From d88420a52a9d1860b47245fdda374162d1c0972c Mon Sep 17 00:00:00 2001
From: Camille GILLOT <gillot.camille@gmail.com>
Date: Thu, 21 Jan 2021 18:21:55 +0100
Subject: [PATCH] Review comment.

---
 compiler/rustc_metadata/src/rmeta/encoder.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 8d985e72711..48524e81f3e 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -783,6 +783,11 @@ fn should_encode_stability(def_kind: DefKind) -> bool {
 
 /// Whether we should encode MIR.
 ///
+/// Computing, optimizing and encoding the MIR is a relatively expensive operation.
+/// We want to avoid this work when not required. Therefore:
+/// - we only compute `mir_for_ctfe` on items with const-eval semantics;
+/// - we skip `optimized_mir` for check runs.
+///
 /// Return a pair, resp. for CTFE and for LLVM.
 fn should_encode_mir(tcx: TyCtxt<'_>, def_id: LocalDefId) -> (bool, bool) {
     match tcx.def_kind(def_id) {