Compile probe does not take Cargo rustc configuration into account

[jonhoo]

The compile probe just invokes rustc directly, without including relevant cargo configuration. This causes problems in environments where that configuration may change whether or not backtrace is available (or compiles):

cargo new anyhow-issue
cd anyhow-issue
mkdir .cargo
echo '[build]' >> .cargo/config.toml
echo 'rustflags = ["-Zallow-features="]' >> .cargo/config.toml
echo 'anyhow = "1"' >> Cargo.toml
rustup override set nightly
cargo check

produces

    Updating crates.io index
  Downloaded anyhow v1.0.41
  Downloaded 1 crate (34.6 KB) in 0.68s
   Compiling anyhow v1.0.41
error[E0725]: the feature `backtrace` is not in the list of allowed features
   --> ~/.cargo/registry/src/github.com-1ecc6299db9ec823/anyhow-1.0.41/src/lib.rs:213:32
    |
213 | #![cfg_attr(backtrace, feature(backtrace))]
    |                                ^^^^^^^^^

error[E0658]: use of unstable library feature 'backtrace'
 --> ~/.cargo/registry/src/github.com-1ecc6299db9ec823/anyhow-1.0.41/src/backtrace.rs:2:33
  |
2 | pub(crate) use std::backtrace::{Backtrace, BacktraceStatus};
  |                                 ^^^^^^^^^
  |
  = note: see issue #53487 <https://github.com/rust-lang/rust/issues/53487> for more information
  = help: add `#![feature(backtrace)]` to the crate attributes to enable
...

Taking everything into account is probably too tall of an order, but the Cargo environment variables should get us pretty far. In particular, if set, anyhow should probably take into account these three:

• CARGO_BUILD_RUSTC
• CARGO_BUILD_RUSTC_WRAPPER
• CARGO_BUILD_RUSTFLAGS

Together, those should encapsulate most of the rustc configuration in cargo.

[dtolnay]

Ah, good call. I would accept a PR to fix this.

[jonhoo]

Ooof, looks like those environment variables are only used to configure said settings. And the variables Cargo sets for build scripts does not include the wrapper nor rustflags. I'll follow up with the Cargo team.

[jonhoo]

Filed rust-lang/cargo#9600

[jonhoo]

Submitted fix in rust-lang/cargo#9601, though this means we'll have to wait a while to actually implement this fix.