unstable feature usage metrics

7 files changed, 87 insertions, 3 deletions

diff --git a/compiler/rustc_driver_impl/messages.ftl b/compiler/rustc_driver_impl/messages.ftl
index 31837e01764..05e11c4527f 100644
--- a/compiler/rustc_driver_impl/messages.ftl
+++ b/compiler/rustc_driver_impl/messages.ftl
@@ -23,3 +23,5 @@ driver_impl_rlink_rustc_version_mismatch = .rlink file was produced by rustc ver
 driver_impl_rlink_unable_to_read = failed to read rlink file: `{$err}`
 
 driver_impl_rlink_wrong_file_type = The input does not look like a .rlink file
+
+driver_impl_unstable_feature_usage = cannot dump feature usage metrics: {$error}
diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index b6f7abed6f3..6c3c81d9f7a 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -50,6 +50,7 @@ use rustc_interface::{Linker, Queries, interface, passes};
 use rustc_lint::unerased_lint_store;
 use rustc_metadata::creader::MetadataLoader;
 use rustc_metadata::locator;
+use rustc_middle::ty::TyCtxt;
 use rustc_parse::{new_parser_from_file, new_parser_from_source_str, unwrap_or_emit_fatal};
 use rustc_session::config::{
     CG_OPTIONS, ErrorOutputType, Input, OutFileName, OutputType, UnstableOptions, Z_OPTIONS,
@@ -102,7 +103,7 @@ mod signal_handler {
 
 use crate::session_diagnostics::{
     RLinkEmptyVersionNumber, RLinkEncodingVersionMismatch, RLinkRustcVersionMismatch,
-    RLinkWrongFileType, RlinkCorruptFile, RlinkNotAFile, RlinkUnableToRead,
+    RLinkWrongFileType, RlinkCorruptFile, RlinkNotAFile, RlinkUnableToRead, UnstableFeatureUsage,
 };
 
 rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
@@ -430,6 +431,10 @@ 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 {
+                queries.global_ctxt()?.enter(|tcxt| dump_feature_usage_metrics(tcxt, metrics_dir));
+            }
+
             if callbacks.after_expansion(compiler, queries) == Compilation::Stop {
                 return early_exit();
             }
@@ -474,6 +479,23 @@ fn run_compiler(
     })
 }
 
+fn dump_feature_usage_metrics(tcxt: TyCtxt<'_>, metrics_dir: &PathBuf) {
+    let output_filenames = tcxt.output_filenames(());
+    let mut metrics_file_name = std::ffi::OsString::from("unstable_feature_usage_metrics-");
+    let mut metrics_path = output_filenames.with_directory_and_extension(metrics_dir, "json");
+    let metrics_file_stem =
+        metrics_path.file_name().expect("there should be a valid default output filename");
+    metrics_file_name.push(metrics_file_stem);
+    metrics_path.pop();
+    metrics_path.push(metrics_file_name);
+    if let Err(error) = tcxt.features().dump_feature_usage_metrics(metrics_path) {
+        // FIXME(yaahc): once metrics can be enabled by default we will want "failure to emit
+        // default metrics" to only produce a warning when metrics are enabled by default and emit
+        // an error only when the user manually enables metrics
+        tcxt.dcx().emit_err(UnstableFeatureUsage { error });
+    }
+}
+
 // Extract output directory and file from matches.
 fn make_output(matches: &getopts::Matches) -> (Option<PathBuf>, Option<OutFileName>) {
     let odir = matches.opt_str("out-dir").map(|o| PathBuf::from(&o));
diff --git a/compiler/rustc_driver_impl/src/session_diagnostics.rs b/compiler/rustc_driver_impl/src/session_diagnostics.rs
index 449878f28c4..e06c56539d1 100644
--- a/compiler/rustc_driver_impl/src/session_diagnostics.rs
+++ b/compiler/rustc_driver_impl/src/session_diagnostics.rs
@@ -1,3 +1,5 @@
+use std::error::Error;
+
 use rustc_macros::{Diagnostic, Subdiagnostic};
 
 #[derive(Diagnostic)]
@@ -93,3 +95,9 @@ pub(crate) struct IceFlags {
 #[derive(Diagnostic)]
 #[diag(driver_impl_ice_exclude_cargo_defaults)]
 pub(crate) struct IceExcludeCargoDefaults;
+
+#[derive(Diagnostic)]
+#[diag(driver_impl_unstable_feature_usage)]
+pub(crate) struct UnstableFeatureUsage {
+    pub error: Box<dyn Error>,
+}
diff --git a/compiler/rustc_feature/Cargo.toml b/compiler/rustc_feature/Cargo.toml
index 9df320e1279..77de7fabd4f 100644
--- a/compiler/rustc_feature/Cargo.toml
+++ b/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
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 8326d0031ea..afd793d71aa 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/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};
@@ -649,6 +651,54 @@ 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.
 ///
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 44721bd889a..bfd38728edb 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1068,7 +1068,7 @@ impl OutputFilenames {
         self.with_directory_and_extension(&self.out_directory, extension)
     }
 
-    fn with_directory_and_extension(&self, directory: &PathBuf, extension: &str) -> PathBuf {
+    pub fn with_directory_and_extension(&self, directory: &PathBuf, extension: &str) -> PathBuf {
         let mut path = directory.join(&self.filestem);
         path.set_extension(extension);
         path
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index f485e8cace5..5d237575991 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/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)"),