regression test for ICE I encountered in my patch.

src/test/ui/nll/user-annotations/issue-54570-bootstrapping.rs

@@ -0,0 +1,31 @@
+// compile-pass
+#![feature(nll)]
+
+// This test is reduced from a scenario pnkfelix encountered while
+// bootstrapping the compiler.
+
+#[derive(Copy, Clone)]
+pub struct Spanned<T> {
+    pub node: T,
+    pub span: Span,
+}
+
+pub type Variant = Spanned<VariantKind>;
+// #[derive(Clone)] pub struct Variant { pub node: VariantKind, pub span: Span, }
+
+#[derive(Clone)]
+pub struct VariantKind { }
+
+#[derive(Copy, Clone)]
+pub struct Span;
+
+pub fn variant_to_span(variant: Variant) {
+    match variant {
+        Variant {
+            span: _span,
+            ..
+        } => { }
+    };
+}
+
+fn main() { }