how.rs/src/context.rs

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

use crate::report::report_write;
use crate::ReportOpts;

/// Provides context furthering the explanation of *how* you got to an error.
#[derive(Debug)]
#[cfg_attr(
    any(feature = "arc-backtrace", feature = "clone-with-caveats"),
    derive(Clone)
)]
pub struct Context {
    pub(crate) detail: Detail,
    pub(crate) extra: Vec<Detail>,
}

impl Context {
    pub(crate) fn new(detail: Detail) -> Self {
        Self {
            detail,
            extra: Vec::new(),
        }
    }

    pub fn detail(&self) -> &Detail {
        &self.detail
    }

    pub fn extra(&self) -> &[Detail] {
        &self.extra
    }

    pub fn detail_mut(&mut self) -> &mut Detail {
        &mut self.detail
    }

    pub fn extra_mut(&mut self) -> &mut [Detail] {
        &mut self.extra
    }

    pub fn push_extra(&mut self, detail: impl IntoContext) {
        let detail = detail.into_context();
        if detail.extra.is_empty() {
            self.extra.push(detail.detail);
        } else {
            self.extra.push(Detail::Context(detail.into()));
        }
    }

    pub fn pop_extra(&mut self) -> Option<Detail> {
        self.extra.pop()
    }
}

impl fmt::Display for Context {
    #[inline(always)]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.detail.fmt(f)?;
        let opts = ReportOpts::default();
        for detail in &self.extra {
            f.write_str("\n")?;
            report_write!(f, &opts.indent().next(), "- {detail}")?;
        }
        Ok(())
    }
}

#[derive(Debug)]
#[cfg_attr(
    any(feature = "arc-backtrace", feature = "clone-with-caveats"),
    derive(Clone)
)]
#[non_exhaustive]
pub enum Detail {
    Str(&'static str),
    String(String),
    Location(Location<'static>),
    #[cfg(feature = "backtrace")]
    Backtrace(PrivateBacktrace),
    Error(PrivateError),
    Context(Box<Context>),
}

impl Detail {
    #[cfg(feature = "backtrace")]
    #[track_caller]
    pub fn backtrace() -> Self {
        use std::backtrace::BacktraceStatus::*;
        let bt = std::backtrace::Backtrace::capture();
        let bt = match bt.status() {
            Disabled => Backtrace::Disabled,
            Unsupported => Backtrace::Unsupported,
            _ => Backtrace::Other(bt.into()),
        };
        Self::Backtrace(PrivateBacktrace(bt))
    }
}

#[derive(Debug, Clone)]
pub struct PrivateError(pub(crate) Arc<dyn std::error::Error + Send + Sync>);

#[cfg(feature = "backtrace")]
#[derive(Debug)]
#[cfg_attr(
    any(feature = "arc-backtrace", feature = "clone-with-caveats"),
    derive(Clone)
)]
pub struct PrivateBacktrace(pub(crate) Backtrace);

// will be replaced with std::backtrace::Backtrace if and when it is Clone
#[cfg(feature = "backtrace")]
#[derive(Debug)]
#[cfg_attr(feature = "arc-backtrace", derive(Clone))]
pub(crate) enum Backtrace {
    Disabled,
    Unsupported,
    #[cfg(feature = "arc-backtrace")]
    Other(Arc<std::backtrace::Backtrace>),
    #[cfg(not(feature = "arc-backtrace"))]
    Other(std::backtrace::Backtrace),
}

#[cfg(all(
    feature = "backtrace",
    feature = "clone-with-caveats",
    not(feature = "arc-backtrace")
))]
impl Clone for Backtrace {
    fn clone(&self) -> Self {
        match self {
            Self::Unsupported => Self::Unsupported,
            _ => Self::Disabled,
        }
    }
}

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(PrivateBacktrace(Backtrace::Unsupported)) => f.write_str(
                "I'd like to show you a backtrace,\n  but it's not supported on your platform",
            ),
            #[cfg(feature = "backtrace")]
            Self::Backtrace(PrivateBacktrace(Backtrace::Disabled)) => {
                f.write_str("If you'd like a backtrace,\n  try again with RUST_BACKTRACE=1")
            }
            #[cfg(feature = "backtrace")]
            Self::Backtrace(PrivateBacktrace(Backtrace::Other(bt))) => {
                f.write_str(if bt.status() == BacktraceStatus::Captured {
                    "Here is the backtrace:\n"
                } else {
                    "I can't tell if backtraces are working,\n  but I'll give it a go:\n"
                })?;
                write!(f, "{}", bt)
            }
            Self::Error(PrivateError(e)) => e.fmt(f),
            Self::Context(c) => c.fmt(f),
        }
    }
}

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

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

    #[inline(always)]
    #[track_caller]
    fn with_caller(self) -> Context {
        self.with(Location::caller())
    }

    #[inline(always)]
    #[track_caller]
    fn with_backtrace(self) -> Context {
        #[cfg(feature = "backtrace")]
        return self.with(Detail::backtrace());
        #[cfg(not(feature = "backtrace"))]
        self.into_context()
    }
}

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(mut self, other: impl IntoContext) -> Self {
        self.push_extra(other);
        self
    }
}

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

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

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 {
        Detail::Location(self).into_context()
    }
}

impl<E> IntoContext for Arc<E>
where
    E: std::error::Error + Send + Sync + 'static,
{
    #[inline]
    fn into_context(self) -> Context {
        Detail::Error(PrivateError(self)).into_context()
    }
}

impl IntoContext for Arc<dyn std::error::Error + Send + Sync> {
    #[inline]
    fn into_context(self) -> Context {
        Detail::Error(PrivateError(self)).into_context()
    }
}

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

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