unstable feature usage metrics

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}

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));

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>,
+}