Improve comments.

Author: cjgillot

compiler/rustc_const_eval/src/const_eval/dummy_machine.rs

@@ -124,8 +124,8 @@ impl<'tcx> interpret::Machine<'tcx> for DummyMachine {
 
     #[inline(always)]
     fn runtime_checks(_ecx: &InterpCx<'tcx, Self>, r: RuntimeChecks) -> InterpResult<'tcx, bool> {
-        // We can't look at `tcx.sess` here as that can differ across crates, which can lead to
-        // unsound differences in evaluating the same constant at different instantiation sites.
+        // Runtime checks have different value depending on the crate they are codegenned in.
+        // Verify we aren't trying to evaluate them in mir-optimizations.
         panic!("compiletime machine evaluated {r:?}")
     }
 

compiler/rustc_middle/src/mir/statement.rs

@@ -694,6 +694,7 @@ impl<'tcx> Operand<'tcx> {
                 local_decls.local_decls()[l.local].source_info.span
             }
             Operand::Constant(c) => c.span,
+            // User code should not contain this operand, so we should not need this span.
             Operand::RuntimeChecks(_) => DUMMY_SP,
         }
     }

compiler/rustc_middle/src/mir/syntax.rs

@@ -1328,8 +1328,8 @@ pub enum Operand<'tcx> {
     /// Constants are already semantically values, and remain unchanged.
     Constant(Box<ConstOperand<'tcx>>),
 
-    /// Special constants whose value depends on the evaluation context. Their value depends on a
-    /// flag on the crate being codegenned.
+    /// Query the compilation session of the current crate for a particular flag. This is not quite
+    /// a const since its value can differ across crates within a single crate graph.
     RuntimeChecks(RuntimeChecks),
 }