Elide stats_facebook from the stats crate in pure-glue

Summary:
Drop the C++ `stats_facebook` backend from `common/rust/shed/stats` in a pure-`glue` build, so a `glue_mode[glue]` ancestor (clippy's `:clippy-glue` / `:clippy_cli`) sheds the fb303 `ServiceData` C++ closure — the runtime-closure win motivating the migration. Glue routing comes from the `stats_glue_backend` factory the consumer registers; this diff is what lets that build link no C++.

Backward-compatible and opt-in: with no `glue_mode` modifier (every existing consumer) the `select()`s resolve to `DEFAULT` — `stats_facebook` stays and no `--cfg` is set — so behavior is unchanged.

- `BUCK`: a `glue_mode[glue]` `select()` sets `--cfg=glue_mode="glue"`, and a second drops `//common/rust/shed/stats/facebook:stats_facebook` from `deps` (kept via `DEFAULT`). No `--check-cfg` is needed — the buck toolchain doesn't run strict cfg-checking, so the cfg is set without a declaration (verified: both modes build warning-free).
- `src/lib.rs`: the only two `stats_facebook` references (`get_default_stats_manager_factory`, `create_singleton_counter`) get a `glue_mode = "glue"` arm routing to the existing no-op, mirroring the `not(fbcode_build)` path.
- `Cargo.toml` (+ autocargo manifest): declare `glue_mode` in the OSS `unexpected_cfgs` check-cfg so the cargo build doesn't warn on the new arms (cargo, unlike buck, runs that lint).

Pure-`glue` clippy (`:clippy-glue`) builds green with this; `inproc` / `inproc_and_glue` are untouched.

Reviewed By: mzlee

Differential Revision: D109851332

fbshipit-source-id: 337fee736173f694e6690ab2751ed312a491bbe6

Author: barracel

common/rust/shed/stats/BUCK

@@ -2,6 +2,9 @@ load("@fbsource//tools/build_defs:rust_library.bzl", "rust_library")
 
 oncall("autocargo")
 
+# Taken only when a binary opts into the `glue_mode[glue]` modifier.
+_GLUE_ONLY = "fbcode//hub_infra/build_defs/glue_mode:glue_mode[glue]"
+
 rust_library(
     name = "stats",
     srcs = glob([
@@ -12,7 +15,10 @@ rust_library(
             "lints": {
                 "rust": {
                     "unexpected_cfgs": {
-                        "check-cfg": ["cfg(fbcode_build)"],
+                        "check-cfg": [
+                            "cfg(fbcode_build)",
+                            'cfg(stats_backend, values("noop"))',
+                        ],
                         "level": "warn",
                     },
                 },
@@ -22,15 +28,23 @@ rust_library(
             },
         },
     },
+    # Pure-`glue` drops the C++ backend (sets the neutral `stats_backend="noop"`
+    # cfg the source reads); no modifier = unchanged.
+    rustc_flags = select({
+        "DEFAULT": [],
+        _GLUE_ONLY: ['--cfg=stats_backend="noop"'],
+    }),
     deps = [
         "fbsource//third-party/rust:futures",
         "fbsource//third-party/rust:tokio",
         "fbsource//third-party/rust:tokio-stream",
         ":stats_traits",
         "//common/rust/shed/fbinit:fbinit",
         "//common/rust/shed/perthread:perthread",
-        "//common/rust/shed/stats/facebook:stats_facebook",
-    ],
+    ] + select({
+        "DEFAULT": ["//common/rust/shed/stats/facebook:stats_facebook"],
+        _GLUE_ONLY: [],
+    }),
 )
 
 rust_library(

common/rust/shed/stats/Cargo.toml

@@ -19,4 +19,4 @@ tokio = { version = "1.52.3", features = ["full", "test-util", "tracing"] }
 tokio-stream = { version = "0.1.18", features = ["fs", "io-util", "net", "signal", "sync", "time"] }
 
 [lints.rust]
-unexpected_cfgs = { level = "warn", check-cfg = ["cfg(fbcode_build)"]}
+unexpected_cfgs = { level = "warn", check-cfg = ["cfg(fbcode_build)", 'cfg(stats_backend, values("noop"))']}

common/rust/shed/stats/src/lib.rs

@@ -90,11 +90,11 @@ pub fn create_stats_manager() -> BoxStatsManager {
 }
 
 fn get_default_stats_manager_factory() -> Box<dyn StatsManagerFactory + Send + Sync> {
-    #[cfg(fbcode_build)]
+    #[cfg(all(fbcode_build, not(stats_backend = "noop")))]
     {
         Box::new(::stats_facebook::ThreadLocalStatsFactory)
     }
-    #[cfg(not(fbcode_build))]
+    #[cfg(any(not(fbcode_build), stats_backend = "noop"))]
     {
         Box::new(crate::noop_stats::NoopStatsFactory)
     }
@@ -104,12 +104,12 @@ fn get_default_stats_manager_factory() -> Box<dyn StatsManagerFactory + Send + S
 /// You probably don't have to use this function, it is made public so that it
 /// might be used by the macros in this crate. It creates a new SingletonCounter.
 pub fn create_singleton_counter(name: String) -> BoxSingletonCounter {
-    #[cfg(fbcode_build)]
+    #[cfg(all(fbcode_build, not(stats_backend = "noop")))]
     {
         Box::new(::stats_facebook::singleton_counter::ServiceDataSingletonCounter::new(name))
     }
 
-    #[cfg(not(fbcode_build))]
+    #[cfg(any(not(fbcode_build), stats_backend = "noop"))]
     {
         let _ = name;
         Box::new(crate::noop_stats::Noop)