Is flate2's rust_backend feature really used?

[est31]

The flate2 dependency is included by cargo in the following way:

cargo/Cargo.toml

Line 34 in 03d7154

flate2 = { version = "1.0.3", features = ["zlib"] }

This includes the on-by-default rust_backend feature, which adds the miniz_oxide dependency to flate2 and the adler32 transitive dependency. In the rustc repo you can find the following note though, saying that adler32 is not used:

rust-lang/rust@61fe2e4#diff-3ac60df36be32d72842bf5351fc2bb1dR43

It might be better to turn off the default features of flate2 so that the unused crate doesn't have to be compiled by people.

[ehuss]

I believe the answer is "no". Removing it saves about 0.1 seconds on my system, so I'm not sure it really matters, but if you want to send a PR to disable default features, it should probably be OK.

[est31]

Can you label it easy and help wanted? Thanks!

[aleksator]

Done. I'll create a PR to rust repository to remove these 2 dependencies from their whitelist once 8023 is merged.

[est31]

@aleksator thanks! Don't forget to also update the cargo submodule

[ehuss]

Done. I'll create a PR to rust repository to remove these 2 dependencies from their whitelist once 8023 is merged.

That won't be necessary. flate2 is used in a number of other projects in the rust repository, which use the default features.

[est31]

Good point. The comment still needs updating because it's not just an unused dep of cargo but used inside the compiler, too.

[aleksator]

@ehuss

That won't be necessary. flate2 is used in a number of other projects in the rust repository, which use the default features.

Certainly true in regards to this constant in deps.rs then:

/// These are exceptions to Rust's permissive licensing policy, and
/// should be considered bugs. Exceptions are only allowed in Rust
/// tooling. It is _crucial_ that no exception crates be dependencies
/// of the Rust runtime (std/test).
const EXCEPTIONS: &[(&str, &str)]

But what about

/// Whitelist of crates rustc is allowed to depend on. Avoid adding to the list if possible.
///
/// This list is here to provide a speed-bump to adding a new dependency to
/// rustc. Please check with the compiler team before adding an entry.
const WHITELIST: &[&str]

? This one is internal for rustc if I understand it correctly, so probably should be changed. Is it?

@est31

Good point. The comment still needs updating because it's not just an unused dep of cargo but used inside the compiler, too.

What would you change it to?