern rand_chacha=/home/dhardy/projects/rand/rand/target/debug/deps/librand_chacha-6decec9409e8722c.rlib --extern rand_core=/home/dhardy/projects/rand/rand/target/debug/deps/librand_core-6c84f106de869675.rlib
error[E0432]: unresolved import `rand::distributions`
  --> examples/monte-carlo.rs:29:11
   |
29 | use rand::distributions::{Distribution, Uniform};
   |           ^^^^^^^^^^^^^ could not find `distributions` in `rand`

error[E0425]: cannot find function `thread_rng` in module `rand`
  --> examples/monte-carlo.rs:33:25
   |
33 |     let mut rng = rand::thread_rng();
   |                         ^^^^^^^^^^ not found in `rand`

warning: unused import: `Distribution`
  --> examples/monte-carlo.rs:29:27
   |
29 | use rand::distributions::{Distribution, Uniform};
   |                           ^^^^^^^^^^^^
   |
   = note: #[warn(unused_imports)] on by default

error: aborting due to 2 previous errors

Some errors occurred: E0425, E0432.
For more information about an error, try `rustc --explain E0425`.

The module is just: pub mod distributions;

[dhardy]

Stranger: if I replace edition=2018 with edition=2015 in the above, then add extern crate rand; to monte-carlo.rs, I can reproduce exactly the same output.

(Setting edition = 2015 in the top Cargo.toml, then cargo clean; cargo test results in the same error, plus a similar error for the monty-hall example.)

[alexcrichton]

The line above extern crate alloc in src/lib.rs erroneously contains a !, configuring out the entire crate by accident.

[dhardy]

Remaining errors:

• trace and debug macros not available in rand_jitter (test: cargo doc --no-deps --all --all-features)

[dhardy]

All checks passed. Thanks @vks!