CARGO environment variable not set when `cargo check` runs through ra

[jplatte]

When trying to use one of SQLx'es query macros (using the master branch), this error currently happens in VSCodium when ra runs cargo check:

proc macro returned error: proc-macro panicked: `CARGO` must be set
proc macro returned error: proc-macro panicked: Lazy instance has previously been poisoned
proc macro returned error: proc-macro panicked: Lazy instance has previously been poisoned
... (one poisoned error for every but the first query macro invocation)

The error comes from here: https://github.com/launchbadge/sqlx/blob/df393128f8b07e3b858e65446b2a508a73bf5135/sqlx-macros/src/query/mod.rs#L33

Since cargo there is only used to run cargo metadata, it would most likely be fine for 99% of the cases to fall back to cargo, but it would be nice if it just worked as-is.

[flodiebold]

It's kind of scary to me that proc macros are depending on this 😕 (Are these env variables at least documented for proc macros?)

On the other hand, why complain about this with a proc macro that also makes an actual database connection...

[jplatte]

Yes, this is documented: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates
But of course it's cargo-specific so not documented for proc-macros.

Ideally proc-macros wouldn't rely on cargo-specific things since some users might want to use a different build system. The most prominent cargo-specific thing a lot of proc-macros do is probably using proc-macro-crate, although I guess most uses of it use a fallback instead of erroring when proc_macro_crate::crate_name fails.