Feature Request: Default to only formatting generated code if rustfmt is available

[casey]

Feature Request

Crates

• tonic_build

Motivation

By using tonic_build with the default settings, we inadvertently added a build-time dependency on rustfmt, which was later noticed by a downstream packager, who requested we remove the dependency to avoid extra build-time dependencies.

Proposal

I would suggest that there be three options regarding formatting of generated code:

1. Don't format generated code
2. Always format generated code
3. Only format generated code if rustfmt is available. (I.e. silently continue if launching rustfmt returns a not found error.)

And that 3 should be the default, since formatted code is nice and low-cost if rustfmt is available, but otherwise not important enough to fail a build.

Alternatives

We could do this ourselves in the build script, i.e. check at build-time for rustfmt and pass true or false to the format method of the builder accordingly. However, it seems like this would be a good default that other users of the crate would benefit from.

[tottoto]

Resolved in #904.