no_std crates should not permit non-no_std dependencies

[zesterer]

I couldn't find this exact problem mentioned elsewhere.

Problem

Cargo rightly produces errors for uses of std within a crate but will not complain about dependencies that rely on std when building for platforms that support std. This means that an implicit std requirement can quietly sneak into a project. It seems rather bizarre that cargo does not catch this issue.

Possible Solution(s)

• Make cargo search for uses of std when building dependencies and produce an error
• Do not permit std as a dependency in the dependency graph for no_std crates
• Only permit dependencies that are explicitly tagged with #![no_std] for no_std crates

[est31]

Current workaround: compile/cargo check for a target that is known to have no standard library.

Also there is the issue that Cargo.toml doesn't contain any info that a crate is no_std. It's specified in the Rust code only.

[epage]

FYI build-std is looking at allowing core/alloc/std being represented in Cargo's dependencies which would allow for this

[epage]

rust-lang/rfcs#3875 is up which will elevate no_std into the dependency tree for analysis.

It does not specify that we will lint the dependency tree for correctness but that could be built on top of it.