commit 8c388e1b816d0a7e5a7d3cc5d213be7f35299cf5
author UebelAndre <andre.brisco@gmail.com> Mon Jan 18 03:53:12 2021 -0800
committer GitHub <noreply@github.com> Mon Jan 18 12:53:12 2021 +0100
tree 921249494490e6ed6661d473af11b073b7c4dd22
parent 9a30c07999a1f41f8775aadffe963fc58bd6d3f5

cargo_build_script now only prints warnings to stderr. (#548)

## Overview
This prevents noisy output from `cargo_build_script` from being printed to the terminal, instead two log files are generated (`stdout`, `stderr`) which contain the raw output of the underlying `build.rs` script

## Details 

Currently users see something similar to the following output when they use `cargo_build_script`:

(note, uplaoded as a file because there's a lot of output)
[big_output.log](https://github.com/bazelbuild/rules_rust/files/5777780/big_output.log)

In general, this masks important information in a sea of arbitrary log lines. The changes in this PR update the output to:
```output
INFO: From CargoBuildScriptRun external/cargo_raze__libssh2_sys__0_2_20/libssh2_sys_build_script.out_dir:
Build Script Warning: libssh2/src/bcrypt_pbkdf.c:84:46: warning: expression does not compute the number of elements in this array; element type is 'uint32_t' (aka 'unsigned int'), not 'uint64_t' (aka 'unsigned long long') [-Wsizeof-array-div]
Build Script Warning:         blf_enc(&state, cdata, sizeof(cdata) / sizeof(uint64_t));
Build Script Warning:                                       ~~~~~  ^
Build Script Warning: libssh2/src/bcrypt_pbkdf.c:65:14: note: array 'cdata' declared here
Build Script Warning:     uint32_t cdata[BCRYPT_BLOCKS];
Build Script Warning:              ^
Build Script Warning: libssh2/src/bcrypt_pbkdf.c:84:46: note: place parentheses around the 'sizeof(uint64_t)' expression to silence this warning
Build Script Warning:         blf_enc(&state, cdata, sizeof(cdata) / sizeof(uint64_t));
Build Script Warning:                                              ^
Build Script Warning: 1 warning generated.
```

And log files are generated for the raw stderr and stdout from the build script, ie:

```output
bazel-out/darwin-fastbuild/bin/external/cargo_raze__libssh2_sys__0_2_20/libssh2_sys_build_script.stderr.log
bazel-out/darwin-fastbuild/bin/external/cargo_raze__libssh2_sys__0_2_20/libssh2_sys_build_script.stdout.log
```

(note, uplaoded as a file because there's a lot of output)
[libssh2_sys_build_script.stderr.log](https://github.com/bazelbuild/rules_rust/files/5777782/libssh2_sys_build_script.stderr.log)

Additionally, the `deps` attribute of `cargo_build_script` now only accepts targets that return the `DepInfo` provider. All other dependencies should be passed via `data`.