Allow specifying defaults for build target types

[dhardy]

Request: allow a feature flag to be required by all benchmarks (whether this acts like dev-dependencies and affects tests too or only benchmarks does not matter).

From what I understand, currently:

• Dev-dependencies cannot be optional. source
• The required-features field can apply to specific targets (e.g. [[bin]] or [[bench]] for a single binary)

Workaround: it appears to be possible to enable the feature for each benchmark independently, as follows:

[[bench]]
name = "distributions"
path = "benches/distributions.rs"
required-features = ["small_rng"]

[[bench]]
name = "generators"
path = "benches/generators.rs"
required-features = ["small_rng"]

...

[ehuss]

I've actually wanted a more general mechanism of specifying any default for all targets of a specific kind. Some examples:

• Set harness = false for all integration tests.
• Set test = true for all examples.
• Set doc = false for all binaries.

I have no idea how that would look in TOML, but I think would be useful in a number of circumstances.

[dhardy]

I think for this (and several other things I've come across) a real scripting language would work better than a declarative language like TOML. (There are several options: an expanded version of build.rs, Lua, a DSL like CMake.)

[weihanglo]

#12430 is a case that could be resolved by a general mechanism mentioned in #6945 (comment). Arguments are:

• Having a bunch of examples could significantly slow down cargo test.
• We already have things like doctest = false to address the slow part of testing. Building examples should also be considered as configurable.

[epage]

When mapping out a path forward for custom test harnesses, I think this feature becomes critical. I'll be proposing to the test team (rust-lang/rfcs#3455) that this Issue be on our short-list.