Added basic failure name (#289)

Author: mitsuhiko

examples/simple.rs

@@ -16,7 +16,7 @@ fn bad_function() -> Result<(), WrappingError> {
 }
 
 fn main() {
-    for cause in Fail::iter_causes(&bad_function().unwrap_err()) {
-        println!("{}", cause);
+    for cause in Fail::iter_chain(&bad_function().unwrap_err()) {
+        println!("{}: {}", cause.name().unwrap_or("Error"), cause);
     }
 }

failure_derive/src/lib.rs

@@ -7,6 +7,7 @@ extern crate synstructure;
 extern crate quote;
 
 use proc_macro2::{TokenStream, Span};
+use syn::LitStr;
 use syn::spanned::Spanned;
 
 #[derive(Debug)]
@@ -40,6 +41,8 @@ fn fail_derive_impl(s: synstructure::Structure) -> Result<TokenStream, Error> {
         quote! { & }
     };
 
+    let ty_name = LitStr::new(&s.ast().ident.to_string(), Span::call_site());
+
     let cause_body = s.each_variant(|v| {
         if let Some(cause) = v.bindings().iter().find(is_cause) {
             quote!(return Some(::failure::AsFail::as_fail(#cause)))
@@ -59,6 +62,10 @@ fn fail_derive_impl(s: synstructure::Structure) -> Result<TokenStream, Error> {
     let fail = s.unbound_impl(
         quote!(::failure::Fail),
         quote! {
+            fn name(&self) -> Option<&str> {
+                Some(concat!(module_path!(), "::", #ty_name))
+            }
+
             #[allow(unreachable_code)]
             fn cause(&self) -> ::failure::_core::option::Option<#make_dyn(::failure::Fail)> {
                 match *self { #cause_body }

src/context.rs

@@ -108,6 +108,10 @@ with_std! {
     }
 
     impl<D: Display + Send + Sync + 'static> Fail for Context<D> {
+        fn name(&self) -> Option<&str> {
+            self.failure.as_cause().and_then(|x| x.name())
+        }
+
         fn cause(&self) -> Option<&Fail> {
             self.failure.as_cause()
         }

src/error/mod.rs

@@ -70,6 +70,11 @@ impl Error {
         self.imp.failure()
     }
 
+    /// Returns the name of the underlying fail.
+    pub fn name(&self) -> Option<&str> {
+        self.as_fail().name()
+    }
+
     /// Returns a reference to the underlying cause of this `Error`. Unlike the
     /// method on `Fail`, this does not return an `Option`. The `Error` type
     /// always has an underlying failure.

src/error_message.rs

@@ -19,7 +19,11 @@ struct ErrorMessage<D: Display + Debug + Sync + Send + 'static> {
     msg: D,
 }
 
-impl<D: Display + Debug + Sync + Send + 'static> Fail for ErrorMessage<D> { }
+impl<D: Display + Debug + Sync + Send + 'static> Fail for ErrorMessage<D> {
+    fn name(&self) -> Option<&str> {
+        Some("failure::ErrorMessage")
+    }
+}
 
 impl<D: Display + Debug + Sync + Send + 'static> Display for ErrorMessage<D> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

src/lib.rs

@@ -106,6 +106,16 @@ with_std! {
 /// `std::error::Error`, and are also `Send`, `Sync`, and `'static`, implement
 /// `Fail` by a blanket impl.
 pub trait Fail: Display + Debug + Send + Sync + 'static {
+    /// Returns the "name" of the error.
+    /// 
+    /// This is typically the type name. Not all errors will implement
+    /// this. This method is expected to be most useful in situations
+    /// where errors need to be reported to external instrumentation systems 
+    /// such as crash reporters.
+    fn name(&self) -> Option<&str> {
+        None
+    }
+
     /// Returns a reference to the underlying cause of this failure, if it
     /// is an error that wraps other errors.
     ///

tests/basic_fail.rs

@@ -0,0 +1,21 @@
+#[macro_use]
+extern crate failure;
+
+use failure::Fail;
+
+#[test]
+fn test_name() {
+    #[derive(Fail, Debug)]
+    #[fail(display = "my error")]
+    struct MyError;
+
+    let err = MyError;
+
+    assert_eq!(err.to_string(), "my error");
+    assert_eq!(err.name(), Some("basic_fail::MyError"));
+
+    let ctx = err.context("whatever");
+
+    assert_eq!(ctx.to_string(), "whatever");
+    assert_eq!(ctx.name(), Some("basic_fail::MyError"));
+}