feat: show sources on alternate display

examples/simple.rs

@@ -0,0 +1,22 @@
+use std::io;
+
+#[derive(Debug, snafu::Snafu)]
+enum OuterError {
+    #[snafu(display("Get failed"))]
+    Get { source: GetError },
+}
+
+#[derive(Debug, snafu::Snafu)]
+enum GetError {
+    #[snafu(display("IO failure"))]
+    Io { source: io::Error },
+}
+
+fn main() {
+    let err = io::Error::new(io::ErrorKind::BrokenPipe, "pipe is broken");
+    let err = GetError::Io { source: err };
+    let err = OuterError::Get { source: err };
+
+    println!("display normal: {err}");
+    println!("display alt   : {err:#}");
+}

snafu-derive/src/lib.rs

@@ -1796,6 +1796,7 @@ impl<'a> quote::ToTokens for DisplayImpl<'a> {
             .collect();
 
         let display = Display {
+            crate_root: &self.0.crate_root,
             arms: &arms,
             original_generics: &self.0.provided_generics_without_defaults(),
             parameterized_error_name: &self.0.parameterized_name(),
@@ -1989,6 +1990,7 @@ impl NamedStructInfo {
         let arm = quote! { #arm };
 
         let display_impl = Display {
+            crate_root: &self.crate_root,
             arms: &[arm],
             original_generics: &original_generics,
             parameterized_error_name: &parameterized_struct_name,

snafu-derive/src/shared.rs

@@ -461,6 +461,7 @@ pub mod display {
     const FORMATTER_ARG: StaticIdent = StaticIdent("__snafu_display_formatter");
 
     pub(crate) struct Display<'a> {
+        pub(crate) crate_root: &'a dyn ToTokens,
         pub(crate) arms: &'a [TokenStream],
         pub(crate) original_generics: &'a [TokenStream],
         pub(crate) parameterized_error_name: &'a dyn ToTokens,
@@ -470,6 +471,7 @@ pub mod display {
     impl ToTokens for Display<'_> {
         fn to_tokens(&self, stream: &mut TokenStream) {
             let Self {
+                crate_root,
                 arms,
                 original_generics,
                 parameterized_error_name,
@@ -487,6 +489,15 @@ pub mod display {
                         match *self {
                             #(#arms),*
                         }
+
+                        if #FORMATTER_ARG.alternate() {
+                            let chain = #crate_root::ChainCompat::new(self).skip(1);
+                            for item in chain {
+                                write!(#FORMATTER_ARG, ": {item}")?;
+                            }
+                        }
+
+                        Ok(())
                     }
                 }
             };
@@ -561,7 +572,7 @@ pub mod display {
 
             let match_arm = quote! {
                 #pattern_ident { #(ref #field_names),* } => {
-                    write!(#FORMATTER_ARG, #format, #shorthand_assignments)
+                    write!(#FORMATTER_ARG, #format, #shorthand_assignments)?;
                 }
             };