"Error: `TypeInfo` cannot be used with -betterC" on a CTFE function

[Richard (Rikki) Andrew Cattermole]

On 10/04/2024 11:21 AM, Liam McGillivray wrote:
> On Sunday, 7 April 2024 at 08:59:55 UTC, Richard (Rikki) Andrew 
> Cattermole wrote:
>> Unfortunately runtime and CTFE are the same target in the compiler.
>>
>> So that function is being used for both, and hence uses GC (appending).
> 
> Are you sure that string appending was really the problem that caused 
> the "TypeInfo" build error? I forgot about this, but I had already had a 
> working CTFE function with string appending before adding the new one 
> that lead to this error. The symbols that it generates could be used in 
> the program compiled with `betterC`.

No, for a string it shouldn't trigger the need for TypeInfo. But that 
wouldn't have worked regardless.

> ```
> string EnumPrefixes(T)(string oldName, string prefix) {
>      string result = "enum " ~ oldName ~ " {\n";
>      static foreach(member; __traits(allMembers, T)) {
>          result ~= "    " ~ prefix ~ member ~ " = " ~ 
> __traits(getMember, T, member).to!int.to!string ~ ",\n";
>      }
>      return result ~ "}\n";
> }
> ```
> 
> The purpose of this was that the enums used by the C library were too 
> verbose. I had changed them from things like `KeyboardKey.KEY_C` to 
> `Key.C`. I wanted to leave the new enums written directly in the module 
> since these were recommended for use, but then generate the old ones 
> with CTFE for backwards compatibility. The function above was used like 
> `mixin(EnumPrefixes!Key("KeyboardKey", "KEY_"));`, and the compiler 
> would allow it even when building with `betterC`.

The string mixin triggers CTFE, if ``EnumPrefixes`` wasn't templated, 
that would cause codegen and hence error. If you called it in a context 
that wasn't CTFE only, it would codegen even with template and would error.

For quite a long time we emitted -betterC errors during semantic, we 
learned that this was all around a bad idea and moved (hopefully all but 
I doubt it) into the glue code. So only if it codegens will it error.