Rejig error hierarchy

[madig]

This experimental PR implements a more layered error hierarchy for better visibility into the error chain.

Example, messing with the x attribute for a point:

Error: failed to load font 'testdata/MutatorSansLightWide.ufo'

Caused by:
    0: failed to load font
    1: failed to load layer 'foreground' from 'testdata/MutatorSansLightWide.ufo/glyphs'
    2: failed to load glyph 'C' from 'testdata/MutatorSansLightWide.ufo/glyphs/C_.glif'
    3: failed to parse glyph data
    4: bad number

It is not entirely possible to have the detailed errors be private, as we expose several public methods that now return them. This made me think: Should we expose e.g. FontInfo::from_file and the LayerSet::load and Layer::load methods as public?

TODO:

• Decide visibility of new errors and therefore maybe limit the public API?
• Rejig writing errors
• Better errors for Glif parsing, GlifLoadError::Parse is a bit non-descript better done in a separate PR

[chrissimpkins]

Looks fantastic Nikolaus 👍

[chrissimpkins]

I came across this in https://github.com/liborty/indxvec/blob/964de9f78a89844bd651f6709aff86f88751ff67/src/lib.rs#L5-L16 in case it happens to be helpful for the error handling revisions:

/// macro `here!()` gives `&str` with the current `file:line path::function` for error messages.
#[macro_export]
macro_rules! here {
    () => {{
        fn f() {}
        fn type_name_of<T>(_: T) -> &'static str {
            std::any::type_name::<T>()
        }
        let name = type_name_of(f); 
        format!("\n{}:{} {}", file!(), line!(), &name[..name.len()-3])
    }}
}

[madig]

Glyph loading errors are a bit finicky. There's dealing with XML, UFO spec compliance and builder logic 🤔

[madig]

LayerSetLoadError sits above LayerLoadError because LayerSet::load is currently public and I want to keep UfoLoadError private for now. Otherwise, I could use UfoLoadError instead I suppose and move the variants over.

[madig]

Hm. I am probably falling into the traps described in https://www.unwoundstack.com/blog/rust-error-handling.html#org579e4a9. I'll try to reduce variants somewhat.

[cmyr]

One little nit, but I think this is an improvement!

[github-actions]

🗜 Bloat check ⚖️

Comparing f922e9a against aca74c2

target	old size	new size	difference
target/release/examples/load_save	1.9 MB	1.9 MB	-1.13 KB (-0.06%)
target/debug/examples/load_save	8.57 MB	8.6 MB	22.25 KB (0.25%)

[chrissimpkins]

Possible to push a new release that includes the updated error handling that was merged over the last couple of months?

[madig]

Already done today 😁

[chrissimpkins]

Already done today 😁

The latest release and tag that I see is https://github.com/linebender/norad/releases/tag/v0.6.0 from Oct 2021?

[chrissimpkins]

It looks like crates.io was updated with a new release (v0.7.0). I checked the tags/releases here. Thanks!