add comments explaining where post-mono const eval errors abort compilation

Author: RalfJung

compiler/rustc_monomorphize/src/collector.rs

@@ -814,13 +814,16 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
         self.super_rvalue(rvalue, location);
     }
 
-    /// This does not walk the constant, as it has been handled entirely here and trying
-    /// to walk it would attempt to evaluate the `ty::Const` inside, which doesn't necessarily
-    /// work, as some constants cannot be represented in the type system.
+    /// This does not walk the MIR of the constant as that is not needed for codegen, all we need is
+    /// to ensure that the constant evaluates successfully and walk the result.
     #[instrument(skip(self), level = "debug")]
     fn visit_constant(&mut self, constant: &mir::ConstOperand<'tcx>, location: Location) {
         let const_ = self.monomorphize(constant.const_);
         let param_env = ty::ParamEnv::reveal_all();
+        // Evaluate the constant. This makes const eval failure a collection-time error (rather than
+        // a codegen-time error). rustc stops after collection if there was an error, so this
+        // ensures codegen never has to worry about failing consts.
+        // (codegen relies on this and ICEs will happen if this is violated.)
         let val = match const_.eval(self.tcx, param_env, None) {
             Ok(v) => v,
             Err(ErrorHandled::Reported(..)) => return,

compiler/rustc_monomorphize/src/partitioning.rs

@@ -1114,6 +1114,9 @@ fn collect_and_partition_mono_items(tcx: TyCtxt<'_>, (): ()) -> (&DefIdSet, &[Co
 
     let (items, usage_map) = collector::collect_crate_mono_items(tcx, collection_mode);
 
+    // If there was an error during collection (e.g. from one of the constants we evaluated),
+    // then we stop here. This way codegen does not have to worry about failing constants.
+    // (codegen relies on this and ICEs will happen if this is violated.)
     tcx.dcx().abort_if_errors();
 
     let (codegen_units, _) = tcx.sess.time("partition_and_assert_distinct_symbols", || {