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# Developing with a local Glean build
FOG uses a release version of Glean, as published on [crates.io][cratesio-glean].
For local development and try runs you can replace this Glean implementation with a local or remote version.
1. To tell `mach` where to find your Glean, patch the top-level {searchfox}`Cargo.toml`
to point at your Glean fork:
```toml
[patch.crates-io]
```
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][cargo-doc].
You can also use a path dependency, for a truly local Glean:
```toml
[patch.crates-io]
glean = { path = "../glean/glean-core/rlb" }
glean-core = { path = "../glean/glean-core" }
```
2. 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 {searchfox}`Cargo.toml`
and {searchfox}`gfx/wr/Cargo.toml` to match the version in your `glean` repo:
```toml
glean = "=MM.m.p"
```
3. Update the Cargo lockfile for the root workspace:
```
cargo update -p glean
```
(You'd think that, if you had to change {searchfox}`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.)
4. 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`:
```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.
5. 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.
6. 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.