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.

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, ")")?;
     }

tests/ui/or-patterns/exhaustiveness-non-exhaustive.rs

@@ -7,11 +7,11 @@ fn main() {
         (0 | 1, 2 | 3) => {}
     }
     match ((0u8,),) {
-        //~^ ERROR non-exhaustive patterns: `((4_u8..=u8::MAX))`
+        //~^ ERROR non-exhaustive patterns: `((4_u8..=u8::MAX,),)`
         ((0 | 1,) | (2 | 3,),) => {}
     }
     match (Some(0u8),) {
-        //~^ ERROR non-exhaustive patterns: `(Some(2_u8..=u8::MAX))`
+        //~^ ERROR non-exhaustive patterns: `(Some(2_u8..=u8::MAX),)`
         (None | Some(0 | 1),) => {}
     }
 }

tests/ui/or-patterns/exhaustiveness-non-exhaustive.stderr

@@ -11,30 +11,30 @@ LL ~         (0 | 1, 2 | 3) => {},
 LL +         (2_u8..=u8::MAX, _) => todo!()
    |
 
-error[E0004]: non-exhaustive patterns: `((4_u8..=u8::MAX))` not covered
+error[E0004]: non-exhaustive patterns: `((4_u8..=u8::MAX,),)` not covered
   --> $DIR/exhaustiveness-non-exhaustive.rs:9:11
    |
 LL |     match ((0u8,),) {
-   |           ^^^^^^^^^ pattern `((4_u8..=u8::MAX))` not covered
+   |           ^^^^^^^^^ pattern `((4_u8..=u8::MAX,),)` not covered
    |
    = note: the matched value is of type `((u8,),)`
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         ((0 | 1,) | (2 | 3,),) => {},
-LL +         ((4_u8..=u8::MAX)) => todo!()
+LL +         ((4_u8..=u8::MAX,),) => todo!()
    |
 
-error[E0004]: non-exhaustive patterns: `(Some(2_u8..=u8::MAX))` not covered
+error[E0004]: non-exhaustive patterns: `(Some(2_u8..=u8::MAX),)` not covered
   --> $DIR/exhaustiveness-non-exhaustive.rs:13:11
    |
 LL |     match (Some(0u8),) {
-   |           ^^^^^^^^^^^^ pattern `(Some(2_u8..=u8::MAX))` not covered
+   |           ^^^^^^^^^^^^ pattern `(Some(2_u8..=u8::MAX),)` not covered
    |
    = note: the matched value is of type `(Option<u8>,)`
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         (None | Some(0 | 1),) => {},
-LL +         (Some(2_u8..=u8::MAX)) => todo!()
+LL +         (Some(2_u8..=u8::MAX),) => todo!()
    |
 
 error: aborting due to 3 previous errors