Tracking a path where an error happens during deserialization

Related to #83, but doesn't solve that specific problem :-(. That
specific error message ("missing field") comes from somewhere else than
this library.

2 files changed, 48 insertions, 4 deletions

diff --git a/src/de.rs b/src/de.rs
index 9ebe2e4..9267338 100644
--- a/src/de.rs
+++ b/src/de.rs
@@ -2,6 +2,7 @@ use config::Config;
 use error::*;
 use serde::de;
 use std::collections::{HashMap, VecDeque};
+use std::iter::Enumerate;
 use value::{Value, ValueKind, ValueWithKey, Table};
 
 // TODO: Use a macro or some other magic to reduce the code duplication here
@@ -284,13 +285,13 @@ impl<'de, 'a> de::Deserializer<'de> for StrDeserializer<'a> {
 }
 
 struct SeqAccess {
-    elements: ::std::vec::IntoIter<Value>,
+    elements: Enumerate<::std::vec::IntoIter<Value>>,
 }
 
 impl SeqAccess {
     fn new(elements: Vec<Value>) -> Self {
         SeqAccess {
-            elements: elements.into_iter(),
+            elements: elements.into_iter().enumerate(),
         }
     }
 }
@@ -303,7 +304,11 @@ impl<'de> de::SeqAccess<'de> for SeqAccess {
         T: de::DeserializeSeed<'de>,
     {
         match self.elements.next() {
-            Some(value) => seed.deserialize(value).map(Some),
+            Some((idx, value)) => {
+                seed.deserialize(value)
+                    .map(Some)
+                    .map_err(|e| e.prepend_index(idx))
+            }
             None => Ok(None),
         }
     }
@@ -349,7 +354,9 @@ impl<'de> de::MapAccess<'de> for MapAccess {
     where
         V: de::DeserializeSeed<'de>,
     {
-        de::DeserializeSeed::deserialize(seed, self.elements.pop_front().unwrap().1)
+        let (key, value) = self.elements.pop_front().unwrap();
+        de::DeserializeSeed::deserialize(seed, value)
+            .map_err(|e| e.prepend_key(key))
     }
 }
 
diff --git a/src/error.rs b/src/error.rs
index 1d348b6..e305750 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -114,6 +114,43 @@ impl ConfigError {
             _ => self,
         }
     }
+
+    fn prepend(self, segment: String, add_dot: bool) -> Self {
+        let concat = |key: Option<String>| {
+            let key = key.unwrap_or_else(String::new);
+            let dot = if add_dot && key.as_bytes().get(0).unwrap_or(&b'[') != &b'[' {
+                "."
+            } else {
+                ""
+            };
+            format!("{}{}{}", segment, dot, key)
+        };
+        match self {
+            ConfigError::Type {
+                origin,
+                unexpected,
+                expected,
+                key,
+            } => {
+                ConfigError::Type {
+                    origin,
+                    unexpected,
+                    expected,
+                    key: Some(concat(key)),
+                }
+            }
+            ConfigError::NotFound(key) => ConfigError::NotFound(concat(Some(key))),
+            _ => self,
+        }
+    }
+
+    pub(crate) fn prepend_key(self, key: String) -> Self {
+        self.prepend(key, true)
+    }
+
+    pub(crate) fn prepend_index(self, idx: usize) -> Self {
+        self.prepend(format!("[{}]", idx), false)
+    }
 }
 
 /// Alias for a `Result` with the error type set to `ConfigError`.