how.rs/src/context.rs

use std::borrow::Cow;
#[cfg(feature = "backtrace")]
use std::backtrace::BacktraceStatus;
use std::fmt;
use std::panic::Location;
use std::sync::Arc;

/// Provides context furthering the explanation of *how* you got to an error.
#[derive(Debug)]
#[cfg_attr(feature = "clone-with-caveats", derive(Clone))]
pub struct Context(pub(crate) ContextInner);

impl fmt::Display for Context {
    #[inline(always)]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.0, f)
    }
}

#[derive(Debug)]
#[cfg_attr(feature = "clone-with-caveats", derive(Clone))]
pub(crate) enum ContextInner {
    Elem(Detail),
    Compound(Vec<Detail>),
}

#[derive(Debug)]
#[cfg_attr(feature = "clone-with-caveats", derive(Clone))]
pub(crate) enum Detail {
    Str(&'static str),
    String(String),
    Location(Location<'static>),
    #[cfg(feature = "backtrace")]
    Backtrace(Backtrace),
    Error(Arc<dyn std::error::Error + Send + Sync>),
}

// will be replaced with std::backtrace::Backtrace if and when it is Clone
#[cfg(feature = "backtrace")]
#[derive(Debug)]
pub(crate) enum Backtrace {
    Disabled,
    Unsupported,
    Other(BacktraceStatus, String),
}

#[cfg(all(feature = "backtrace", feature = "clone-with-caveats"))]
impl Clone for Backtrace {
    fn clone(&self) -> Self {
        Backtrace::Disabled
    }
}

impl fmt::Display for ContextInner {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Elem(elem) => fmt::Display::fmt(elem, f),
            Self::Compound(elems) => {
                let mut elems = elems.iter();
                if let Some(elem) = elems.next() {
                    fmt::Display::fmt(elem, f)?;
                    for elem in elems {
                        write!(f, "\n- {elem}")?;
                    }
                }
                Ok(())
            }
        }
    }
}

impl fmt::Display for Detail {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Str(s) => f.write_str(s),
            Self::String(s) => f.write_str(s),
            Self::Location(l) => write!(f, "At {l}"),
            #[cfg(feature = "backtrace")]
            Self::Backtrace(Backtrace::Unsupported) => f.write_str("\nI'd like to show you a backtrace,\n  but it's not supported on your platform"),
            #[cfg(feature = "backtrace")]
            Self::Backtrace(Backtrace::Disabled) => f.write_str("\nIf you'd like a backtrace,\n  try again with RUST_BACKTRACE=1"),
            #[cfg(feature = "backtrace")]
            Self::Backtrace(Backtrace::Other(status, bt)) => {
                f.write_str(if *status == BacktraceStatus::Captured {
                    "\nHere is the backtrace:\n"
                } else {
                    "\nI can't tell if backtraces are working,\n  but I'll give it a go:\n"
                })?;
                write!(f, "{}", bt)
            },
            Self::Error(e) => e.fmt(f),
        }
    }
}

pub trait IntoContext {
    fn into_context(self) -> Context;

    #[inline(always)]
    fn with(self, other: impl IntoContext) -> Context
    where
        Self: Sized,
    {
        self.into_context().with(other)
    }
}

impl IntoContext for Context {
    #[inline(always)]
    fn into_context(self) -> Context {
        self
    }

    /// Chains another piece of context that is a child from a hierarchical perspective.
    #[track_caller]
    #[inline]
    fn with(self, other: impl IntoContext) -> Self {
        let other = other.into_context().0;
        Context(ContextInner::Compound(match self.0 {
            ContextInner::Compound(mut elems) => {
                match other {
                    ContextInner::Elem(elem) => elems.push(elem),
                    ContextInner::Compound(mut elems1) => elems.append(&mut elems1),
                };
                elems
            }
            ContextInner::Elem(elem) => match other {
                ContextInner::Elem(elem1) => vec![elem, elem1],
                ContextInner::Compound(mut elems) => {
                    elems.insert(0, elem);
                    elems
                }
            },
        }))
    }
}

impl IntoContext for String {
    #[inline(always)]
    fn into_context(self) -> Context {
        Context(ContextInner::Elem(Detail::String(self)))
    }
}

impl IntoContext for &'static str {
    #[inline(always)]
    fn into_context(self) -> Context {
        Context(ContextInner::Elem(Detail::Str(self)))
    }
}

impl IntoContext for Cow<'static, str> {
    #[inline]
    fn into_context(self) -> Context {
        match self {
            Cow::Borrowed(s) => s.into_context(),
            Cow::Owned(s) => s.into_context(),
        }
    }
}

impl<'a> IntoContext for &'a Location<'static> {
    #[inline]
    fn into_context(self) -> Context {
        Location::into_context(*self)
    }
}

impl IntoContext for Location<'static> {
    #[inline]
    fn into_context(self) -> Context {
        Context(ContextInner::Elem(Detail::Location(self)))
    }
}

impl<C, F> IntoContext for F
where
    C: IntoContext,
    F: FnOnce() -> C,
{
    #[inline(always)]
    fn into_context(self) -> Context {
        self().into_context()
    }
}