Make promote_consts emit the errors when required promotion fails

src/librustc_mir/transform/promote_consts.rs

@@ -80,6 +80,17 @@ pub enum Candidate {
     Argument { bb: BasicBlock, index: usize },
 }
 
+impl Candidate {
+    /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`.
+    fn forces_explicit_promotion(&self) -> bool {
+        match self {
+            Candidate::Ref(_) |
+            Candidate::Repeat(_) => false,
+            Candidate::Argument { .. } => true,
+        }
+    }
+}
+
 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> {
     let attrs = tcx.get_attrs(def_id);
     let attr = attrs.iter().find(|a| a.check_name(sym::rustc_args_required_const))?;
@@ -727,16 +738,22 @@ pub fn validate_candidates(
     };
 
     candidates.iter().copied().filter(|&candidate| {
-        validator.explicit = match candidate {
-            Candidate::Ref(_) |
-            Candidate::Repeat(_) => false,
-            Candidate::Argument { .. } => true,
-        };
+        validator.explicit = candidate.forces_explicit_promotion();
 
         // FIXME(eddyb) also emit the errors for shuffle indices
         // and `#[rustc_args_required_const]` arguments here.
 
-        validator.validate_candidate(candidate).is_ok()
+        let is_promotable = validator.validate_candidate(candidate).is_ok();
+        match candidate {
+            Candidate::Argument { bb, index } if !is_promotable => {
+                let span = body[bb].terminator().source_info.span;
+                let msg = format!("argument {} is required to be a constant", index + 1);
+                tcx.sess.span_err(span, &msg);
+            }
+            _ => ()
+        }
+
+        is_promotable
     }).collect()
 }
 

src/librustc_mir/transform/qualify_consts.rs

@@ -1599,20 +1599,12 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
                     // This is not a problem, because the argument explicitly
                     // requests constness, in contrast to regular promotion
                     // which happens even without the user requesting it.
-                    // We can error out with a hard error if the argument is not
-                    // constant here.
+                    //
+                    // `promote_consts` is responsible for emitting the error if
+                    // the argument is not promotable.
                     if !IsNotPromotable::in_operand(self, arg) {
                         debug!("visit_terminator_kind: candidate={:?}", candidate);
                         self.promotion_candidates.push(candidate);
-                    } else {
-                        if is_shuffle {
-                            span_err!(self.tcx.sess, self.span, E0526,
-                                      "shuffle indices are not constant");
-                        } else {
-                            self.tcx.sess.span_err(self.span,
-                                &format!("argument {} is required to be a constant",
-                                         i + 1));
-                        }
                     }
                 }
             }