Add option enabling MIR inlining independently of mir-opt-level

compiler/rustc_interface/src/tests.rs

@@ -557,6 +557,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(function_sections, Some(false));
     tracked!(human_readable_cgu_names, true);
     tracked!(inline_in_all_cgus, Some(true));
+    tracked!(inline_mir, Some(true));
     tracked!(inline_mir_threshold, 123);
     tracked!(inline_mir_hint_threshold, 123);
     tracked!(insert_sideeffect, true);

compiler/rustc_mir/src/transform/inline.rs

@@ -37,21 +37,27 @@ struct CallSite<'tcx> {
     source_info: SourceInfo,
 }
 
+/// Returns true if MIR inlining is enabled in the current compilation session.
+crate fn is_enabled(tcx: TyCtxt<'_>) -> bool {
+    if tcx.sess.opts.debugging_opts.instrument_coverage {
+        // Since `Inline` happens after `InstrumentCoverage`, the function-specific coverage
+        // counters can be invalidated, such as by merging coverage counter statements from
+        // a pre-inlined function into a different function. This kind of change is invalid,
+        // so inlining must be skipped. Note: This check is performed here so inlining can
+        // be disabled without preventing other optimizations (regardless of `mir_opt_level`).
+        return false;
+    }
+
+    if let Some(enabled) = tcx.sess.opts.debugging_opts.inline_mir {
+        return enabled;
+    }
+
+    tcx.sess.opts.debugging_opts.mir_opt_level >= 2
+}
+
 impl<'tcx> MirPass<'tcx> for Inline {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-        // If you change this optimization level, also change the level in
-        // `mir_drops_elaborated_and_const_checked` for the call to `mir_inliner_callees`.
-        // Otherwise you will get an ICE about stolen MIR.
-        if tcx.sess.opts.debugging_opts.mir_opt_level < 2 {
-            return;
-        }
-
-        if tcx.sess.opts.debugging_opts.instrument_coverage {
-            // Since `Inline` happens after `InstrumentCoverage`, the function-specific coverage
-            // counters can be invalidated, such as by merging coverage counter statements from
-            // a pre-inlined function into a different function. This kind of change is invalid,
-            // so inlining must be skipped. Note: This check is performed here so inlining can
-            // be disabled without preventing other optimizations (regardless of `mir_opt_level`).
+        if !is_enabled(tcx) {
             return;
         }
 

compiler/rustc_mir/src/transform/mod.rs

@@ -429,8 +429,7 @@ fn mir_drops_elaborated_and_const_checked<'tcx>(
         let def = ty::WithOptConstParam::unknown(did);
 
         // Do not compute the mir call graph without said call graph actually being used.
-        // Keep this in sync with the mir inliner's optimization level.
-        if tcx.sess.opts.debugging_opts.mir_opt_level >= 2 {
+        if inline::is_enabled(tcx) {
             let _ = tcx.mir_inliner_callees(ty::InstanceDef::Item(def));
         }
     }

compiler/rustc_session/src/options.rs

@@ -957,6 +957,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         (default: no)"),
     incremental_verify_ich: bool = (false, parse_bool, [UNTRACKED],
         "verify incr. comp. hashes of green query instances (default: no)"),
+    inline_mir: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "enable MIR inlining (default: no)"),
     inline_mir_threshold: usize = (50, parse_uint, [TRACKED],
         "a default MIR inlining threshold (default: 50)"),
     inline_mir_hint_threshold: usize = (100, parse_uint, [TRACKED],