cargo should provide builtin support to let build.rs detect feature is supported

[loynoir]

Problem

There is a

• 1 ~ 2 weeks project, with 1 ~ 2 weeks cache

• recent RA v0.3.1705

• deep dependency tree have recent version, thiserror-1.0.50 anyhow-1.0.75, same author dtolnay

I encounter same old bug on thiserror-1.0.50 anyhow-1.0.75.

• #![feature] may not be used on the stable release channel`

This bug was happening one year ago, but then fix.

In conclusion, this is maybe a mix problem of

• build.rs workaround to detect feature, such as error_generic_member_access, is supported

• RA rustc wrapper for performance

• sccache

• cargo build cache

And I think the core problem is

• cargo does not provide builtin support to let build.rs detect feature is supported, so dev invent themself.

Proposed Solution

cargo should provide builtin support to let build.rs detect feature is supported

Notes

Related

dtolnay/thiserror#192

https://github.com/dtolnay/thiserror/blob/e9ea67c7e251764c3c2d839b6c06d9f35b154647/build.rs#L12

https://github.com/dtolnay/anyhow/blob/05e413219e97f101d8f39a90902e5c5d39f951fe/build.rs#L17

rust-lang/rust-analyzer#15057

[epage]

Cargo has no knowledge of #![feature]s and doing something regarding this would be more of a language-level feature, like rust-lang/rust#64797. With this in mind, I'm going to close this issue. If there is something I missed, let us know and we can reconsider opening this. If you are looking for next steps, maybe try posting about this idea on internals.rust-lang.org/.

Personally, I think auto-detecting nightly features is a mistake and instead feature flags should be used to enable them explicitly.

[loynoir]

Generally, I think implement which makes less assumption is better than implement which makes more assumption.

So, personally, I agree with @epage .

According to my understanding, epage suggest

Personally, I think auto-detecting nightly features is a mistake and instead feature flags should be used to enable them explicitly.

anyhow = { version = "xxx", features = ["nightly"] }

But, @dtolnay does not agree.

So, I still insist

• rustc or cargo should provide builtin support to let build.rs detect feature is supported

Which will make build system robust, with less assumption.

[epage]

Note that I did not close this because of my opinion but that cargo isn't in a place to solve this.

[loynoir]

Tired. Leave it for next one who fall into cargo clean workaround.