add helper methods for accessing struct tail

2 files changed, 18 insertions, 2 deletions

diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index c100c45b61a..9a28122535d 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -2028,6 +2028,22 @@ impl VariantDef {
 
         &self.fields[FieldIdx::from_u32(0)]
     }
+
+    /// Returns the last field in this variant, if present.
+    #[inline]
+    pub fn tail_opt(&self) -> Option<&FieldDef> {
+        self.fields.raw.last()
+    }
+
+    /// Returns the last field in this variant.
+    ///
+    /// # Panics
+    ///
+    /// Panics, if the variant has no fields.
+    #[inline]
+    pub fn tail(&self) -> &FieldDef {
+        self.tail_opt().expect("expected unsized ADT to have a tail field")
+    }
 }
 
 impl PartialEq for VariantDef {
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
index 720d770eed4..e2e4a2dbdc8 100644
--- a/compiler/rustc_middle/src/ty/util.rs
+++ b/compiler/rustc_middle/src/ty/util.rs
@@ -230,7 +230,7 @@ impl<'tcx> TyCtxt<'tcx> {
                     if !def.is_struct() {
                         break;
                     }
-                    match def.non_enum_variant().fields.raw.last() {
+                    match def.non_enum_variant().tail_opt() {
                         Some(field) => {
                             f();
                             ty = field.ty(self, substs);
@@ -304,7 +304,7 @@ impl<'tcx> TyCtxt<'tcx> {
                 (&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs))
                     if a_def == b_def && a_def.is_struct() =>
                 {
-                    if let Some(f) = a_def.non_enum_variant().fields.raw.last() {
+                    if let Some(f) = a_def.non_enum_variant().tail_opt() {
                         a = f.ty(self, a_substs);
                         b = f.ty(self, b_substs);
                     } else {