Developing with a local Glean build

FOG uses a release version of Glean, as published on crates.io.

For local development and try runs you can replace this Glean implementation with a local or remote version.

To tell mach where to find your Glean, patch the top-level Cargo.toml to point at your Glean fork:

[patch.crates-io]
glean = { git = "https://github.com/myfork/glean", branch = "my-feature-branch" }
glean-core = { git = "https://github.com/myfork/glean", branch = "my-feature-branch" }

Both crates need to be specified to ensure they are in sync.

You can specify the exact code to use by branch, tag or rev (Git commit). See the cargo documentation for details.

You can also use a path dependency, for a truly local Glean:

[patch.crates-io]
glean = { path = "../glean/glean-core/rlb" }
glean-core = { path = "../glean/glean-core" }

If the version of the Glean SDK in the patched repository is not semver-compatible with the versions required by the workspaces within mozilla-central, trying to update the Cargo lockfile at this time will result in an error like
```
warning: Patch `glean vMM.m.p (/path/to/glean/glean-core/rlb)` was not used in the crate graph.
```
In this case, change the glean crate version in both Cargo.toml and gfx/wr/Cargo.toml to match the version in your glean repo:
```
glean = "=MM.m.p"
```
Update the Cargo lockfile for the root workspace:
```
cargo update -p glean
```
(You’d think that, if you had to change gfx/wr/Cargo.toml in step 2, you’d have to cargo update in gfx/wr too, but you’d be wrong. It’s not needed, and if you try it’ll just error about crates.io not knowing about your local version of Glean.)
Mozilla’s supply-chain management policy requires that third-party software (which includes the Glean SDK because it is distributed as though it is third-party) be audited and certified as safe. Your local Glean SDK is not coming from a source that has been vetted. If you try to vendor right now, ./mach vendor rust may complain something like:
```
Vet error: There are some issues with your policy.audit-as-crates-io entries
```
To allow your local Glean crates to be treated as crates.io-sourced crates for vetting, add the following sections to the top of supply-chain/config.toml:
```
[policy.glean]
audit-as-crates-io = true

[policy.glean-core]
audit-as-crates-io = true
```
Note: Do not attempt to check these changes in. @supply-chain-reviewers may become cross as they r- your patch.
Vendor the changed crates:
```
./mach vendor rust
```
Note: If you’re using a path dependency, mach vendor rust doesn’t actually change files. Instead it pulls the files directly from the location on disk you specify.
Finally, build Firefox:
```
./mach build
```

A remote git reference works for try runs as well, but a path dependency will not.

Please ensure you do not land a non-release version of Glean.