Fixes #2946 (#2949)

* Fixes #2946

* Update string_mutable.zig

* Fixes #2948

---------

Co-authored-by: Jarred Sumner <709451+Jarred-Sumner@users.noreply.github.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/src/string_mutable.zig b/src/string_mutable.zig
index 2b8d80429..e6738ae7f 100644
--- a/src/string_mutable.zig
+++ b/src/string_mutable.zig
@@ -109,7 +109,12 @@ pub const MutableString = struct {
         }
 
         if (needs_gap) {
-            var mutable = try MutableString.initCopy(allocator, str[0..start_i]);
+            var mutable = try MutableString.initCopy(allocator, if (start_i == 0)
+                // the first letter can be a non-identifier start
+                // https://github.com/oven-sh/bun/issues/2946
+                "_"
+            else
+                str[0..start_i]);
             needs_gap = false;
 
             var slice = str[start_i..];
@@ -137,6 +142,10 @@ pub const MutableString = struct {
                 has_needed_gap = true;
             }
 
+            if (comptime bun.Environment.allow_assert) {
+                std.debug.assert(js_lexer.isIdentifier(mutable.list.items));
+            }
+
             return try mutable.list.toOwnedSlice(allocator);
         }