chore(ci): speed up nightly builds

.github/actions/setup-rust/action.yml

@@ -13,6 +13,14 @@ inputs:
     description: Whether to restore the Rust cache
     required: false
     default: 'true'
+  cache-namespace:
+    description: >
+      Cache namespace. Workflows that compile with different profile settings
+      MUST use different namespaces — cargo fingerprints every dependency by its
+      profile, so sharing one would mean each workflow evicts artifacts the other
+      cannot use.
+    required: false
+    default: release-cache
 runs:
   using: composite
   steps:
@@ -29,7 +37,7 @@ runs:
       with:
         key: ${{ inputs.target }}
         save-if: ${{ inputs.save-cache }}
-        shared-key: release-cache
+        shared-key: ${{ inputs.cache-namespace }}
         # Release builds are long and can fail late (signing, notarization,
         # upload). Without this the whole compile is thrown away on those runs.
         cache-on-failure: true

.github/workflows/oneclient_nightly.yml

@@ -10,6 +10,22 @@ concurrency:
   group: oneclient-nightly-${{ github.ref }}
   cancel-in-progress: true
 
+# Nightlies are unsigned test builds, so they do not need the release profile's
+# whole-program optimization — and that optimization is exactly what makes them
+# slow. `lto = "fat"` + `codegen-units = 1` force a single-threaded re-optimization
+# of the entire dependency graph at link time, on every run, which no cache can
+# ever skip. Thin LTO keeps nearly all of the runtime benefit at a fraction of the
+# link cost, and 16 codegen units lets the whole compile use every runner core.
+#
+# These are cargo's documented profile overrides, so `target/release` stays the
+# output directory and every path below is unaffected. The shipped release binary
+# is untouched: oneclient_release.yml still builds the fat-LTO profile from
+# Cargo.toml. That difference is why the two workflows must not share a cache
+# namespace — see `cache-namespace` on the Setup Rust step.
+env:
+  CARGO_PROFILE_RELEASE_LTO: thin
+  CARGO_PROFILE_RELEASE_CODEGEN_UNITS: '16'
+
 jobs:
   build:
     permissions:
@@ -55,6 +71,10 @@ jobs:
           target: ${{ matrix.target }}
           save-cache: true
           restore-cache: true
+          # Separate namespace: nightly's thin-LTO dependency artifacts have
+          # different cargo fingerprints than the release workflow's fat-LTO ones,
+          # so a shared cache would just thrash between the two.
+          cache-namespace: nightly-cache
 
       - name: (Linux) Install system dependencies
         if: matrix.platform == 'ubuntu-22.04'