Edition key should be per-target, not per-package

[nrc]

Possibly optionally?

[alexcrichton]

Seems plausible to me! That way existing crates could migrate one Target at a time instead of all at once

[dtolnay]

@dwijnand or @nrc, could you clarify for someone who is not familiar with Cargo what counts as a "target"? Would this allow selecting an edition at the granularity of individual top-level test files, or would that be a different feature request?

[[test]]
name = "rust2018"
edition = "2018"

I am interested in being able to test bitflags's support for being used in 2018-style macro imports, while keeping our existing 2015-style tests. See bitflags/bitflags#167.

[alexcrichton]

@dtolnay your exact PR to bitflags is actually what @dwijnand's PR is enabling.

"Targets" in Cargo are basically the same thing as "rustc invocations" or crates, so things like unit tests, libraries, binaries, examples, unit tests for librararies, etc, are all different targets.