Expose Detail

- Add some accessor methods on `Context`
- Wrap some variants in private structs to prevent people from depending
  on them
- Add some #[inline]

Cargo.toml

@@ -6,6 +6,7 @@ edition = "2021"
 [features]
 default = []
 backtrace = []
+extra-backtrace = ["backtrace"]
 clone-with-caveats = []
 termination = ["dep:ansee"]
 

src/context.rs

@@ -20,6 +20,26 @@ 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 pop_extra(&mut self) -> Option<Detail> {
+        self.extra.pop()
+    }
 }
 
 impl fmt::Display for Context {
@@ -37,15 +57,39 @@ impl fmt::Display for Context {
 
 #[derive(Debug)]
 #[cfg_attr(feature = "clone-with-caveats", derive(Clone))]
-pub(crate) enum Detail {
+#[non_exhaustive]
+pub enum Detail {
     Str(&'static str),
     String(String),
     Location(Location<'static>),
     #[cfg(feature = "backtrace")]
-    Backtrace(Backtrace),
+    Backtrace(PrivateBacktrace),
-    Error(Arc<dyn std::error::Error + Send + Sync>),
+    Error(PrivateError),
 }
 
+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,
+            status => Backtrace::Other(status, bt.to_string()),
+        };
+        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(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)]
@@ -69,11 +113,11 @@ impl fmt::Display for Detail {
             Self::String(s) => f.write_str(s),
             Self::Location(l) => write!(f, "At {l}"),
             #[cfg(feature = "backtrace")]
-            Self::Backtrace(Backtrace::Unsupported) => f.write_str("I'd like to show you a backtrace,\n  but it's not supported on your platform"),
+            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(Backtrace::Disabled) => f.write_str("If you'd like a backtrace,\n  try again with RUST_BACKTRACE=1"),
+            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(Backtrace::Other(status, bt)) => {
+            Self::Backtrace(PrivateBacktrace(Backtrace::Other(status, bt))) => {
                 f.write_str(if *status == BacktraceStatus::Captured {
                     "Here is the backtrace:\n"
                 } else {
@@ -81,7 +125,7 @@ impl fmt::Display for Detail {
                 })?;
                 write!(f, "{}", bt)
             },
-            Self::Error(e) => e.fmt(f),
+            Self::Error(PrivateError(e)) => e.fmt(f),
         }
     }
 }

src/explain.rs

@@ -2,8 +2,7 @@ use std::panic::Location;
 use std::sync::Arc;
 
 use crate::{How, IntoContext, Context, Detail};
-#[cfg(feature = "backtrace")]
+use crate::context::PrivateError;
-use crate::context::Backtrace;
 
 pub trait Explain {
     type Output;
@@ -21,23 +20,14 @@ impl Explain for How {
     fn context(mut self, context: impl IntoContext) -> Self {
         let mut context = context.into_context();
 
-        context.extra.reserve(if cfg!(feature = "backtrace") {
+        context.extra.reserve(if cfg!(feature = "extra-backtrace") {
             2
         } else {
             1
         });
         context.extra.push(Detail::Location(*Location::caller()));
-        #[cfg(feature = "backtrace")]
+        #[cfg(feature = "extra-backtrace")]
-        context.extra.push({
+        context.extra.push(Detail::backtrace());
-            use std::backtrace::BacktraceStatus::*;
-            let bt = std::backtrace::Backtrace::capture();
-            let bt = match bt.status() {
-                Disabled => Backtrace::Disabled,
-                Unsupported => Backtrace::Unsupported,
-                status => Backtrace::Other(status, bt.to_string()),
-            };
-            Detail::Backtrace(bt)
-        });
         self.push_context(context);
         self
     }
@@ -83,7 +73,7 @@ where
 
     #[track_caller]
     fn context(self, context: impl IntoContext) -> Self::Output {
-        How::new(Context::new(Detail::Error(self)))
+        How::new(Context::new(Detail::Error(PrivateError(self))))
             .context(context)
     }
 }

src/how.rs

@@ -25,17 +25,23 @@ impl How {
     #[track_caller]
     pub fn new(context: impl IntoContext) -> Self {
         let location = Location::caller();
-        Self(Box::new(HowInner {
+        #[allow(unused_mut)]
+        let mut how = Self(Box::new(HowInner {
             location,
             context: Vec::with_capacity(4),
         }))
-        .context(context)
+        .context(context);
+        #[cfg(all(feature = "backtrace", not(feature = "extra-backtrace")))]
+        how.top_mut().extra.push(Detail::backtrace());
+        how
     }
 
+    #[inline]
     pub fn location(&self) -> &'static Location {
         self.0.location
     }
 
+    #[inline]
     pub fn top(&self) -> &Context {
         // SAFETY: we only ever push values into context, and the constructor ensures that there
         // is at least 1 value in context.
@@ -50,6 +56,7 @@ impl How {
         }
     }
 
+    #[inline]
     pub fn bottom(&self) -> &Context {
         // SAFETY: we only ever push values into context, and the constructor ensures that there
         // is at least 1 value in context.
@@ -64,6 +71,37 @@ impl How {
         }
     }
 
+    #[inline]
+    pub fn top_mut(&mut self) -> &mut Context {
+        // SAFETY: we only ever push values into context, and the constructor ensures that there
+        // is at least 1 value in context.
+        let o = self.0.context.iter_mut().next();
+        if cfg!(debug_assertions) {
+            o.unwrap()
+        } else {
+            #[allow(unsafe_code)]
+            unsafe {
+                o.unwrap_unchecked()
+            }
+        }
+    }
+
+    #[inline]
+    pub fn bottom_mut(&mut self) -> &mut Context {
+        // SAFETY: we only ever push values into context, and the constructor ensures that there
+        // is at least 1 value in context.
+        let o = self.0.context.iter_mut().next_back();
+        if cfg!(debug_assertions) {
+            o.unwrap()
+        } else {
+            #[allow(unsafe_code)]
+            unsafe {
+                o.unwrap_unchecked()
+            }
+        }
+    }
+
+    #[inline]
     pub fn into_context(self) -> impl Iterator<Item = Context> {
         self.0.context.into_iter()
     }
@@ -75,6 +113,7 @@ impl How {
         b.finish()
     }
 
+    #[inline]
     pub(crate) fn push_context(&mut self, context: Context) {
         self.0.context.push(context);
     }
@@ -90,6 +129,7 @@ impl Clone for HowInner {
         }
     }
 
+    #[inline]
     fn clone_from(&mut self, source: &Self) {
         self.location = source.location;
         self.context.clone_from(&source.context);

src/lib.rs

@@ -3,8 +3,7 @@
 #![feature(doc_auto_cfg)]
 
 mod context;
-pub(crate) use context::Detail;
+pub use context::{Context, Detail, IntoContext};
-pub use context::{Context, IntoContext};
 
 mod report;
 pub use report::{Report, ReportOpts};