This can just be chars.next().map(FormatPreconversor::from_char)

Hum... this still returns None if chars.next() is None?

Asking because FormatPreconversor::from_char is not prepared to handle None as input.

Another aspect: // Should fail instead?

I do not know how to raise exceptions. Ideally, {!x} should fail, but this code will pass for now:

# CPy 3.6
>>> '{!x}'.format(123)
Traceback (most recent call last):
  File "<ipython-input-2-b294f18f6498>", line 1, in <module>
    '{!x}'.format(123)
ValueError: Unknown conversion specifier x

# RustPython
>>>>> '{!x}'.format(123)
'123'

next().map(FormatPreconversor::from_char) would just map the inner value of the Option if present – would do exactly what you did (pass the inner value to from_char() if it exists, or return None if the Option is None).

To raise an exception, you’d need to signal an error somehow (eg. return a Result<Option<FormatPreconversor>, String> instead of an Option<FormatPreconversor>) and then check for the result in call_object_format() and in case of an error, return a new vm error there (eg. by return Err(vm.new_value_error(…))). Or just by mapping the error:

let (preconversor, new_format_spec) = FormatPreconversor::parse_and_consume(format_spec);
let preconversor = preconversor.map_err(|msg| vm.new_value_error(msg))?;
// …

But then you’d probably want to return Result<(Option<FormatPreconversor>, &str), String> rather than (Result<Option<FormatPreconversor>, String>, &str), so that it’d look like:

let (preconversor, new_format_spec) = FormatPreconversor::parse_and_consume(format_spec)
    .map_err(|msg| vm.new_value_error(msg))?;

See https://doc.rust-lang.org/std/result/