diff --git a/lib/std/zig.zig b/lib/std/zig.zig
index fa45af129c3878233727e4e4ab26611c0a9529c1..d113129b7e7aa48b042fed5d6949e64a2dcf2984 100644
--- a/lib/std/zig.zig
+++ b/lib/std/zig.zig
@@ -534,6 +534,11 @@ pub fn readSourceFileToEndAlloc(gpa: Allocator, file_reader: *std.fs.File.Reader
     var buffer: std.ArrayListAlignedUnmanaged(u8, .@"2") = .empty;
     defer buffer.deinit(gpa);
 
+    if (file_reader.getSize()) |size| {
+        const casted_size = std.math.cast(u32, size) orelse return error.StreamTooLong;
+        try buffer.ensureTotalCapacityPrecise(gpa, casted_size);
+    } else |_| {}
+
     try file_reader.interface.appendRemaining(gpa, .@"2", &buffer, .limited(max_src_size));
 
     // Detect unsupported file types with their Byte Order Mark