Clearer error message when line-length goes beyond threshold

crates/ruff_linter/src/line_width.rs

@@ -14,7 +14,7 @@ use ruff_text_size::TextSize;
 /// The length of a line of text that is considered too long.
 ///
 /// The allowed range of values is 1..=320
-#[derive(Clone, Copy, Debug, Eq, PartialEq, serde::Serialize, serde::Deserialize)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq, serde::Serialize)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct LineLength(
     #[cfg_attr(feature = "schemars", schemars(range(min = 1, max = 320)))] NonZeroU16,
@@ -46,6 +46,21 @@ impl fmt::Display for LineLength {
     }
 }
 
+impl<'de> serde::Deserialize<'de> for LineLength {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let value = u16::deserialize(deserializer)?;
+        Self::try_from(value).map_err(|_| {
+            serde::de::Error::custom(format!(
+                "line-length must be between 1 and {} (got {value})",
+                Self::MAX,
+            ))
+        })
+    }
+}
+
 impl CacheKey for LineLength {
     fn cache_key(&self, state: &mut CacheKeyHasher) {
         state.write_u16(self.0.get());

crates/ruff_workspace/src/pyproject.rs

@@ -266,7 +266,6 @@ mod tests {
     use crate::pyproject::{Pyproject, Tools, find_settings_toml, parse_pyproject_toml};
 
     #[test]
-
     fn deserialize() -> Result<()> {
         let pyproject: Pyproject = toml::from_str(r"")?;
         assert_eq!(pyproject.tool, None);
@@ -456,6 +455,19 @@ other-attribute = 1
             .is_err()
         );
 
+        let invalid_line_length = toml::from_str::<Pyproject>(
+            r"
+[tool.ruff]
+line-length = 500
+",
+        )
+        .expect_err("Deserialization should have failed for a too large line-length");
+
+        assert_eq!(
+            invalid_line_length.message(),
+            "line-length must be between 1 and 320 (got 500)"
+        );
+
         Ok(())
     }