Better error for trailing commas in using

[c42f]

Leaving a trailing comma after the last element of using is a mistake I've made a few times. The parser only catches this error later, resulting in a fairly poor error message. For example, it can gobble up the function keyword in the next part of the file:

julia> using A: b,

       function foo()
       end

ERROR: ParseError:
Error: extra tokens after end of expression
@ REPL[4]:3:9
using A: b,

function foo()
--------^^^^^^---
end

[pfitzseb]

This is made slightly trickier by the fact that function can be used as a valid identifier there:

julia> module A
       b = 2
       var"function" = 3
       end
Main.A

julia> using .A: b,
       function

julia> using .A: b,
       function foo()
ERROR: syntax: extra token "foo" after end of expression
Stacktrace:
 [1] top-level scope
   @ none:1

Then again, this is a really good use case for post-hoc diagnostics instead of limited lookahead during parsing :)

[c42f]

Yes exactly 👍 The same kind of problem happens for struct definitions or other keywords like if.