fix(md077): reject a configured indent of 0

An indent of 0 asks for continuation content at the list marker's own
column, which is not continuation content at all: it ends the list item.
The requirement was accepted, so correctly indented bodies were reported as
"Continuation line over-indented (expected 0, found 2)" and `fix` would have
dissolved the list the rule exists to keep consistent. It is now rejected at
parse time with an explanation, and MD077 falls back to its defaults.

Author: rvben

src/rules/md077_list_continuation_indent/md077_config.rs

@@ -1,6 +1,6 @@
 use super::ContinuationStyle;
 use crate::rule_config_serde::RuleConfig;
-use serde::{Deserialize, Serialize};
+use serde::{Deserialize, Deserializer, Serialize};
 
 /// Configuration for MD077 (List continuation content indentation)
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, Default)]
@@ -10,13 +10,30 @@ pub struct MD077Config {
     #[serde(default)]
     pub style: ContinuationStyle,
     /// Fixed continuation indent relative to the list marker, e.g. `4` for
-    /// MkDocs-style documents. When set, overrides the content-column-derived
-    /// requirement (`content_col`, or `max(content_col, 4)` under the MkDocs
-    /// flavor). `None` keeps the default content-column behavior.
-    #[serde(default)]
+    /// MkDocs-style documents. When set, it replaces the content-column-derived
+    /// requirement, except that under a flavor requiring strict list indentation
+    /// it may only raise the 4-space minimum, never lower it. `None` keeps the
+    /// default content-column behavior.
+    #[serde(default, deserialize_with = "deserialize_indent")]
     pub indent: Option<usize>,
 }
 
+/// Rejects `indent = 0`. A continuation at the marker column is not continuation
+/// content at all: it ends the list item, so requiring it would make `fix`
+/// dissolve the list it is meant to keep consistent.
+fn deserialize_indent<'de, D>(deserializer: D) -> Result<Option<usize>, D::Error>
+where
+    D: Deserializer<'de>,
+{
+    match Option::<usize>::deserialize(deserializer)? {
+        Some(0) => Err(serde::de::Error::custom(
+            "Invalid indent 0: continuation content at the list marker's own column \
+             leaves the list item. Use 1 or greater.",
+        )),
+        other => Ok(other),
+    }
+}
+
 impl RuleConfig for MD077Config {
     const RULE_NAME: &'static str = "MD077";
 }
@@ -60,4 +77,21 @@ mod tests {
         assert_eq!(parsed.style, ContinuationStyle::Aligned);
         assert_eq!(parsed.indent, Some(4));
     }
+
+    #[test]
+    fn rejects_zero_indent() {
+        let err = toml::from_str::<MD077Config>("indent = 0").unwrap_err().to_string();
+        assert!(
+            err.contains("leaves the list item"),
+            "zero must be rejected with an explanation, got: {err}"
+        );
+        // Control: the neighbouring value parses, so the rejection is about 0
+        // and not about the key being unreadable.
+        assert_eq!(toml::from_str::<MD077Config>("indent = 1").unwrap().indent, Some(1));
+    }
+
+    #[test]
+    fn rejects_negative_indent() {
+        assert!(toml::from_str::<MD077Config>("indent = -1").is_err());
+    }
 }