Dim unchanged comments as a crude syntax highlighting technique

CHANGELOG.md

@@ -12,6 +12,9 @@ for instructions.
 
 ### Display
 
+Unchanged comments are now dimmed, to distinguish them from other
+unchanged code.
+
 Characters that don't have a position in the parsed syntax tree are
 now displayed in purple, to make bugs more obvious. Previously they
 were dimmed.

src/style.rs

@@ -1,10 +1,6 @@
 //! Apply colours and styling to strings.
 
-use crate::{
-    lines::{codepoint_len, substring_by_codepoint, LineNumber},
-    positions::SingleLineSpan,
-    syntax::{MatchKind, MatchedPos},
-};
+use crate::{lines::{codepoint_len, substring_by_codepoint, LineNumber}, positions::SingleLineSpan, syntax::{HighlightKind, MatchKind, MatchedPos}};
 use colored::*;
 use std::{cmp::min, collections::HashMap};
 
@@ -13,6 +9,7 @@ struct Style {
     foreground: Color,
     background: Option<Color>,
     bold: bool,
+    dimmed: bool,
 }
 
 impl Style {
@@ -21,6 +18,9 @@ impl Style {
         if self.bold {
             res = res.bold();
         }
+        if self.dimmed {
+            res = res.dimmed();
+        }
         if let Some(background) = self.background {
             res = res.on_color(background);
         };
@@ -99,10 +99,11 @@ pub fn apply_colors(s: &str, is_lhs: bool, positions: &[MatchedPos]) -> String {
     let mut styles = vec![];
     for pos in positions {
         let style = match pos.kind {
-            MatchKind::Unchanged { .. } => Style {
+            MatchKind::Unchanged { highlight, .. } => Style {
                 foreground: Color::White,
                 background: None,
                 bold: false,
+                dimmed: highlight == HighlightKind::Comment,
             },
             MatchKind::Novel | MatchKind::ChangedCommentPart => Style {
                 foreground: if is_lhs {
@@ -112,11 +113,13 @@ pub fn apply_colors(s: &str, is_lhs: bool, positions: &[MatchedPos]) -> String {
                 },
                 background: None,
                 bold: true,
+                dimmed: false,
             },
             MatchKind::UnchangedCommentPart { .. } => Style {
                 foreground: if is_lhs { Color::Red } else { Color::Green },
                 background: None,
                 bold: false,
+                dimmed: false,
             },
         };
         for line_pos in &pos.pos {

src/syntax.rs

@@ -476,9 +476,16 @@ impl<'a> Hash for Syntax<'a> {
     }
 }
 
+#[derive(PartialEq, Eq, Debug, Clone, Copy)]
+pub enum HighlightKind {
+    Normal,
+    Comment,
+}
+
 #[derive(PartialEq, Eq, Debug, Clone)]
 pub enum MatchKind {
     Unchanged {
+        highlight: HighlightKind,
         opposite_pos: (Vec<SingleLineSpan>, Vec<SingleLineSpan>),
     },
     Novel,
@@ -576,6 +583,7 @@ fn split_comment_words(
 impl MatchedPos {
     fn new(
         ck: ChangeKind,
+        is_comment: bool,
         pos: Vec<SingleLineSpan>,
         prev_opposite_pos: Vec<SingleLineSpan>,
     ) -> Vec<Self> {
@@ -610,7 +618,14 @@ impl MatchedPos {
                     Atom { position, .. } => (position.clone(), position.clone()),
                 };
 
-                MatchKind::Unchanged { opposite_pos }
+                MatchKind::Unchanged {
+                    highlight: if is_comment {
+                        HighlightKind::Comment
+                    } else {
+                        HighlightKind::Normal
+                    },
+                    opposite_pos,
+                }
             }
             Novel => MatchKind::Novel,
         };
@@ -683,6 +698,7 @@ fn change_positions_<'a>(
 
                 positions.extend(MatchedPos::new(
                     change,
+                    false,
                     open_position.clone(),
                     prev_opposite_pos.clone(),
                 ));
@@ -708,11 +724,17 @@ fn change_positions_<'a>(
                 }
                 positions.extend(MatchedPos::new(
                     change,
+                    false,
                     close_position.clone(),
                     prev_opposite_pos.clone(),
                 ));
             }
-            Atom { info, position, .. } => {
+            Atom {
+                info,
+                position,
+                is_comment,
+                ..
+            } => {
                 let change = info
                     .change
                     .get()
@@ -733,6 +755,7 @@ fn change_positions_<'a>(
                 }
                 positions.extend(MatchedPos::new(
                     change,
+                    *is_comment,
                     position.clone(),
                     prev_opposite_pos.clone(),
                 ));