feat(core): create rule to enforce sentence case

harper-core/src/linting/lint_group.rs

@@ -156,6 +156,7 @@ use super::safe_to_save::SafeToSave;
 use super::save_to_safe::SaveToSafe;
 use super::semicolon_apostrophe::SemicolonApostrophe;
 use super::sentence_capitalization::SentenceCapitalization;
+use super::sentence_casing::SentenceCasing;
 use super::shoot_oneself_in_the_foot::ShootOneselfInTheFoot;
 use super::simple_past_to_past_participle::SimplePastToPastParticiple;
 use super::since_duration::SinceDuration;
@@ -623,6 +624,7 @@ impl LintGroup {
         insert_expr_rule!(SafeToSave, true);
         insert_expr_rule!(SaveToSafe, true);
         insert_expr_rule!(SemicolonApostrophe, true);
+        insert_struct_rule!(SentenceCasing, false);
         insert_expr_rule!(ShootOneselfInTheFoot, true);
         insert_expr_rule!(SimplePastToPastParticiple, true);
         insert_expr_rule!(SinceDuration, true);

harper-core/src/linting/mod.rs

@@ -167,6 +167,7 @@ mod safe_to_save;
 mod save_to_safe;
 mod semicolon_apostrophe;
 mod sentence_capitalization;
+mod sentence_casing;
 mod shoot_oneself_in_the_foot;
 mod simple_past_to_past_participle;
 mod since_duration;

