Rollup merge of #130236 - yaahc:unstable-feature-usage, r=estebank

unstable feature usage metrics

example output

```
test-lib on  master [?] is 📦 v0.1.0 via 🦀 v1.80.1
❯ cat src/lib.rs
───────┬───────────────────────────────────────────────────────
       │ File: src/lib.rs
───────┼───────────────────────────────────────────────────────
   1   │ #![feature(unix_set_mark)]
   2   │ pub fn add(left: u64, right: u64) -> u64 {
   3   │     left + right
   4   │ }
   5   │
   6   │ #[cfg(test)]
   7   │ mod tests {
   8   │     use super::*;
   9   │
  10   │     #[test]
  11   │     fn it_works() {
  12   │         let result = add(2, 2);
  13   │         assert_eq!(result, 4);
  14   │     }
  15   │ }
───────┴───────────────────────────────────────────────────────

test-lib on  master [?] is 📦 v0.1.0 via 🦀 v1.80.1
❯ cargo +stage1 rustc -- -Zmetrics-dir=$PWD/metrics
   Compiling test-lib v0.1.0 (/home/yaahc/tmp/test-lib)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.08s

test-lib on  master [?] is 📦 v0.1.0 via 🦀 v1.80.1
❯ cat metrics/unstable_feature_usage.json
───────┬─────────────────────────────────────────────────────────────────────
       │ File: metrics/unstable_feature_usage.json
───────┼─────────────────────────────────────────────────────────────────────
   1   │ {"lib_features":[{"symbol":"unix_set_mark"}],"lang_features":[]}
   ```

   related to https://github.com/rust-lang/rust/issues/129485

compiler/rustc_feature/Cargo.toml

@@ -7,4 +7,6 @@ edition = "2021"
 # tidy-alphabetical-start
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_span = { path = "../rustc_span" }
+serde = { version = "1.0.125", features = [ "derive" ] }
+serde_json = "1.0.59"
 # tidy-alphabetical-end

compiler/rustc_feature/src/unstable.rs

@@ -1,5 +1,7 @@
 //! List of the unstable feature gates.
 
+use std::path::PathBuf;
+
 use rustc_data_structures::fx::FxHashSet;
 use rustc_span::Span;
 use rustc_span::symbol::{Symbol, sym};
@@ -651,6 +653,54 @@ declare_features! (
     // -------------------------------------------------------------------------
 );
 
+impl Features {
+    pub fn dump_feature_usage_metrics(
+        &self,
+        metrics_path: PathBuf,
+    ) -> Result<(), Box<dyn std::error::Error>> {
+        #[derive(serde::Serialize)]
+        struct LibFeature {
+            symbol: String,
+        }
+
+        #[derive(serde::Serialize)]
+        struct LangFeature {
+            symbol: String,
+            since: Option<String>,
+        }
+
+        #[derive(serde::Serialize)]
+        struct FeatureUsage {
+            lib_features: Vec<LibFeature>,
+            lang_features: Vec<LangFeature>,
+        }
+
+        let metrics_file = std::fs::File::create(metrics_path)?;
+        let metrics_file = std::io::BufWriter::new(metrics_file);
+
+        let lib_features = self
+            .enabled_lib_features
+            .iter()
+            .map(|EnabledLibFeature { gate_name, .. }| LibFeature { symbol: gate_name.to_string() })
+            .collect();
+
+        let lang_features = self
+            .enabled_lang_features
+            .iter()
+            .map(|EnabledLangFeature { gate_name, stable_since, .. }| LangFeature {
+                symbol: gate_name.to_string(),
+                since: stable_since.map(|since| since.to_string()),
+            })
+            .collect();
+
+        let feature_usage = FeatureUsage { lib_features, lang_features };
+
+        serde_json::to_writer(metrics_file, &feature_usage)?;
+
+        Ok(())
+    }
+}
+
 /// Some features are not allowed to be used together at the same time, if
 /// the two are present, produce an error.
 ///