Fix HIR pretty printing of let else

compiler/rustc_hir_pretty/src/lib.rs

@@ -911,6 +911,10 @@ impl<'a> State<'a> {
         if let Some(els) = els {
             self.nbsp();
             self.word_space("else");
+            // containing cbox, will be closed by print-block at `}`
+            self.cbox(0);
+            // head-box, will be closed by print-block after `{`
+            self.ibox(0);
             self.print_block(els);
         }
 

src/test/ui/unpretty/pretty-let-else.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Zunpretty=hir
+// check-pass
+
+#![feature(let_else)]
+
+fn foo(x: Option<u32>) {
+    let Some(_) = x else { panic!() };
+}
+
+fn main() {}

src/test/ui/unpretty/pretty-let-else.stdout

@@ -0,0 +1,18 @@
+// compile-flags: -Zunpretty=hir
+// check-pass
+
+#![feature(let_else)]
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+#[macro_use]
+extern crate std;
+
+fn foo(x:
+        Option<u32>) {
+        let Some(_) = x else
+            {
+
+            { ::std::rt::begin_panic("explicit panic") }
+        };
+    }
+fn main() { }