Add timestamp to unstable feature usage metrics

compiler/rustc_feature/src/unstable.rs

@@ -1,6 +1,7 @@
 //! List of the unstable feature gates.
 
 use std::path::PathBuf;
+use std::time::{SystemTime, UNIX_EPOCH};
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_span::{Span, Symbol, sym};
@@ -681,11 +682,13 @@ impl Features {
     ) -> Result<(), Box<dyn std::error::Error>> {
         #[derive(serde::Serialize)]
         struct LibFeature {
+            timestamp: u128,
             symbol: String,
         }
 
         #[derive(serde::Serialize)]
         struct LangFeature {
+            timestamp: u128,
             symbol: String,
             since: Option<String>,
         }
@@ -699,10 +702,20 @@ impl Features {
         let metrics_file = std::fs::File::create(metrics_path)?;
         let metrics_file = std::io::BufWriter::new(metrics_file);
 
+        let now = || {
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .expect("system time should always be greater than the unix epoch")
+                .as_nanos()
+        };
+
         let lib_features = self
             .enabled_lib_features
             .iter()
-            .map(|EnabledLibFeature { gate_name, .. }| LibFeature { symbol: gate_name.to_string() })
+            .map(|EnabledLibFeature { gate_name, .. }| LibFeature {
+                symbol: gate_name.to_string(),
+                timestamp: now(),
+            })
             .collect();
 
         let lang_features = self
@@ -711,6 +724,7 @@ impl Features {
             .map(|EnabledLangFeature { gate_name, stable_since, .. }| LangFeature {
                 symbol: gate_name.to_string(),
                 since: stable_since.map(|since| since.to_string()),
+                timestamp: now(),
             })
             .collect();