Default feature specification

[ehuss]

This issue is for working through the implementation issues for determining the default set of cfg values for the standard library.

Currently the default set is driven by the rust build system, which uses the config.toml file which is configured by various parts of the CI scripts. The build system sets Cargo features (such as here)
to control how the standard library is built.

The build system also sets environment variables (like LLVM_CONFIG for sanitizers), which are picked up by build scripts.

Currently Cargo has no way of knowing which features or environment variables to set. They change per platform, and based on build configuration settings.

Some rough ideas on how to approach this:

• Hard-code the values in Cargo. The MVP implementation will likely take this route, but it is not a realistic option.
• Include some configuration file in the source distribution that declares the default features used per platform. This may be difficult to generate.
• Make the user explicitly declare the features they need.

[SimonSapin]

Make the user explicitly declare the features they need.

This sounds like #4. But even if there is such a choice available, the default should likely be the same configuration that was used to build the compiler.

[Ericson2314]

Hard-code the values in Cargo. The MVP implementation will likely take this route, but it is not a realistic option.

This sounds like a bad idea even for the MVP.

Make the user explicitly declare the features they need.

Or even leave it to them to declare env vars and things where the build.rs picks it up. This is an easier MVP, and fine for most tasks. This also encourages us to triage what complexity is really needed, rather than leave it cluttering the Cargo codebase by default.

Remember that the users that need std-aware Cargo most probably won't even use std.