docs(architecture): adjudicate the io-metrics leaf dependency on the s3-ops contract crate (#6683)

ARCHITECTURE.md

@@ -115,8 +115,13 @@ default build (lifecycle:
 1. **Layers flow downward.** Server → Admin/App → Storage → ecstore → rio/io-core.
    No upward imports.
 
-2. **Leaf crates have zero internal dependencies.** `config`, `credentials`, `crypto`,
-   `io-metrics`, and `madmin` should depend only on external crates.
+2. **Leaf crates depend only on external crates, with one adjudicated exception.**
+   `config`, `credentials`, `crypto`, `io-metrics`, and `madmin` take no internal
+   dependency except `io-metrics → rustfs-s3-ops` (transitively `rustfs-s3-types`),
+   a pure contract crate with no I/O and no global state. Adjudicated in
+   rustfs/backlog#1834 and pinned by the leaf allowlist in
+   `scripts/check_architecture_migration_rules.sh`; any other internal dependency
+   fails the guard ([crate boundaries](docs/architecture/crate-boundaries.md)).
    - ✅ RESOLVED: the historical `utils → config` and `common → filemeta`/`madmin`
      edges were removed; do not reintroduce them (see Known Structural Issues).
 

docs/architecture/crate-boundaries.md

@@ -35,6 +35,19 @@ wire types still live in `rustfs-filemeta`. This keeps the temporary dependency
 centralized until those wire contracts can move without introducing a
 `rustfs-replication` / `rustfs-storage-api` cycle.
 
+Leaf crates carry exactly one adjudicated allowed edge:
+`io-metrics -> rustfs-s3-ops` (transitively `rustfs-s3-types`). Both are pure
+contract crates — types and enums only, no I/O, no global state, no non-contract
+internal dependencies — so `io-metrics` reuses the `S3Operation` vocabulary
+instead of copying it. The allowance covers that edge and nothing else: the
+leaf-crate allowlist in `scripts/check_architecture_migration_rules.sh` fails any
+other `rustfs-*` dependency in `config`, `credentials`, `crypto`, `io-metrics`,
+or `madmin`. Adjudicated in
+[`rustfs/backlog#1834`](https://github.com/rustfs/backlog/issues/1834); a further
+exception must meet the same criterion — pure contract crate, no I/O, no globals,
+no non-contract internal dependencies — and land its guard allowlist entry
+alongside the dependency.
+
 Dependency direction also applies to compile-time source reads:
 `include_str!`/`include!` of a `.rs` file must not resolve outside the
 including crate's own directory (`scripts/check_layer_dependencies.sh`

scripts/check_architecture_migration_rules.sh

@@ -5454,9 +5454,11 @@ require_source_contains \
 # --- Leaf crates must stay free of internal dependencies (backlog#1834) ---
 # ARCHITECTURE.md invariant 2 names config, credentials, crypto, io-metrics,
 # and madmin as leaf crates that depend only on external crates. Allowlist:
-# io-metrics -> rustfs-s3-ops (contract crate; leaf-allowance adjudication is
-# tracked as backlog#1834 PR2). Adding any other rustfs-* dependency to a leaf
-# crate needs a maintainer decision, not a quiet Cargo.toml edit.
+# io-metrics -> rustfs-s3-ops. That edge is DECIDED in backlog#1834: allowed as a
+# pure-contract-crate exception (types/enums only, no I/O, no globals, no
+# non-contract internal deps), narrowed to exactly this edge. Adding any other
+# rustfs-* dependency to a leaf crate needs its own adjudication, not a quiet
+# Cargo.toml edit.
 LEAF_CRATE_DEP_HITS_FILE="${TMP_DIR}/leaf_crate_dep_hits.txt"
 : >"$LEAF_CRATE_DEP_HITS_FILE"
 (
@@ -5478,7 +5480,7 @@ LEAF_CRATE_DEP_HITS_FILE="${TMP_DIR}/leaf_crate_dep_hits.txt"
 )
 
 if [[ -s "$LEAF_CRATE_DEP_HITS_FILE" ]]; then
-  report_failure "leaf crates (config/credentials/crypto/io-metrics/madmin) must not depend on internal rustfs-* crates (allowlist: io-metrics -> rustfs-s3-ops, backlog#1834): $(paste -sd '; ' "$LEAF_CRATE_DEP_HITS_FILE")"
+  report_failure "leaf crates (config/credentials/crypto/io-metrics/madmin) must not depend on internal rustfs-* crates (sole adjudicated exception, decided in backlog#1834: io-metrics -> rustfs-s3-ops, a pure contract crate; any new edge needs its own adjudication): $(paste -sd '; ' "$LEAF_CRATE_DEP_HITS_FILE")"
 fi
 
 # --- ecstore module-level lint blankets (backlog#1823 step 9) ---