unstable feature usage metrics

Cargo.lock

@@ -3684,6 +3684,8 @@ version = "0.0.0"
 dependencies = [
  "rustc_data_structures",
  "rustc_span",
+ "serde",
+ "serde_json",
 ]
 
 [[package]]

compiler/rustc_driver_impl/src/lib.rs

@@ -60,6 +60,7 @@ use rustc_session::lint::{Lint, LintId};
 use rustc_session::output::collect_crate_types;
 use rustc_session::{EarlyDiagCtxt, Session, config, filesearch};
 use rustc_span::FileName;
+use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::source_map::FileLoader;
 use rustc_target::json::ToJson;
 use rustc_target::spec::{Target, TargetTuple};
@@ -430,6 +431,16 @@ fn run_compiler(
             // Make sure name resolution and macro expansion is run.
             queries.global_ctxt()?.enter(|tcx| tcx.resolver_for_lowering());
 
+            if let Some(metrics_dir) = &sess.opts.unstable_opts.metrics_dir {
+                compiler.enter(|queries| -> Result<(), ErrorGuaranteed> {
+                    queries.global_ctxt()?.enter(|tcxt| {
+                        let crate_id = tcxt.stable_crate_id(LOCAL_CRATE);
+                        tcxt.features().dump_feature_usage_metrics(metrics_dir, crate_id)
+                    });
+                    Ok(())
+                })?;
+            }
+
             if callbacks.after_expansion(compiler, queries) == Compilation::Stop {
                 return early_exit();
             }

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

@@ -1,7 +1,10 @@
 //! List of the unstable feature gates.
 
+use std::path::Path;
+
 use rustc_data_structures::fx::FxHashSet;
 use rustc_span::Span;
+use rustc_span::def_id::StableCrateId;
 use rustc_span::symbol::{Symbol, sym};
 
 use super::{Feature, to_nonzero};
@@ -649,6 +652,60 @@ declare_features! (
     // -------------------------------------------------------------------------
 );
 
+impl Features {
+    pub fn dump_feature_usage_metrics(&self, metrics_dir: &Path, _crate_id: StableCrateId) {
+        #[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>,
+        }
+
+        // TODO (DECIDE): How fine grained do we want to track feature usage?
+        //      Jane Preference: i want to track usage for code that gets used, not code
+        //      that is in development, but I don't know how we'd hook into this for code
+        //      that doesn't participate in the crates.io ecosystem, those crates won't even
+        //      necessarily have releases or versioning norms that match other crates, so we
+        //      may have to just track per compilation and aggressively collapse metrics to
+        //      avoid unnecessary disk usage.
+        // TODO avoid filename collisions between runs
+        // let path = format!("unstable_feature_usage-{crate_id:?}.json");
+        let feature_metrics_file = metrics_dir.join("unstable_feature_usage.json");
+        println!("{}", feature_metrics_file.display());
+        let feature_metrics_file = std::fs::File::create(feature_metrics_file).unwrap();
+        let feature_metrics_file = std::io::BufWriter::new(feature_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 };
+
+        let _ = serde_json::to_writer(feature_metrics_file, &feature_usage);
+    }
+}
+
 /// Some features are not allowed to be used together at the same time, if
 /// the two are present, produce an error.
 ///

compiler/rustc_session/src/options.rs

@@ -1889,7 +1889,7 @@ options! {
     meta_stats: bool = (false, parse_bool, [UNTRACKED],
         "gather metadata statistics (default: no)"),
     metrics_dir: Option<PathBuf> = (None, parse_opt_pathbuf, [UNTRACKED],
-        "stores metrics about the errors being emitted by rustc to disk"),
+        "the directory metrics emitted by rustc are dumped into (implicitly enables default set of metrics)"),
     mir_emit_retag: bool = (false, parse_bool, [TRACKED],
         "emit Retagging MIR statements, interpreted e.g., by miri; implies -Zmir-opt-level=0 \
         (default: no)"),

tests/run-make/unstable-feature-usage-metrics/lib.rs

@@ -0,0 +1,9 @@
+#![feature(ascii_char)] // random lib feature
+#![feature(box_patterns)] // random lang feature
+
+// picked arbitrary unstable features, just need a random lib and lang feature, ideally ones that
+// won't be stabilized any time soon so we don't have to update this test
+
+fn main() {
+    println!("foobar");
+}

tests/run-make/unstable-feature-usage-metrics/rmake.rs

@@ -0,0 +1,63 @@
+//! This test checks if unstable feature usage metric dump files `unstable-feature-usage*.json` work
+//! as expected.
+//!
+//! - Basic sanity checks on a default ICE dump.
+//!
+//! See <https://github.com/rust-lang/rust/issues/129485>.
+//!
+//! # Test history
+//!
+//! - forked from dump-ice-to-disk test, which has flakeyness issues on i686-mingw, I'm assuming
+//! those will be present in this test as well on the same platform
+
+//@ ignore-windows
+//FIXME(#128911): still flakey on i686-mingw.
+
+use std::path::{Path, PathBuf};
+
+use run_make_support::{
+    cwd, has_extension, has_prefix, rfs, run_in_tmpdir, rustc, serde_json, shallow_find_files,
+};
+
+fn find_feature_usage_metrics<P: AsRef<Path>>(dir: P) -> Vec<PathBuf> {
+    shallow_find_files(dir, |path| {
+        has_prefix(path, "unstable_feature_usage") && has_extension(path, "json")
+    })
+}
+
+fn main() {
+    test_metrics_dump();
+}
+
+#[track_caller]
+fn test_metrics_dump() {
+    run_in_tmpdir(|| {
+        let metrics_dir = cwd().join("metrics");
+        rustc()
+            .input("lib.rs")
+            .env("RUST_BACKTRACE", "short")
+            .arg(format!("-Zmetrics-dir={}", metrics_dir.display()))
+            .run();
+        let mut metrics = find_feature_usage_metrics(&metrics_dir);
+        let json_path =
+            metrics.pop().expect("there should be exactly metrics file in the output directory");
+
+        assert_eq!(
+            0,
+            metrics.len(),
+            "there should be exactly one metrics file in the output directory"
+        );
+
+        let message = rfs::read_to_string(json_path);
+        let parsed: serde_json::Value =
+            serde_json::from_str(&message).expect("metrics should be dumped as json");
+        let expected = serde_json::json!(
+            {
+                "lib_features":[{"symbol":"ascii_char"}],
+                "lang_features":[{"symbol":"box_patterns","since":null}]
+            }
+        );
+
+        assert_eq!(expected, parsed);
+    });
+}