Cargo misleads users when RUSTC_BOOTSTRAP set by build script

[Mark-Simulacrum]

Problem

Starting with 1.52 (currently in beta), Cargo will prevent crates from setting RUSTC_BOOTSTRAP via build scripts. However, that prevention can be by-passed with an explicit RUSTC_BOOTSTRAP=1 in the environment. Cargo misleadingly tells users that RUSTC_BOOTSTRAP=<crate-name> will be sufficient to bypass the error.

Steps

Cargo.toml:

[package]
name = "foo"
version = "0.1.0"
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
obfstr = "=0.2.1" # pin to a version that sets this variable

Running with the bootstrap variable setup as Cargo asks:

$ RUSTC_BOOTSTRAP=obfstr cargo +beta build
   Compiling obfstr v0.2.1
error: Cannot set `RUSTC_BOOTSTRAP=1` from build script of `obfstr v0.2.1`.
note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.
help: If you're sure you want to do this in your project, set the environment variable `RUSTC_BOOTSTRAP=obfstr` before running cargo instead.

Possible Solution(s)

Likely we should either:

• enable nightly features in Cargo if any value of rustc_bootstrap is defined
• adjust the message to indicate =1 (but @joshtriplett IIRC was against this approach)

[ehuss]

Link PR #9181 cc @jyn514

Sorry, I'm a bit confused about what exactly is misleading here (there are definitely some confusing bits, I just want to clarify what is misleading to you).

enable nightly features in Cargo if any value of rustc_bootstrap is defined

I think that is exactly what the change is trying to prevent? I'm uncertain what this means.

adjust the message to indicate =1

I think the message is saying, remove RUSTC_BOOTSTRAP from the build script, and use RUSTC_BOOTSTRAP=obfstr outside of cargo to make it work in a targeted fashion. I think the intent here is to limit the scope of what gets nightly features enabled, and that must be an external opt-in. Using =1 is kinda against the intent of this exercise I think.

I just noticed that externally setting RUSTC_BOOTSTRAP=obfstr without changing the build script still fails. That seems like that should be changed to be allowed. That might make the error message make more sense?

[jyn514]

Ugh, I really really wish there were a way to test stable things from nightly channels. Yes, this is a bug and I intended RUSTC_BOOTSTRAP=crate_name to turn the error into a warning.

[jyn514]

I think the message is saying, remove RUSTC_BOOTSTRAP from the build script, and use RUSTC_BOOTSTRAP=obfstr outside of cargo to make it work in a targeted fashion. I think the intent here is to limit the scope of what gets nightly features enabled, and that must be an external opt-in. Using =1 is kinda against the intent of this exercise I think.

The whole point of rust-lang/rust#77802 was so that you don't have to modify the original build script.

[jyn514]

Ok I think I see the issue - cargo treats RUSTC_BOOTSTRAP different than rustc used to: it only allows nightly features when RUSTC_BOOTSTRAP=1, not on any other value:

cargo/src/cargo/core/features.rs

Lines 885 to 889 in e870eac

	if let Ok(staging) = env::var("RUSTC_BOOTSTRAP") {
	if staging == "1" {
	return "dev".to_string();
	}
	}

@ehuss do you want cargo to keep that behavior? If so the fix is simple enough, I can just check if RUSTC_BOOTSTRAP is set to any value in custom_build.rs.

[jyn514]

Err wait no, any value is incorrect, it should only allow it if RUSTC_BOOTSTRAP contains the crate name. Otherwise RUSTC_BOOTSTRAP=crate is no different from RUSTC_BOOTSTRAP=1.