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.