Encapsulate the printing of WitnessPat

This hides the fact that we print `WitnessPat` by converting it to `thir::Pat`
and then printing that.

Author: Zalathar

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -1077,7 +1077,7 @@ fn report_non_exhaustive_match<'p, 'tcx>(
     let suggested_arm = if suggest_the_witnesses {
         let pattern = witnesses
             .iter()
-            .map(|witness| cx.hoist_witness_pat(witness).to_string())
+            .map(|witness| cx.print_witness_pat(witness))
             .collect::<Vec<String>>()
             .join(" | ");
         if witnesses.iter().all(|p| p.is_never_pattern()) && cx.tcx.features().never_patterns {
@@ -1195,13 +1195,13 @@ fn joined_uncovered_patterns<'p, 'tcx>(
     witnesses: &[WitnessPat<'p, 'tcx>],
 ) -> String {
     const LIMIT: usize = 3;
-    let pat_to_str = |pat: &WitnessPat<'p, 'tcx>| cx.hoist_witness_pat(pat).to_string();
+    let pat_to_str = |pat: &WitnessPat<'p, 'tcx>| cx.print_witness_pat(pat);
     match witnesses {
         [] => bug!(),
-        [witness] => format!("`{}`", cx.hoist_witness_pat(witness)),
+        [witness] => format!("`{}`", cx.print_witness_pat(witness)),
         [head @ .., tail] if head.len() < LIMIT => {
             let head: Vec<_> = head.iter().map(pat_to_str).collect();
-            format!("`{}` and `{}`", head.join("`, `"), cx.hoist_witness_pat(tail))
+            format!("`{}` and `{}`", head.join("`, `"), cx.print_witness_pat(tail))
         }
         _ => {
             let (head, tail) = witnesses.split_at(LIMIT);

compiler/rustc_pattern_analysis/src/errors.rs

@@ -26,19 +26,13 @@ impl Uncovered {
     where
         'tcx: 'p,
     {
-        let witness_1 = cx.hoist_witness_pat(witnesses.get(0).unwrap()).to_string();
+        let witness_1 = cx.print_witness_pat(witnesses.get(0).unwrap());
         Self {
             span,
             count: witnesses.len(),
             // Substitute dummy values if witnesses is smaller than 3. These will never be read.
-            witness_2: witnesses
-                .get(1)
-                .map(|w| cx.hoist_witness_pat(w).to_string())
-                .unwrap_or_default(),
-            witness_3: witnesses
-                .get(2)
-                .map(|w| cx.hoist_witness_pat(w).to_string())
-                .unwrap_or_default(),
+            witness_2: witnesses.get(1).map(|w| cx.print_witness_pat(w)).unwrap_or_default(),
+            witness_3: witnesses.get(2).map(|w| cx.print_witness_pat(w)).unwrap_or_default(),
             witness_1,
             remainder: witnesses.len().saturating_sub(3),
         }

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -811,9 +811,17 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
 
         Pat { ty: ty.inner(), span: DUMMY_SP, kind }
     }
+
+    /// Prints a [`WitnessPat`] to an owned string, for diagnostic purposes.
+    pub fn print_witness_pat(&self, pat: &WitnessPat<'p, 'tcx>) -> String {
+        // This works by converting the witness pattern back to a `thir::Pat`
+        // and then printing that, but callers don't need to know that.
+        self.hoist_witness_pat(pat).to_string()
+    }
+
     /// Convert back to a `thir::Pat` for diagnostic purposes. This panics for patterns that don't
     /// appear in diagnostics, like float ranges.
-    pub fn hoist_witness_pat(&self, pat: &WitnessPat<'p, 'tcx>) -> Pat<'tcx> {
+    fn hoist_witness_pat(&self, pat: &WitnessPat<'p, 'tcx>) -> Pat<'tcx> {
         let cx = self;
         let is_wildcard = |pat: &Pat<'_>| matches!(pat.kind, PatKind::Wild);
         let mut subpatterns = pat.iter_fields().map(|p| Box::new(cx.hoist_witness_pat(p)));