Different default features for crate binary and library

[emilk]

Problem

I have a crate that is both a library and a binary. I want to be able to specify a default set of features for users of the library and the binary. For instance, people who write mycrate = "*" in their Cargo.toml get a small set of features (and fast compilation times), but when typing cargo install mycrate they get most or all of the features enabled.

This is currently not possible, due to the single [features] default = […] field in Cargo.toml.

Proposed Solution

The [lib] and [[bin]] sections already have a required-features = […] field. I propose we also add a default-features = […] field that, if present, will override the default field in [features]:

[package]
name = "mycrate"

[[bin]]
# Make most features opt-out for users of the binary
default-features = ["features", "that", "binary", "users", "often", "want"]

[lib]
default-features = [] # Make all features opt-in for users of the library

[[test]]
# We don't set `default-features` here, so the same features from `[features] default` are used


[features]
# default features for any target that hasn't set `default-features` (tests, examples and benches):
default = ["foo", "bar"]

Notes

No response

[weihanglo]

Thanks for the issue report.
RFC 3374 should cover the cargo install use case. This also looks like a duplicate of #10409.

[weihanglo]

Close in favour of #10409. Thanks.

[emilk]

Thank you!