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

1 files changed, 23 insertions, 1 deletions

diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index d2333454f28..c270ce16726 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -51,6 +51,7 @@ 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,
@@ -103,7 +104,7 @@ 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" }
@@ -431,6 +432,10 @@ 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();
             }
@@ -475,6 +480,23 @@ 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));