feat: support underscore expressions

Author: calebcartwright

src/formatting/expr.rs

@@ -439,6 +439,7 @@ pub(crate) fn format_expr(
             }
         }
         ast::ExprKind::Await(_) => rewrite_chain(expr, context, shape),
+        ast::ExprKind::Underscore => Some("_".to_owned()),
         ast::ExprKind::Err => None,
     };
 

src/formatting/utils.rs

@@ -524,7 +524,8 @@ pub(crate) fn is_block_expr(context: &RewriteContext<'_>, expr: &ast::Expr, repr
         | ast::ExprKind::Ret(..)
         | ast::ExprKind::Tup(..)
         | ast::ExprKind::Type(..)
-        | ast::ExprKind::Yield(None) => false,
+        | ast::ExprKind::Yield(None)
+        | ast::ExprKind::Underscore => false,
     }
 }
 

tests/source/expr.rs

@@ -567,3 +567,13 @@ fn foo() {
     }
         .await;
 }
+
+fn underscore() {
+        _= 1;
+        _;
+        [  _,a,_  ] = [1, 2, 3];
+    (a,   _) = (8, 9);
+    TupleStruct(  _, a) = TupleStruct(2, 2);
+
+    let _  : usize = foo(_, _);
+}

tests/target/expr.rs

@@ -661,3 +661,13 @@ fn foo() {
     }
     .await;
 }
+
+fn underscore() {
+    _ = 1;
+    _;
+    [_, a, _] = [1, 2, 3];
+    (a, _) = (8, 9);
+    TupleStruct(_, a) = TupleStruct(2, 2);
+
+    let _: usize = foo(_, _);
+}