Auto merge of #47690 - estebank:for-block-277, r=nikomatsakis

For E0277 on `for` loops, point at the "head" expression

When E0277's span points at a `for` loop, the actual issue is in the
element being iterated. Instead of pointing at the entire loop, point
only at the first line (when possible) so that the span ends in the
element for which E0277 was triggered.

src/librustc_errors/emitter.rs

@@ -767,6 +767,7 @@ fn fix_multispan_in_std_macros(&mut self,
                         }
                         // Check to make sure we're not in any <*macros>
                         if !cm.span_to_filename(def_site).is_macros() &&
+                           !trace.macro_decl_name.starts_with("desugaring of ") &&
                            !trace.macro_decl_name.starts_with("#[") ||
                            always_backtrace {
                             new_labels.push((trace.call_site,