Ref T784. Allow differential changesets to be marked as generated with a DifferentialDiffProperty. The expectation is that arc diff will submit an arc:generated diff property (which will be populated out of the local .arcconfig file). These paths will then be automatically marked as generated when rendering the differential changeset.

Currently, the only way to achieve this is with differential.generated-paths. The main issue with differential.generated-paths is that it is a global setting and cannot be varied across repositories. A natural extension from differential.generated-paths would be to move this configuration into the per-repository configuration, however I feel that this approach would be too inflexible to be useful. Specifically, this setting would need to be updated whenever generated paths are added/removed/changed for a particular repository (although I would expect this to be a fairly infrequent occurrence).

Unfortunately, this functionality will not be retrofitted to existing differentials. We could possibly add a migration which populates arc:generated for all existing differentials based on the value of differential.generated-paths, although I'm not convinced that this is worth pursuing.

Uploaded a diff (using arc diff) and set paths for arc:generated with arc call-conduit 'differential.setdiffproperty'. Loaded the differential in the UI and noticed that the paths were marked as generated as expected.

One interesting thing to note (I'm not sure if I would consider this to be a bug or expected behavior) is that lint messages takes a higher precedence over generated paths. If a path matches a regular expression in arc:generated but has some linter messages associated with that path, then the file will not be shown as generated in the UI. This is mainly an issue until after D11537.