Couldn't we create a private version of PyUnicode_DecodeUnicodeEscape, to be called by ast.c, which passes back invalid escape info? Then have the actual warning raised by ast.c, which knows enough about the context to generate a better error/warning. I think we'd only be able to report on he first error in a string, though, but I haven't thought it all the way through.

I believe we'd only need to modify decode_unicode_with_escapes() in ast.c, which is called for both regular strings and f-string. And of course PyUnicode_DecodeUnicodeEscape would have to call the new private version and do the right thing.