There is a logical issue around what counts as leading white-space.
There is code which does a subtraction assuming that no errors will be reported
inside the leading whitespace. However we compute the length of
that whitespace with std::char::is_whitespace and not
rustc_lexer::is_whitespace. The former will include a no-break space while
later will excluded it. We can only safely make the assumption that no errors
will be reported in whitespace if it is all "Rust Standard" whitespace.
Indeed an error does occur in unicode whitespace if it contains a no-break
space.
