How to write to a pixel buffer line by line in glium?

Question

When uploading a decoded image to a pixel buffer, sometimes the lines contain a stride. That is, the lines are bigger than the image lines, for performance reasons (alignment). So if we want to upload the image called slice, we either upload it entirely if it has no stride (linesize==width) or we upload line by line:

    //If the linesize is equal to width then we don't need to respect stride
    if linesize == width as usize {
        pixel_buffer.write(slice);
    } else {
        for line in 0..height {
            let current_line_start = (line * width) as usize;
            let s = &slice[(current_line_start)..(current_line_start + linesize)];
            pixel_buffer.write(s);
        }
    }

However, pixel_buffer.write(s) fails if s.len() is not the same as pixel_buffer's size.

Reading https://docs.rs/glium/0.30.0/glium/texture/pixel_buffer/struct.PixelBuffer.html, it's not clear how to write by pieces. It looks like map_write might do it:

pub fn map_write(&mut self) -> WriteMapping<'_, T>

Maps the buffer in memory for writing only.

But how do I write to WriteMapping?

PS: I think this is not the solution because I'd have to wait for the GPU map to occur. I just want to send data to the pixel buffer as fast as possible