Crates which use a specific rustc version fail to build

[GuillaumeGomez]

Crate name

rustc-tools

Build failure link

https://docs.rs/crate/rustc-tools/0.3.1/builds/1050150

Additional details

No response

[syphar]

@GuillaumeGomez

I'm likely missing something, but where is the rustc version defined in rustc-tools?

Also, how would you imagine this to work? In the end, we only can work with nightly versions, and at the very least we always have to have a "minimum nightly" with the features / fixes we need, so a crate needing a specific nightly will never work reliably. Apart from the complexity to manage the different toolchains on the buildserver.

[GuillaumeGomez]

The version is in https://github.com/GuillaumeGomez/rustc-tools/blob/master/rust-toolchain. When this file exists, cargo downloads the related version automatically and use it.

Also, how would you imagine this to work? In the end, we only can work with nightly versions, and at the very least we always have to have a "minimum nightly" with the features / fixes we need, so a crate needing a specific nightly will never work reliably. Apart from the complexity to manage the different toolchains on the buildserver.

Isn't it possible to download and use a specific version if one is specified? No need to keep the compiler around, we can remove it as soon as the crate was documented. Unfortunately, for rustc plugins, you always need a specific compiler version otherwise it simply doesn't compile.

[syphar]

Isn't it possible to download and use a specific version if one is specified?

theoretically yes, but since we rely on specific docs.rs functionality in the nightly rustc version, you might break the build when rust-toolchain defines a nightly that is too old.
Also stable or beta would not have our features .

[GuillaumeGomez]

Can't we use it and if it fails, we consider it's ok?

[syphar]

Can't we use it and if it fails, we consider it's ok?

We had bugs in nightly versions that lead to silent failures, like #2351 or this issue.

So IMO this would only work if we define a "minimum" nightly that we can work with.