Support gracefully failing if Cargo.toml is not found (or not supported by build system)

[landaire]

At my job we use https://buck2.build/ as our build system, including for Rust crates. Unfortunately some dependencies seem to use this crate to document features in a manner which cannot be disabled, which makes ingesting the crates difficult.

While I plan on making a PR to these crates, this method doesn't scale well and it would be nice if there were an env var check or something similar to gracefully make the macro a no-op in such scenarios.

[ogoffart]

The reason for not failing gracefully was that if the Cargo.toml file cannot be found, this is most likely some problem and the documentation would not be rendered properly and it would be hard to diagnose why.

But perhaps this could be changed to a warning, or to output a warning string.

[landaire]

Oh sorry, to clarify I'm suggesting just something that can override the behavior to be a graceful fail. I'll also say that this is not critical for me as shortly after filing this I managed to find a workaround in our build system by ensuring Cargo.toml from the upstream crate is available in its build directory during compilation.