Cargo should tell `build.rs` what the Rust edition is

[vandenoever]

Describe the problem you are trying to solve
Syntax between Rust 2015 and Rust 2018 is different. Generating code in build.rs should know what edition of Rust to generate code for.
The code could parse Cargo.toml to figure this out, but it's cumbersome.

Describe the solution you'd like
An environment variable e.g. RUST_EDITION=2018 would help.

[ehuss]

This may be complicated by the fact that targets within a package can be in different editions. I imagine just including the package edition would cover 99.9% of the situations, but there should be at least a note in the documentation about this potential pitfall.

[ehuss]

See also: https://internals.rust-lang.org/t/editions-and-generated-code/9054

[epage]

This may be complicated by the fact that targets within a package can be in different editions.

I had to go look at the docs because I found this so surprising.

[nyurik]

Turns out I rushed to implementation with #14873 when this feature hasn't been settled yet... I do think we should simply provide the root-level edition, even if edition can be set on each target (why is it even possible is beyond me). And I do agree that in theory compiler may want to allow per-mod edition too, allowing auto-generated code to use an older edition than the overall package.

[epage]

My concern with CARGO_PKG_EDITION is for people creating build script libraries. It looks like the tool for the job and but it could fail at that job. It feels like a misleading promise from the Cargo to provide it. For me, I think I'd only consider it if we were to deprecate per-target editions and remove them in a future edition. I think the reason per-target editions exist is to allow more gradual migration. There is also a small chance of someone wanting to provide edition-specific examples but that seems unlikely. In considering targets vs packages, I lean towards packages.

There would be other areas that could potentially benefit from per-mod or per-block editions, like with doctests which are allowed to be per-edition.

[nyurik]

I created an issue to deprecate per-target edition support:

• Deprecate per-target editions #15188

[nyurik]

When running build.rs, does Cargo know which specific target it intends to build? I.e. would Cargo execute build.rs multiple times when building both the [lib] and [bin] targets on the same crate? And/or would it optimize running build.rs if both of the targets have the same requirements/hash?