Adds serde_path_to_error to have more user-friendly error messages

examples/invalid_metrics_variant.yaml

@@ -0,0 +1,28 @@
+generator:
+  - http:
+      seed: [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53,
+             59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131]
+      headers:
+        content-type: "application/x-protobuf"
+      target_uri: "http://127.0.0.1:4318/v1/metrics"
+      bytes_per_second: "6 MiB"
+      parallel_connections: 5
+      method:
+        post:
+          maximum_prebuild_cache_size_bytes: "512 MiB"
+          variant: "opentelemetry_metrics"
+
+blackhole:
+  - http:
+      binding_addr: "127.0.0.1:9091"
+      body_variant: "nothing"
+  - http:
+      binding_addr: "127.0.0.1:9092"
+      body_variant: "nothing"
+  - http:
+      binding_addr: "127.0.0.1:9093"
+      body_variant: "nothing"
+
+target_metrics:
+  - prometheus:
+      uri: "http://127.0.0.1:5000/telemetry"

lading/Cargo.toml

@@ -70,6 +70,7 @@ serde = { workspace = true }
 serde_json = { workspace = true }
 serde_qs = { version = "0.15", default-features = false }
 serde_yaml = { version = "0.9" }
+serde_path_to_error = { version = "0.1" }
 thiserror = { workspace = true }
 tokio = { workspace = true, features = [
   "rt",

lading/src/bin/lading.rs

@@ -50,6 +50,8 @@
     LadingObserver(#[from] lading::observer::Error),
     #[error("Failed to deserialize Lading config: {0}")]
     SerdeYaml(#[from] serde_yaml::Error),
+    #[error("Configuration error at {path}: {message}")]
+    ConfigPath { path: String, message: String },
     #[error("Lading failed to sync servers {0}")]
     Send(#[from] tokio::sync::broadcast::error::SendError<Option<i32>>),
     #[error("Parsing Prometheus address failed: {0}")]
@@ -267,9 +269,22 @@
 }
 
 fn parse_config(contents: &str) -> Result<Config, Error> {
-    serde_yaml::from_str(contents).map_err(|err| {
-        error!("Configuration validation failed: {}", err);
-        Error::SerdeYaml(err)
+    let deserializer = serde_yaml::Deserializer::from_str(contents);
+    serde_path_to_error::deserialize(deserializer).map_err(|err| {
+        let path_info = err.path().to_string();
+        let inner_message = err.inner().to_string();
+
+        // Clean up the message to avoid redundancy - remove the path info if it's already in the inner message
+        let message = if inner_message.contains(&path_info) {
+            inner_message
+        } else {
+            format!("{} (at {})", inner_message, path_info)
+        };
+        error!("Configuration validation failed: {}", message);
+        Error::ConfigPath {
+            path: path_info,
+            message,
+        }
     })
 }