abi_check: tweak some comments

Author: RalfJung

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

@@ -35,8 +35,6 @@ fn do_check_simd_vector_abi<'tcx>(
     is_call: bool,
     loc: impl Fn() -> (Span, HirId),
 ) {
-    // We check this on all functions, including those using the "Rust" ABI.
-    // For the "Rust" ABI it would be a bug if the lint ever triggered, but better safe than sorry.
     let feature_def = tcx.sess.target.features_for_correct_vector_abi();
     let codegen_attrs = tcx.codegen_fn_attrs(def_id);
     let have_feature = |feat: Symbol| {
@@ -123,8 +121,9 @@ fn do_check_wasm_abi<'tcx>(
     is_call: bool,
     loc: impl Fn() -> (Span, HirId),
 ) {
-    // Only proceed for `extern "C" fn` on wasm32-unknown-unknown (same check as what `adjust_for_foreign_abi` uses to call `compute_wasm_abi_info`),
-    // and only proceed if `wasm_c_abi_opt` indicates we should emit the lint.
+    // Only proceed for `extern "C" fn` on wasm32-unknown-unknown (same check as what
+    // `adjust_for_foreign_abi` uses to call `compute_wasm_abi_info`), and only proceed if
+    // `wasm_c_abi_opt` indicates we should emit the lint.
     if !(tcx.sess.target.arch == "wasm32"
         && tcx.sess.target.os == "unknown"
         && tcx.wasm_c_abi_opt() == WasmCAbi::Legacy { with_lint: true }
@@ -157,8 +156,15 @@ fn check_instance_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) {
     else {
         // An error will be reported during codegen if we cannot determine the ABI of this
         // function.
+        tcx.dcx().delayed_bug("ABI computation failure should lead to compilation failure");
         return;
     };
+    // Unlike the call-site check, we do also check "Rust" ABI functions here.
+    // This should never trigger, *except* if we start making use of vector registers
+    // for the "Rust" ABI and the user disables those vector registers (which should trigger a
+    // warning as that's clearly disabling a "required" target feature for this target).
+    // Using such a function is where disabling the vector register actually can start leading
+    // to soundness issues, so erroring here seems good.
     let loc = || {
         let def_id = instance.def_id();
         (
@@ -179,7 +185,8 @@ fn check_call_site_abi<'tcx>(
     loc: impl Fn() -> (Span, HirId) + Copy,
 ) {
     if callee.fn_sig(tcx).abi().is_rustic_abi() {
-        // we directly handle the soundness of Rust ABIs
+        // We directly handle the soundness of Rust ABIs -- so let's skip the majority of
+        // call sites to avoid a perf regression.
         return;
     }
     let typing_env = ty::TypingEnv::fully_monomorphized();