Internal compiler

Protobuf is the most stable and widely adopted interface description language available today - it's why Buf is concentrating its initial efforts on Protobuf. But Protobuf has never had an officially published Protobuf grammar - there are proto2 and proto3 specs published, but neither actually cover all edge cases, of which there are many (especially around options).

In effect, the official Protobuf "grammar" is the protoc implementation - this has been the only codified representation of what Protobuf is, and the only way to properly parse Protobuf messages and produce FileDescriptorSets suitable for stub generation.

Additionally, there are many situations outside of stub generation that rely on proper Protobuf parsing, such as linting and breaking change detection. All existing Protobuf tooling has gone one of two routes:

Use a third-party Protobuf parser instead of protoc that produces non-FileDescriptorSet results. There are many third-party Protobuf parsers in existence, however no parser has been able to reliably cover all edge cases of the grammar, inevitably there are breakdowns that either result in parse errors, or an invalid representation of Protobuf sources. The edge cases in the Protobuf grammar are so numerous, that some of the most popular third-party parsers actually get around the problem by happily parsing invalid Protobuf, resulting in being unable to make a decision from these parsers as to whether or not a file is valid.
Shell out to (or build against) protoc. This results in both accurate parsing, and FileDescriptorSet production, however this method presents a number of issues. First, actually managing external protoc installs becomes problematic - it makes any tooling reliant on either managing protoc installation itself, or relying on protoc being deterministically installed. Second, parsing protoc's output is difficult, as there is no structured output format, both warnings and errors are printed to stderr, and the warning and error output changes between minor releases. To accurately parse protoc, tooling needs to handle every release of protoc as it comes out, which makes any such tooling unmaintainable. Additionally, protoc has different behavior depending on the location of the Well-Known Types.

We find neither of these solutions to be tenable in the long-term for a tool that aims to manage your Protobuf schema. Therefore, we've taken a different route.

The internal compiler quite literally replaces protoc outside of the built-in plugins (--java_out, --cpp_out, etc.). The resulting FileDescriptorSets are tested for equivalence to protoc, including both proto2 and proto3 definitions, imports, FileDescriptorProto ordering, SourceCodeInfo, and custom options. (We even took what we learned from building this compiler to write a comprehensive language specification!)

The resulting FileDescriptorSets are almost byte equivalent to protoc, in fact - under most scenarios without SourceCodeInfo, you can actually compare the byte representation of a serialized FileDescriptorSet produced by buf and by protoc, and they are equal. There are two known exceptions that make this not always the case:

buf produces slightly different SourceCodeInfo for the special json_name and default field options, to be consistent with SourceCodeInfo of other options. In protoc, the SourceCodeInfo for these special options is inconsistent.
buf represents custom/unknown options slightly differently on the wire, although when deserialized, the result is equivalent for consumers of FileDescriptorSets. The underlying compiler provides a work-around (see method CanonicalProto of linker.Result), but buf does not enable it because it has both a performance and usability cost, and it has zero effect on any actual usage.

Besides removing the need to manually manage protoc and the Well-Known Types (which buf handles in all cases), buf's compiler is faster than protoc in most scenarios. buf parses your .proto files across all available cores, and re-orders the result to match protoc's ordering as a post-processing task. As an example, buf can compile all 3,944 .proto files in googleapis in about 0.9s, on a 2022 MacBook Pro with an M1 Max chip, as opposed to about 1.6s for protoc v21.7 on the same machine.

We know this is all a series of big claims. There have been many claims in the Protobuf community about producing non-protoc-based parsing, so this is one of the reasons that we enable protoc output to be buf input. If you don't trust us, then use protoc as your compiler instead, no problem.

It's also one of the reasons we've exposed buf build as we have - you can produce FileDescriptorSets yourself and pass them to your Protobuf plugins to verify that the resulting stubs are equivalent. There is one known exception with docs generated based on json_name, see this issue to track this being updated within protoc.

Instead of this...

# Adjust -I as necessary; this example includes the current directory.
rm -rf java
mkdir java
protoc -I . --java_out=java $(find . -name '*.proto')

...you can use buf's compiler to generate your stubs with the --descriptor_set_in flag of protoc:

# We need to do "buf build | buf ls-files -" instead of "buf ls-files"
# to make sure that the filenames are root
rm -rf java
mkdir java
buf build -o - | protoc --descriptor_set_in=/dev/stdin --java_out=java $(buf ls-files)

This results in protoc's internal parser not being used at all, so you can verify our claims further. If you do find an issue, contact us.

Having this new compiler is a key component of Buf's future. Right now, it enables reliable linting, breaking change detection, generation, and the BSR. In the future, it enables a lot of other real-time possibilities for us.