try_v2 0.3.3

Provides a derive macro for `Try` ([try_trait_v2](https://rust-lang.github.io/rfcs/3058-try-trait-v2.html))
Documentation 
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use std::fmt::Display;

use proc_macro::TokenStream as TokenStream1;
use proc_macro2::{Span, TokenStream as TokenStream2};

pub(crate) type DiagnosticStream = DiagnosticResult<TokenStream2>;

#[derive(Debug)]
pub(crate) enum DiagnosticResult<T> {
    Ok(T),
    Err(Diagnostic),
}

impl<T> DiagnosticResult<T> {
    pub(crate) fn error<S: Display>(message: S) -> Self {
        Self::Err(Diagnostic {
            level: Level::Error,
            message: message.to_string(),
            spans: vec![Span::call_site()],
            children: vec![],
        })
    }
    pub(crate) fn add_help<S: Display>(mut self, span: Span, message: S) -> Self {
        let Self::Err(ref mut diagnostic) = self else {
            todo!()
        };
        diagnostic.children.push(Diagnostic {
            level: Level::Help,
            message: message.to_string(),
            spans: vec![span],
            children: vec![],
        });
        self
    }
    #[allow(unused)]
    pub(crate) fn unwrap(self) -> T {
        let Self::Ok(t) = self else {
            panic!("Called unwrap on a not-OK value")
        };
        t
    }
}

#[derive(Debug, Clone)]
pub(crate) struct Diagnostic {
    level: Level,
    message: String,
    spans: Vec<Span>,
    children: Vec<Diagnostic>,
}

#[derive(Debug, Clone, Copy, PartialEq)]
enum Level {
    Error,
    #[expect(unused)]
    Warning,
    #[expect(unused)]
    Note,
    Help,
}

impl From<Level> for proc_macro::Level {
    fn from(level: Level) -> Self {
        match level {
            Level::Error => Self::Error,
            Level::Help => Self::Help,
            Level::Note => Self::Note,
            Level::Warning => Self::Warning,
        }
    }
}

impl<T> std::ops::Try for DiagnosticResult<T> {
    type Output = T;

    type Residual = DiagnosticResult<!>;

    fn from_output(output: Self::Output) -> Self {
        Self::Ok(output)
    }

    fn branch(self) -> std::ops::ControlFlow<Self::Residual, Self::Output> {
        match self {
            Self::Ok(t) => std::ops::ControlFlow::Continue(t),
            Self::Err(d) => std::ops::ControlFlow::Break(DiagnosticResult::Err(d)),
        }
    }
}

impl<T> std::ops::FromResidual<DiagnosticResult<!>> for DiagnosticResult<T> {
    fn from_residual(residual: DiagnosticResult<!>) -> Self {
        match residual {
            DiagnosticResult::Err(residual) => DiagnosticResult::Err(residual),
        }
    }
}

impl<T> std::ops::FromResidual<Result<std::convert::Infallible, DiagnosticResult<T>>>
    for DiagnosticResult<T>
{
    fn from_residual(result: Result<std::convert::Infallible, DiagnosticResult<T>>) -> Self {
        match result {
            Err(e) => e,
        }
    }
}

impl Diagnostic {
    fn add_as_child(self, parent: proc_macro::Diagnostic) -> proc_macro::Diagnostic {
        let msg = self.message.clone();
        match self.level {
            Level::Error => parent.span_error(self.as_spans(), msg),
            Level::Warning => parent.span_warning(self.as_spans(), msg),
            Level::Note => parent.span_note(self.as_spans(), msg),
            Level::Help => parent.span_help(self.as_spans(), msg),
        }
    }
}

impl Diagnostic {
    fn as_spans(&self) -> Vec<proc_macro::Span> {
        self.spans.iter().map(|span| span.unwrap()).collect()
    }
}

impl From<DiagnosticStream> for TokenStream1 {
    fn from(result: DiagnosticStream) -> Self {
        match result {
            DiagnosticResult::Ok(t) => t.into(),
            DiagnosticResult::Err(diagnostic) => {
                // MSV: unwrap requires rustc 1.29+ *without* semver exempt features
                let spans = diagnostic.as_spans();
                let mut pm_diagnostic = proc_macro::Diagnostic::spanned(
                    spans,
                    diagnostic.level.into(),
                    diagnostic.message,
                );
                for child in diagnostic.children {
                    pm_diagnostic = child.add_as_child(pm_diagnostic);
                }
                pm_diagnostic.emit();
                TokenStream1::new()
            }
        }
    }
}