May Swift source code be in UTF-16 in XCode?

Using Swift
,
1

I am able to compile in Swift a very simple source file encoded in UTF-8 (with or without BOM).

But when I try to build the same source file but now encoded in UTF-16, I get an error : "Invalid Swift parseable output (malformed JSON)".

The file (main.swift) :

import Foundation
let str : String = "abc"
print (str)

What is happening ? What JSON is malformed?

Note that I can see the UTF-16 file in the code editor, I can edit it, but I cannot build it.

In Xcode > Preferences > Text Editing, I set "Default Text Encoding" to UTF-16BE/LE but to no avail.

Any help is appreciated, does Swift support only UTF-8 source files?

1 Like
2

Yes, Swift only supports UTF-8 input (in any environment, not just Xcode). That assumption's woven through the compiler in a few ways, mainly assuming that some content can be copied directly from the source file into output, but supporting UTF-16 in particular would be tricky because of the 16-bit code units, which would require reworking the lexer quite a bit, which could affect parsing speed (but might not be significant).

Do you have a particular reason why it'd be useful to accept source files encoded as UTF-16?

2 Likes
3

Thank you, Jordan.

No, I don't have a pressing need to accept source files encoded as UTF-16, I was just testing how XCode would behave under various circumstances.

1 Like
4

I have one ancillary question: what is the purpose of the Xcode > Preferences > Text Editing > Default Text Encoding if using anything but UTF-8 may cause a problem?

1 Like
5

Xcode supports languages other than Swift.

3 Likes
6

That sounds rather off the mark as an error message. I wonder where it comes from.

7

Moreover, you can edit files that are not source code from within Xcode.

1 Like
8

Yeah, it'd be worth filing a bug against Xcode and/or Swift to make sure the error message is good. I suspect some of the null bytes from the source file are making it into the output unescaped somehow (UTF-16-encoded ASCII characters have all zeroes in the high byte).

9

When compiling a file encoded in UTF-16, the Swift 6.2.1 compiler fails with the following error:

input files must be encoded as UTF-8 instead of UTF-16

swift compiler, error input files must be encoded as UTF-8 instead of UTF-16
swift compiler, error input files must be encoded as UTF-8 instead of UTF-161464×228 33.4 KB

10

@Avi Yes, I agree that some compilers accept standards other than Unicode or different Unicode encodings. Have you ever seen Xcode’s Preferences → Text Editing → Default Text Encoding setting actually be respected when creating a new file? I tested it with Swift, Objective-C, and C projects, and in every case the new files were created as UTF-8, even when my Default Text Encoding was set to UTF-16.

In my experience with Xcode 26.1, the Text Encoding and Line Endings features are broken. The File Inspector's Text Encoding field does not detect a file’s actual encoding, and the conversion dialog asks for confirmation but produces no changes — Git shows no diffs afterward. The Xcode settings related to these features also appear to be non-functional. I have set UTF-16 as default encoding but new files are always UTF-8. I posted a thread about this issue on the Apple Developer Forums but have not received any response. For detecting and converting file encodings or line endings, I currently use Visual Studio Code.

The Xcode editor, if a file is encoded in UTF-16 shows an error, project does not compiles and shows an error

input files must be encoded as UTF-8 instead of UTF-16

Xcode error input files must be encoded as UTF-8 instead of UTF-16
Xcode error input files must be encoded as UTF-8 instead of UTF-162520×1158 233 KB