[BUG] Panic in unsafe `fill_rect_fast`

[agersant]

Describe the bug

When using Rust nightly compiler, attempting to draw a line is likely to cause a panic (depending on color / line coordinates):

thread 'main' panicked at 'misaligned pointer dereference: address must be a multiple of 0x8 but is 0x1ab590c7fd3', [redacted]\plotters-bitmap-0.3.2\src\bitmap_pixel\rgb.rs:215:29

The offending line is:

plotters/plotters-bitmap/src/bitmap_pixel/rgb.rs

Line 215 in acf5158

*ptr = d1;

I believe the relevant Nightly change is rust-lang/rust#98112, but this seems to be a plotters bug and not a rustc regression (from my limited understanding).

To Reproduce

use plotters::prelude::*;

fn main() {
    let mut backend = BitMapBackend::new("test.png", (256, 256));
    backend.draw_line((1, 0), (100, 0), &RED).unwrap();
}

Sample project: https://github.com/agersant/plotter-panic

Version Information
Plotters 0.3.4

[nbdd0121]

That code does seem very broken:

• The transmute call is wrong, because (u64, u64, u64) does not have any layout guarantees.
• ptr comes from *mut [u8; 24], which doesn't have the correct alignment for u64.

[pbdeuchler]

I'm hitting this as well, but only when plotting a line series with a certain number of data points... i.e. purely adding more points to the same exact line "fixes" the issue.

[noahdahlman]

I'm getting this as well, most of the examples I tried in the docs fail when using bitmap backend. I tried this example in a sample project and always got a panic, no issues when switching to SVG backend. Only fails on nightly (I'm using nightly-x86_64-unknown-linux-gnu ), stable works fine.

use plotters::prelude::*;
fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut backend = BitMapBackend::new("1.png", (300, 200));
    backend.draw_rect((50, 50), (200, 150), &RED, true)?;
    backend.present()?;
    Ok(())
}

[Berrysoft]

I also trigger this issue when trying to use plotters with slint with nightly toolchain. Anyway this code is broken, because you should use write_unaligned.

[jayvdb]

This now occurs on stable v1.70

[Berrysoft]

We need a new release.

[ktjd123]

I'm having error at blend_rect_fast 😭