clean up witness printing for tuple-like constructors

By construction, `subpatterns` contains all fields in order. Witness
patterns are constructed with all fields in order by
`WitnessPat::wild_from_ctor` and `WitnessStack::apply_constructor`, and
the order is preserved at `write_struct_like`'s call-site in
`print_witness_pat`. It's thus no longer necessary to go looking for
fields or handle missing fields.

Author: dianne

compiler/rustc_pattern_analysis/src/rustc/print.rs

@@ -101,23 +101,8 @@ pub(crate) fn write_struct_like<'tcx>(
     let num_fields = variant_and_name.as_ref().map_or(subpatterns.len(), |(v, _)| v.fields.len());
     if num_fields != 0 || variant_and_name.is_none() {
         write!(f, "(")?;
-        for i in 0..num_fields {
-            write!(f, "{}", start_or_comma())?;
-
-            // Common case: the field is where we expect it.
-            if let Some(p) = subpatterns.get(i) {
-                if p.field.index() == i {
-                    write!(f, "{}", p.pattern)?;
-                    continue;
-                }
-            }
-
-            // Otherwise, we have to go looking for it.
-            if let Some(p) = subpatterns.iter().find(|p| p.field.index() == i) {
-                write!(f, "{}", p.pattern)?;
-            } else {
-                write!(f, "_")?;
-            }
+        for FieldPat { pattern, .. } in subpatterns {
+            write!(f, "{}{pattern}", start_or_comma())?;
         }
         if matches!(ty.kind(), ty::Tuple(..)) && num_fields == 1 {
             write!(f, ",")?;