harper-core/src/linting/sentence_casing.rs

@@ -0,0 +1,233 @@
+use super::Suggestion;
+use super::{Lint, LintKind, Linter};
+use crate::document::Document;
+use crate::{Token, TokenStringExt};
+
+/// A linter that checks for words that are capitalized but shouldn't be
+/// (i.e., not at the start of a sentence/heading and not proper nouns).
+pub struct SentenceCasing;
+
+impl Default for SentenceCasing {
+    fn default() -> Self {
+        Self
+    }
+}
+
+impl SentenceCasing {
+    /// Check a sequence of tokens for incorrect capitalization.
+    /// `first_word_idx` is the index of the first word that should be capitalized.
+    fn check_tokens(&self, tokens: &[Token], document: &Document, lints: &mut Vec<Lint>) {
+        // Get the index of the first word in the sequence
+        let first_word_idx = tokens.iter().position(|t| t.kind.is_word());
+
+        let Some(first_word_idx) = first_word_idx else {
+            return;
+        };
+
+        // Check all words after the first one
+        for (idx, token) in tokens.iter().enumerate() {
+            // Skip the first word (it should be capitalized)
+            if idx <= first_word_idx {
+                continue;
+            }
+
+            // Only check actual words
+            if !token.kind.is_word() {
+                continue;
+            }
+
+            // Check if the word is capitalized
+            let word_chars = document.get_span_content(&token.span);
+            let Some(first_char) = word_chars.first() else {
+                continue;
+            };
+
+            // Skip if not capitalized
+            if !first_char.is_uppercase() {
+                continue;
+            }
+
+            // Skip proper nouns - these should be capitalized
+            if token.kind.is_proper_noun() {
+                continue;
+            }
+
+            // Skip words that are all uppercase (likely acronyms/initialisms)
+            if word_chars
+                .iter()
+                .all(|c| !c.is_alphabetic() || c.is_uppercase())
+            {
+                continue;
+            }
+
+            // Skip words after a colon (might be starting a new clause)
+            if let Some(prev_non_ws) = tokens[..idx].iter().rev().find(|t| !t.kind.is_whitespace())
+                && prev_non_ws.kind.is_punctuation()
+            {
+                let prev_chars = document.get_span_content(&prev_non_ws.span);
+                if prev_chars == [':'] {
+                    continue;
+                }
+            }
+
+            // Skip single-letter capitalizations (often used for proper context like "Plan A")
+            if word_chars.len() == 1 {
+                continue;
+            }
+
+            // Skip words after opening quotes (might be a quoted sentence start)
+            if let Some(prev_non_ws) = tokens[..idx].iter().rev().find(|t| !t.kind.is_whitespace())
+                && prev_non_ws.kind.is_quote()
+            {
+                continue;
+            }
+
+            // Check if this word follows a sentence terminator within the same sequence
+            // (This handles cases where parsing might not have split sentences correctly)
+            let has_terminator_before = tokens[first_word_idx + 1..idx]
+                .iter()
+                .any(|t| t.kind.is_sentence_terminator());
+
+            if has_terminator_before {
+                continue;
+            }
+
+            // Create the lowercase suggestion
+            let mut replacement_chars = word_chars.to_vec();
+            replacement_chars[0] = replacement_chars[0].to_ascii_lowercase();
+
+            lints.push(Lint {
+                span: token.span,
+                lint_kind: LintKind::Capitalization,
+                suggestions: vec![Suggestion::ReplaceWith(replacement_chars)],
+                priority: 63,
+                message: "This word is capitalized but does not appear to be a proper noun. Consider using lowercase.".to_string(),
+            });
+        }
+    }
+}
+
+impl Linter for SentenceCasing {
+    fn lint(&mut self, document: &Document) -> Vec<Lint> {
+        let mut lints = Vec::new();
+
+        // Check headings
+        for heading in document.iter_headings() {
+            self.check_tokens(heading, document, &mut lints);
+        }
+
+        // Check regular sentences (but skip those in headings)
+        for paragraph in document.iter_paragraphs() {
+            // Skip paragraphs that are headings (they're already checked above)
+            if paragraph.iter().any(|t| t.kind.is_heading_start()) {
+                continue;
+            }
+
+            for sentence in paragraph.iter_sentences() {
+                self.check_tokens(sentence, document, &mut lints);
+            }
+        }
+
+        lints
+    }
+
+    fn description(&self) -> &'static str {
+        "Flags words that are capitalized mid-sentence or mid-heading but are not proper nouns."
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::super::tests::{assert_lint_count, assert_suggestion_result};
+    use super::SentenceCasing;
+
+    #[test]
+    fn catches_mid_sentence_capital() {
+        assert_lint_count(
+            "The quick Brown fox jumps over the lazy dog.",
+            SentenceCasing,
+            1,
+        );
+    }
+
+    #[test]
+    fn allows_proper_nouns() {
+        assert_lint_count("I visited Paris last summer.", SentenceCasing, 0);
+    }
+
+    #[test]
+    fn allows_sentence_start() {
+        assert_lint_count("The fox is quick. The dog is lazy.", SentenceCasing, 0);
+    }
+
+    #[test]
+    fn allows_acronyms() {
+        assert_lint_count("The NASA mission was successful.", SentenceCasing, 0);
+    }
+
+    #[test]
+    fn allows_after_colon() {
+        assert_lint_count("Here is the answer: True or false.", SentenceCasing, 0);
+    }
+
+    #[test]
+    fn allows_single_letter() {
+        assert_lint_count("This is plan A for the mission.", SentenceCasing, 0);
+    }
+
+    #[test]
+    fn fixes_capitalization() {
+        assert_suggestion_result(
+            "The quick Brown fox.",
+            SentenceCasing,
+            "The quick brown fox.",
+        );
+    }
+
+    #[test]
+    fn allows_names() {
+        assert_lint_count("I talked to John yesterday.", SentenceCasing, 0);
+    }
+
+    #[test]
+    fn multiple_errors() {
+        assert_lint_count(
+            "The Quick Brown Fox jumps over the Lazy Dog.",
+            SentenceCasing,
+            4,
+        );
+    }
+
+    #[test]
+    fn allows_quoted_start() {
+        assert_lint_count("She said \"Hello there\" to him.", SentenceCasing, 0);
+    }
+
+    // Heading tests
+
+    #[test]
+    fn catches_heading_mid_word_capital() {
+        // Markdown heading with incorrect capitalization
+        assert_lint_count("# The Quick Brown Fox", SentenceCasing, 3);
+    }
+
+    #[test]
+    fn allows_heading_proper_nouns() {
+        assert_lint_count("# A trip to Paris", SentenceCasing, 0);
+    }
+
+    #[test]
+    fn allows_heading_start_capital() {
+        assert_lint_count("# Introduction to the topic", SentenceCasing, 0);
+    }
+
+    #[test]
+    fn fixes_heading_capitalization() {
+        assert_suggestion_result("# The Quick fox", SentenceCasing, "# The quick fox");
+    }
+
+    #[test]
+    fn heading_with_acronym() {
+        assert_lint_count("# Working with NASA and SpaceX", SentenceCasing, 0);
+    }
+}