Disallow any unknown fields in configuration files

This will break backwards compatibility of configurations, but
ensures that a previously-configured setting won't get dropped
arbitrarily. Pretty much worth it, I think.

src/config.rs

@@ -29,6 +29,7 @@ pub(crate) static DEFAULT_PATH: Lazy<PathBuf> =
 
 #[allow(clippy::struct_excessive_bools)]
 #[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 pub(crate) struct Config {
     #[serde(default = "false_fn")]
     pub(crate) conduit_compat: bool,
@@ -77,6 +78,7 @@ pub(crate) struct Config {
 }
 
 #[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 pub(crate) struct ServerDiscovery {
     /// Server-server discovery configuration
     #[serde(default)]
@@ -88,6 +90,7 @@ pub(crate) struct ServerDiscovery {
 
 /// Server-server discovery configuration
 #[derive(Debug, Default, Deserialize)]
+#[serde(deny_unknown_fields)]
 pub(crate) struct ServerServerDiscovery {
     /// The alternative authority to make server-server API requests to
     pub(crate) authority: Option<OwnedServerName>,
@@ -95,6 +98,7 @@ pub(crate) struct ServerServerDiscovery {
 
 /// Client-server discovery configuration
 #[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 pub(crate) struct ClientServerDiscovery {
     /// The base URL to make client-server API requests to
     pub(crate) base_url: Url,
@@ -104,6 +108,7 @@ pub(crate) struct ClientServerDiscovery {
 }
 
 #[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 pub(crate) struct TlsConfig {
     pub(crate) certs: String,
     pub(crate) key: String,
@@ -114,6 +119,7 @@ pub(crate) struct TlsConfig {
 )]
 #[serde(rename_all = "snake_case")]
 #[strum(serialize_all = "snake_case")]
+#[serde(deny_unknown_fields)]
 pub(crate) enum ListenComponent {
     Client,
     Federation,
@@ -129,6 +135,7 @@ impl ListenComponent {
 
 #[derive(Clone, Debug, Deserialize)]
 #[serde(tag = "type", rename_all = "snake_case")]
+#[serde(deny_unknown_fields)]
 pub(crate) enum ListenTransport {
     Tcp {
         #[serde(default = "default_address")]
@@ -171,6 +178,7 @@ impl Display for ListenTransport {
 }
 
 #[derive(Clone, Debug, Deserialize)]
+// Incompatible with deny_unknown_fields due to serde(flatten).
 pub(crate) struct ListenConfig {
     #[serde(default = "ListenComponent::all_components")]
     pub(crate) components: HashSet<ListenComponent>,
@@ -194,6 +202,7 @@ impl Display for ListenConfig {
 }
 
 #[derive(Copy, Clone, Default, Debug, Deserialize, clap::ValueEnum)]
+#[serde(deny_unknown_fields)]
 #[serde(rename_all = "snake_case")]
 pub(crate) enum LogFormat {
     /// Multiple lines per event, includes all information
@@ -222,6 +231,7 @@ impl Display for LogFormat {
 }
 
 #[derive(Clone, Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 #[serde(default)]
 pub(crate) struct TurnConfig {
     pub(crate) username: String,
@@ -244,6 +254,7 @@ impl Default for TurnConfig {
 }
 
 #[derive(Clone, Copy, Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 #[serde(rename_all = "lowercase")]
 pub(crate) enum DatabaseBackend {
     #[cfg(feature = "rocksdb")]
@@ -264,6 +275,7 @@ impl Display for DatabaseBackend {
 }
 
 #[derive(Clone, Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 pub(crate) struct DatabaseConfig {
     pub(crate) backend: DatabaseBackend,
     pub(crate) path: String,
@@ -275,12 +287,14 @@ pub(crate) struct DatabaseConfig {
 }
 
 #[derive(Clone, Debug, Default, Deserialize)]
+#[serde(deny_unknown_fields)]
 #[serde(default)]
 pub(crate) struct MetricsConfig {
     pub(crate) enable: bool,
 }
 
 #[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 #[serde(default)]
 pub(crate) struct OtelTraceConfig {
     pub(crate) enable: bool,
@@ -301,6 +315,7 @@ impl Default for OtelTraceConfig {
 }
 
 #[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 #[serde(default)]
 pub(crate) struct FlameConfig {
     pub(crate) enable: bool,
@@ -319,6 +334,7 @@ impl Default for FlameConfig {
 }
 
 #[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 #[serde(default)]
 pub(crate) struct LogConfig {
     pub(crate) filter: EnvFilterClone,
@@ -339,6 +355,7 @@ impl Default for LogConfig {
 }
 
 #[derive(Debug, Default, Deserialize)]
+#[serde(deny_unknown_fields)]
 #[serde(default)]
 pub(crate) struct ObservabilityConfig {
     /// Prometheus metrics
@@ -352,6 +369,7 @@ pub(crate) struct ObservabilityConfig {
 }
 
 #[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
 #[serde(default)]
 pub(crate) struct FederationConfig {
     pub(crate) enable: bool,

tests/integrations/snapshots/integrations__check_config__invalid_keys@stderr.snap

@@ -8,5 +8,5 @@ Error: failed to validate configuration
     Caused by: TOML parse error at line 1, column 1
   |
 1 | some_name = "example.com"
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^
-missing field `server_name`
+  | ^^^^^^^^^
+unknown field `some_name`, expected one of `conduit_compat`, `listen`, `tls`, `server_name`, `server_discovery`, `database`, `federation`, `cache_capacity_modifier`, `pdu_cache_capacity`, `cleanup_second_interval`, `max_request_size`, `allow_registration`, `registration_token`, `allow_encryption`, `allow_room_creation`, `serve_media_unauthenticated`, `default_room_version`, `proxy`, `jwt_secret`, `observability`, `turn`, `emergency_password`