diff --git a/lib/std/base64.zig b/lib/std/base64.zig
index 0b91a1cf0a..112410a32d 100644
--- a/lib/std/base64.zig
+++ b/lib/std/base64.zig
@@ -38,8 +38,8 @@ pub const Base64Encoder = struct {
     }
 
     /// dest.len must be what you get from ::calcSize.
-    pub fn encode(encoder: *const Base64Encoder, dest: []u8, source: []const u8) void {
-        assert(dest.len == Base64Encoder.calcSize(source.len));
+    pub fn encode(encoder: *const Base64Encoder, dest: []u8, source: []const u8) []const u8 {
+        assert(dest.len >= Base64Encoder.calcSize(source.len));
 
         var i: usize = 0;
         var out_index: usize = 0;
@@ -78,6 +78,7 @@ pub const Base64Encoder = struct {
             dest[out_index] = encoder.pad_char;
             out_index += 1;
         }
+        return dest[0..out_index];
     }
 };
 
@@ -398,8 +399,7 @@ fn testAllApis(expected_decoded: []const u8, expected_encoded: []const u8) !void
     // Base64Encoder
     {
         var buffer: [0x100]u8 = undefined;
-        var encoded = buffer[0..Base64Encoder.calcSize(expected_decoded.len)];
-        standard_encoder.encode(encoded, expected_decoded);
+        const encoded = standard_encoder.encode(&buffer, expected_decoded);
         testing.expectEqualSlices(u8, expected_encoded, encoded);
     }
 
diff --git a/lib/std/build/write_file.zig b/lib/std/build/write_file.zig
index 923df960de..9bbe8830a3 100644
--- a/lib/std/build/write_file.zig
+++ b/lib/std/build/write_file.zig
@@ -72,7 +72,7 @@ pub const WriteFileStep = struct {
         var digest: [48]u8 = undefined;
         hash.final(&digest);
         var hash_basename: [64]u8 = undefined;
-        fs.base64_encoder.encode(&hash_basename, &digest);
+        _ = fs.base64_encoder.encode(&hash_basename, &digest);
         self.output_dir = try fs.path.join(self.builder.allocator, &[_][]const u8{
             self.builder.cache_root,
             "o",
diff --git a/lib/std/fs.zig b/lib/std/fs.zig
index 4c346a2c89..6880940c03 100644
--- a/lib/std/fs.zig
+++ b/lib/std/fs.zig
@@ -83,7 +83,7 @@ pub fn atomicSymLink(allocator: *Allocator, existing_path: []const u8, new_path:
     tmp_path[dirname.len] = path.sep;
     while (true) {
         crypto.random.bytes(rand_buf[0..]);
-        base64_encoder.encode(tmp_path[dirname.len + 1 ..], &rand_buf);
+        _ = base64_encoder.encode(tmp_path[dirname.len + 1 ..], &rand_buf);
 
         if (cwd().symLink(existing_path, tmp_path, .{})) {
             return cwd().rename(tmp_path, new_path);
@@ -153,15 +153,14 @@ pub const AtomicFile = struct {
     ) InitError!AtomicFile {
         var rand_buf: [RANDOM_BYTES]u8 = undefined;
         var tmp_path_buf: [TMP_PATH_LEN:0]u8 = undefined;
-        // TODO: should be able to use TMP_PATH_LEN here.
-        tmp_path_buf[base64.Base64Encoder.calcSize(RANDOM_BYTES)] = 0;
 
         while (true) {
             crypto.random.bytes(rand_buf[0..]);
-            base64_encoder.encode(&tmp_path_buf, &rand_buf);
+            const tmp_path = base64_encoder.encode(&tmp_path_buf, &rand_buf);
+            tmp_path_buf[tmp_path.len] = 0;
 
             const file = dir.createFile(
-                &tmp_path_buf,
+                tmp_path,
                 .{ .mode = mode, .exclusive = true },
             ) catch |err| switch (err) {
                 error.PathAlreadyExists => continue,
diff --git a/lib/std/testing.zig b/lib/std/testing.zig
index 63054892db..8e17c4e9f8 100644
--- a/lib/std/testing.zig
+++ b/lib/std/testing.zig
@@ -305,7 +305,7 @@ pub fn tmpDir(opts: std.fs.Dir.OpenDirOptions) TmpDir {
     var random_bytes: [TmpDir.random_bytes_count]u8 = undefined;
     std.crypto.random.bytes(&random_bytes);
     var sub_path: [TmpDir.sub_path_len]u8 = undefined;
-    std.fs.base64_encoder.encode(&sub_path, &random_bytes);
+    _ = std.fs.base64_encoder.encode(&sub_path, &random_bytes);
 
     var cwd = getCwdOrWasiPreopen();
     var cache_dir = cwd.makeOpenPath("zig-cache", .{}) catch