add some comments explaining how the required_consts stuff fits together

2 files changed, 6 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/constant.rs b/compiler/rustc_codegen_ssa/src/mir/constant.rs
index fde4e85f966..558f64fffc2 100644
--- a/compiler/rustc_codegen_ssa/src/mir/constant.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/constant.rs
@@ -21,6 +21,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     }
 
     pub fn eval_mir_constant(&self, constant: &mir::ConstOperand<'tcx>) -> mir::ConstValue<'tcx> {
+        // `MirUsedCollector` visited all constants before codegen began, so if we got here there
+        // can be no more constants that fail to evaluate.
         self.monomorphize(constant.const_)
             .eval(self.cx.tcx(), ty::ParamEnv::reveal_all(), Some(constant.span))
             .expect("erroneous constant not captured by required_consts")
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 06cdc47610d..d0b799e087b 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -209,6 +209,10 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         caller_location: None,
     };
 
+    // It may seem like we should iterate over `required_consts` to ensure they all successfully
+    // evaluate; however, the `MirUsedCollector` already did that during the collection phase of
+    // monomorphization so we don't have to do it again.
+
     fx.per_local_var_debug_info = fx.compute_per_local_var_debug_info(&mut start_bx);
 
     let memory_locals = analyze::non_ssa_locals(&fx);