Make thir::Pat not implement fmt::Display directly

This gives a clearer view of the (diagnostic) code that expects to be able to
print THIR patterns, and makes it possible to experiment with requiring some
kind of context (for ID lookup) when printing patterns.

compiler/rustc_middle/src/thir.rs

@@ -1073,8 +1073,33 @@ impl<'tcx> PatRangeBoundary<'tcx> {
     }
 }
 
-impl<'tcx> fmt::Display for Pat<'tcx> {
+impl<'tcx> Pat<'tcx> {
+    /// Prints a [`Pat`] to an owned string, for user-facing diagnostics.
+    ///
+    /// If we ever switch over to storing subpatterns as `PatId`, this will also
+    /// need to take a context that can resolve IDs to subpatterns.
+    pub fn to_string(&self) -> String {
+        format!("{}", self.display())
+    }
+
+    /// Used internally by [`fmt::Display`] for [`PatDisplay`].
+    fn display(&self) -> PatDisplay<'_, 'tcx> {
+        PatDisplay { pat: self }
+    }
+}
+
+/// Wrapper around [`&Pat<'tcx>`][`Pat`] that implements [`fmt::Display`].
+///
+/// If we ever switch over to storing subpatterns as `PatId`, this will also
+/// need to hold a context that can resolve IDs to subpatterns.
+struct PatDisplay<'pat, 'tcx> {
+    pat: &'pat Pat<'tcx>,
+}
+
+impl<'pat, 'tcx> fmt::Display for PatDisplay<'pat, 'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let &Self { pat } = self;
+
         // Printing lists is a chore.
         let mut first = true;
         let mut start_or_continue = |s| {
@@ -1087,20 +1112,22 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
         };
         let mut start_or_comma = || start_or_continue(", ");
 
-        match self.kind {
+        match pat.kind {
             PatKind::Wild => write!(f, "_"),
             PatKind::Never => write!(f, "!"),
-            PatKind::AscribeUserType { ref subpattern, .. } => write!(f, "{subpattern}: _"),
+            PatKind::AscribeUserType { ref subpattern, .. } => {
+                write!(f, "{}: _", subpattern.display())
+            }
             PatKind::Binding { name, mode, ref subpattern, .. } => {
                 f.write_str(mode.prefix_str())?;
                 write!(f, "{name}")?;
                 if let Some(ref subpattern) = *subpattern {
-                    write!(f, " @ {subpattern}")?;
+                    write!(f, " @ {}", subpattern.display())?;
                 }
                 Ok(())
             }
             PatKind::Variant { ref subpatterns, .. } | PatKind::Leaf { ref subpatterns } => {
-                let variant_and_name = match self.kind {
+                let variant_and_name = match pat.kind {
                     PatKind::Variant { adt_def, variant_index, .. } => ty::tls::with(|tcx| {
                         let variant = adt_def.variant(variant_index);
                         let adt_did = adt_def.did();
@@ -1113,7 +1140,7 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                         };
                         Some((variant, name))
                     }),
-                    _ => self.ty.ty_adt_def().and_then(|adt_def| {
+                    _ => pat.ty.ty_adt_def().and_then(|adt_def| {
                         if !adt_def.is_enum() {
                             ty::tls::with(|tcx| {
                                 Some((adt_def.non_enum_variant(), tcx.def_path_str(adt_def.did())))
@@ -1138,11 +1165,11 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                                 continue;
                             }
                             let name = variant.fields[p.field].name;
-                            write!(f, "{}{}: {}", start_or_comma(), name, p.pattern)?;
+                            write!(f, "{}{}: {}", start_or_comma(), name, p.pattern.display())?;
                             printed += 1;
                         }
 
-                        let is_union = self.ty.ty_adt_def().is_some_and(|adt| adt.is_union());
+                        let is_union = pat.ty.ty_adt_def().is_some_and(|adt| adt.is_union());
                         if printed < variant.fields.len() && (!is_union || printed == 0) {
                             write!(f, "{}..", start_or_comma())?;
                         }
@@ -1161,14 +1188,14 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                         // 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)?;
+                                write!(f, "{}", p.pattern.display())?;
                                 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)?;
+                            write!(f, "{}", p.pattern.display())?;
                         } else {
                             write!(f, "_")?;
                         }
@@ -1179,45 +1206,45 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                 Ok(())
             }
             PatKind::Deref { ref subpattern } => {
-                match self.ty.kind() {
+                match pat.ty.kind() {
                     ty::Adt(def, _) if def.is_box() => write!(f, "box ")?,
                     ty::Ref(_, _, mutbl) => {
                         write!(f, "&{}", mutbl.prefix_str())?;
                     }
-                    _ => bug!("{} is a bad Deref pattern type", self.ty),
+                    _ => bug!("{} is a bad Deref pattern type", pat.ty),
                 }
-                write!(f, "{subpattern}")
+                write!(f, "{}", subpattern.display())
             }
             PatKind::DerefPattern { ref subpattern, .. } => {
-                write!(f, "deref!({subpattern})")
+                write!(f, "deref!({})", subpattern.display())
             }
             PatKind::Constant { value } => write!(f, "{value}"),
             PatKind::InlineConstant { def: _, ref subpattern } => {
-                write!(f, "{} (from inline const)", subpattern)
+                write!(f, "{} (from inline const)", subpattern.display())
             }
             PatKind::Range(ref range) => write!(f, "{range}"),
             PatKind::Slice { ref prefix, ref slice, ref suffix }
             | PatKind::Array { ref prefix, ref slice, ref suffix } => {
                 write!(f, "[")?;
                 for p in prefix.iter() {
-                    write!(f, "{}{}", start_or_comma(), p)?;
+                    write!(f, "{}{}", start_or_comma(), p.display())?;
                 }
                 if let Some(ref slice) = *slice {
                     write!(f, "{}", start_or_comma())?;
                     match slice.kind {
                         PatKind::Wild => {}
-                        _ => write!(f, "{slice}")?,
+                        _ => write!(f, "{}", slice.display())?,
                     }
                     write!(f, "..")?;
                 }
                 for p in suffix.iter() {
-                    write!(f, "{}{}", start_or_comma(), p)?;
+                    write!(f, "{}{}", start_or_comma(), p.display())?;
                 }
                 write!(f, "]")
             }
             PatKind::Or { ref pats } => {
                 for pat in pats.iter() {
-                    write!(f, "{}{}", start_or_continue(" | "), pat)?;
+                    write!(f, "{}{}", start_or_continue(" | "), pat.display())?;
                 }
                 Ok(())
             }