This a backend for the Pigweed logging facet, pw_log.
Facets are resolved globally for a build, but different parts of the Fuchsia build (within the same toolchain) have different logging requirements. For example drivers and command-line tools. To address this the backend //third_party/pigweed/backends/pw_log contains the interface and some common code but depends on an implementation. These must be linked into the target that uses Pigweed code that uses logging.
The current targets are:
//third_party/pigweed/backends/pw_log:dfv1 : uses the DFV1 logging APIs.
//third_party/pigweed/backends/pw_log:printf : uses libc printf to send messages to stdout.
More targets can be added in the future by adding more source_sets that provide the C symbol:
void pw_log_fuchsia_impl(int level, const char* module_name, const char* file_name, int line_number, const char* message)