get rid of NoMirFor error variant

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -402,8 +402,6 @@ impl fmt::Display for UndefinedBehaviorInfo<'_> {
 pub enum UnsupportedOpInfo {
     /// Free-form case. Only for errors that are never caught!
     Unsupported(String),
-    /// Could not find MIR for a function.
-    NoMirFor(DefId),
     /// Encountered a pointer where we needed raw bytes.
     ReadPointerAsBytes,
     //
@@ -421,7 +419,6 @@ impl fmt::Display for UnsupportedOpInfo {
         match self {
             Unsupported(ref msg) => write!(f, "{}", msg),
             ReadExternStatic(did) => write!(f, "cannot read from extern static ({:?})", did),
-            NoMirFor(did) => write!(f, "no MIR body is available for {:?}", did),
             ReadPointerAsBytes => write!(f, "unable to turn pointer into raw bytes",),
             ThreadLocalStatic(did) => write!(f, "cannot access thread local static ({:?})", did),
         }

compiler/rustc_mir/src/const_eval/machine.rs

@@ -212,7 +212,9 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 if ecx.tcx.is_ctfe_mir_available(def.did) {
                     Ok(ecx.tcx.mir_for_ctfe_opt_const_arg(def))
                 } else {
-                    throw_unsup!(NoMirFor(def.did))
+                    let path = ecx.tcx.def_path_str(def.did);
+                    Err(ConstEvalErrKind::NeedsRfc(format!("calling extern function `{}`", path))
+                        .into())
                 }
             }
             _ => Ok(ecx.tcx.instance_mir(instance)),
@@ -247,20 +249,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
             }
         }
         // This is a const fn. Call it.
-        Ok(Some(match ecx.load_mir(instance.def, None) {
-            Ok(body) => body,
-            Err(err) => {
-                if let err_unsup!(NoMirFor(did)) = err.kind() {
-                    let path = ecx.tcx.def_path_str(*did);
-                    return Err(ConstEvalErrKind::NeedsRfc(format!(
-                        "calling extern function `{}`",
-                        path
-                    ))
-                    .into());
-                }
-                return Err(err);
-            }
-        }))
+        Ok(Some(ecx.load_mir(instance.def, None)?))
     }
 
     fn call_intrinsic(