Rollup merge of #145234 - dianne:1-tuple-witnesses, r=jackh726

match exhaustiveness diagnostics: show a trailing comma on singleton tuple consructors in witness patterns (and clean up a little)

Constructor patterns of type `(T,)` are written `(pat,)`, not `(pat)`. However, exhaustiveness/usefulness diagnostics would print them as `(pat)` when e.g. providing a witness of non-exhaustiveness and suggesting adding arms to make matches exhaustive; this would result in an error when applied.
rust-analyzer already prints the trailing comma, so it doesn't need changing.

This also includes some cleanup in the second commit, with justification in the commit message.

1 files changed, 5 insertions, 17 deletions

diff --git a/compiler/rustc_pattern_analysis/src/rustc/print.rs b/compiler/rustc_pattern_analysis/src/rustc/print.rs
index 7649f72f868..cdf62c2553d 100644
--- a/compiler/rustc_pattern_analysis/src/rustc/print.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc/print.rs
@@ -101,23 +101,11 @@ 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, ",")?;
         }
         write!(f, ")")?;
     }