Allow setting custom line numbers

[jyn514]

This is a bit of an edge case ... In C there is a #line macro that allows setting the current line number, it looks like this:

void f();
int main() {
    #line 1
    return f();
}
<stdin>:1:12: error: returning 'void' from a function with incompatible result type
      'int'

Note how the error message says the error occurs on line 1, not line 4.

Currently, it looks like the lines in a file are calculated once and can't be modified. Would it be possible to make the line numbers configurable?

This has the unfortunate complication that there's no longer a 1-1 mapping between the line numbers displayed to the user and the lines in a file: in my example above, there are 2 lines with line number 1. However, this corresponds pretty closely to the current difference between LineIndex (internal) and LineNumber (user-facing) so I don't forsee it being a giant deal.

[brendanzab]

Ahh wow! That is a pretty challenging use case? Any idea how we might be able to support this?

[jyn514]

It looks like for compute_line_starts() we currently store a Vec, where finding the start of the line is found by indexing into the vec using the line index. I think this is fine: the frontend wants to know the span for the physical line, not for the logical line.

The only thing that needs to change is LineIndex::number(). Currently that just returns self.0 + 1. Maybe instead we could store a BTreeMap of physical lines -> offsets, where the offset records the difference between the physical line and the logical line (line changes within a column is way more than I need and would be harder, I don't think we need to support it). For example, given the following code:

// blank
#line 1
int main() {}
#line 10
int f() {}

the offsets would look like { 2: -1, 4: 6 }. Then to find the logical line for a physical line, you look up the first entry before and add that value to the physical line. So to get the logical line for physical line 6 (int f() {}), we'd find the first entry before was 4: 6 and return 6 + 6 = 12 for the logical line.

Let me know if that sounds doable.

[jyn514]

Oh forgot to mention: the API would look something like Files::add_line_offset(&mut self, file: FileId, byte_index: ByteIndex) and we would look up the appropriate physical line number for that index. I have no opinions on what should happen if it's called multiple times for the same line, maybe just overwrite the existing offset?

[brendanzab]

@Marwes would this help you out with macro support on Gluon?

[brendanzab]

Wondering if we could have a Files::line_number method:

pub trait Files {
    ⋮
    /// The line number of the line at the given line index.
    fn line_number(&self, id: Self::FileId, line_index: usize) -> Option<usize>;
    ⋮
}