config: warn on unknown keys instead of dropping them silently

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Id06a7e2c96cfdcb69a21fa0416d988696a6a6964

Cargo.lock

@@ -42,6 +42,7 @@ dependencies = [
  "pound",
  "rustix",
  "serde",
+ "serde_ignored",
  "smithay-client-toolkit",
  "thiserror",
  "toml",
@@ -488,6 +489,16 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "serde_ignored"
+version = "0.1.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "115dffd5f3853e06e746965a20dcbae6ee747ae30b543d91b0e089668bb07798"
+dependencies = [
+ "serde",
+ "serde_core",
+]
+
 [[package]]
 name = "serde_spanned"
 version = "1.1.1"

Cargo.toml

@@ -23,6 +23,7 @@ rustix = { version = "1.1.4", features = [
   "fs",
 ] }
 serde = { version = "1.0.228", features = ["derive"] }
+serde_ignored = "0.1.12"
 smithay-client-toolkit = "0.20.0"
 thiserror = "2.0.18"
 toml = "0.9"

src/config.rs

@@ -195,8 +195,26 @@ impl Config {
         if !path.exists() {
             return Self::default();
         }
-        match std::fs::read_to_string(&path) {
-            Ok(text) => match toml::from_str(&text) {
+        let text = match std::fs::read_to_string(&path) {
+            Ok(text) => text,
+            Err(err) => {
+                tracing::warn!("read config {}: {err}; using defaults", path.display());
+                return Self::default();
+            }
+        };
+        // Deserialize through serde_ignored so a typo'd key (`font-sze`) is
+        // reported instead of silently dropped, while still loading: unknown
+        // keys stay tolerated, keeping forward-compatibility with newer configs.
+        let de = match toml::Deserializer::parse(&text) {
+            Ok(de) => de,
+            Err(err) => {
+                tracing::warn!("config {}: {err}; using defaults", path.display());
+                return Self::default();
+            }
+        };
+        match serde_ignored::deserialize(de, |key| {
+            tracing::warn!("config {}: unknown key `{key}` ignored", path.display());
+        }) {
             Ok(config) => {
                 tracing::info!("loaded config from {}", path.display());
                 config
@@ -205,11 +223,6 @@ impl Config {
                 tracing::warn!("config {}: {err}; using defaults", path.display());
                 Self::default()
             }
-            },
-            Err(err) => {
-                tracing::warn!("read config {}: {err}; using defaults", path.display());
-                Self::default()
-            }
         }
     }
 }
@@ -253,4 +266,25 @@ mod tests {
         assert_eq!(c.main.term, "beer");
         assert_eq!(c.scrollback.lines, 10_000);
     }
+
+    #[test]
+    fn unknown_keys_are_reported_but_still_load() {
+        let toml = r##"
+            [main]
+            font-sze = 14
+            font = "JetBrains Mono"
+
+            [made-up]
+            x = 1
+        "##;
+        let mut unknown = Vec::new();
+        let de = toml::Deserializer::parse(toml).unwrap();
+        let c: Config =
+            serde_ignored::deserialize(de, |key| unknown.push(key.to_string())).unwrap();
+        // The typo and the bogus table are both surfaced...
+        assert!(unknown.iter().any(|k| k == "main.font-sze"), "{unknown:?}");
+        assert!(unknown.iter().any(|k| k == "made-up"), "{unknown:?}");
+        // ...yet the valid key still loaded.
+        assert_eq!(c.main.font, "JetBrains Mono");
+    }
 }