Add swift_prefix to managed mode

[bufdev]

Fixes #4213.

This PR adds swift_prefix to managed mode, e.g.

version: v2
inputs:
  - directory: api
managed:
  enabled: true
  override:
    - path: user/v1
      file_option: swift_prefix
      value: UserV1

If managed.enabled: true is set, but no override is provided, the
default behavior for swift_prefix is to take the pascal case of the
package name and use a _ delimited prefix, e.g. foo.empty -> Foo_Empty_.
This emulates the default behavior for apple/swift when swift_prefix
is not set.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Dec 12, 2025, 6:34 PM

[bufdev]

This TODO

[doriable]

Pushed an update for this TODO, thanks for setting this PR up!

[doriable]

Will open a PR for docs so that we have the docs ready to go when this feature is merged and released.

[bufdev]

Just confirming you checked this is right (I'm not sure if there's anything special to verify or. Ot based on what protoc's behavior is, maybe not)

[doriable]

I actually had a question about this/wanted your thoughts on the behaviour.

I checked against apple/swift, and I think the following behaviour makes sense before and after this change:

• When the package is not set and swift_prefix is not set, there will be no prefix (same before and after)
• When swift_prefix is set (in the file before the change and by managed mode after the change), the struct will get the configured prefix.

The only thing I'm unsure about is the default behaviour/what we want to set the default value for swift_prefix in managed mode to be, since swift_prefix is an optional file option for apple/swift. So when a file does not have swift_prefix at all, apple/swift will still generate code and will set the prefix of each struct to a _ delimited version of the package in pascal case, e.g.

syntax = "proto3";

package weather.acme.v1;

message Foo {...}

struct Weather_Acme_V1_Foo: ...

So I wasn't sure if we wanted to set an opinionated version of this, e.g. WeatherAcmeV1, or if we should effectively treat swift_prefix as unset when managed.enabled: true but no override is given to swift_prefix. I'm starting to think we should err on the side of the latter, just pushed up an update, but interested to get your opinion on this.