Enforce at least one space after comment symbol

fixtures/small/class_comment_expected.rb

@@ -1,6 +1,6 @@
-#adsf
+# adsf
 class Foo
-  #but this one doesn't get a proceeding newline
+  # but this one doesn't get a proceeding newline
   a
 end
 

fixtures/small/comment_at_least_one_leading_space_actual.rb

@@ -0,0 +1,40 @@
+#! /usr/bin/env ruby
+# frozen_string_literal: true
+
+#### #### AUTHOR: coder204coolgizmos ### ##
+
+##DATE: 2026-06-27
+##   Cool Gizmos, Inc.
+
+#### ####
+###----------
+#: asdf
+#| asdf
+#** doc doc
+#
+#
+#This is a class that frobs a gizmo. It:
+#  * frobs the sprog
+#  * then turns the sprog into the gizmo.
+#### #
+#### #
+#Keep this class here.
+class Foo
+  #inner class comment
+
+  #method description comment
+  def self.b(&_blk)
+    yield
+  end
+
+  #method description comment
+  def self.a
+    #some method comment
+
+    x = b do
+      #some comment inside the block
+      y = 4
+      # some other comment
+    end
+  end
+end

fixtures/small/comment_at_least_one_leading_space_expected.rb

@@ -0,0 +1,40 @@
+#! /usr/bin/env ruby
+# frozen_string_literal: true
+
+#### #### AUTHOR: coder204coolgizmos ### ##
+
+## DATE: 2026-06-27
+##   Cool Gizmos, Inc.
+
+#### ####
+###----------
+#: asdf
+#| asdf
+#** doc doc
+#
+#
+# This is a class that frobs a gizmo. It:
+#  * frobs the sprog
+#  * then turns the sprog into the gizmo.
+#### #
+#### #
+# Keep this class here.
+class Foo
+  # inner class comment
+
+  # method description comment
+  def self.b(&_blk)
+    yield
+  end
+
+  # method description comment
+  def self.a
+    # some method comment
+
+    x = b do
+      # some comment inside the block
+      y = 4
+      # some other comment
+    end
+  end
+end

fixtures/small/empty_comments_expected.rb

@@ -1,8 +1,8 @@
 def foo
-  #a
-  #b
-  #c
-  #d
+  # a
+  # b
+  # c
+  # d
 end
 
 if a

fixtures/small/inline_comments_expected.rb

@@ -15,9 +15,9 @@ module MathsAndPhysics
   #   degree_codes:       The corresponding JAC codes to the subject(s)
   #                       the teacher completed their degree in.
   CONST = [
-    #maths
+    # maths
     a,
-    #physics
+    # physics
     b,
 
     # foreign language

fixtures/small/rspec_its_expected.rb

@@ -2,19 +2,19 @@
   "a" \
     "b"
 ) do
-  #hi
+  # hi
 end
 
 it "a" do
-  #hi
+  # hi
 end
 
 it "a", flag: true do
-  #hi
+  # hi
 end
 
 it "a", flag: true, other: "b", another: false do
-  #hi
+  # hi
 end
 
 it("a", flag: true, other: "b", another: false) { 1 }

fixtures/small/start_of_file_comments_expected.rb

@@ -1,5 +1,5 @@
 #!/usr/bin/env ruby
-#frozen_string_literal: true
+# frozen_string_literal: true
 
 def foo
   a

fixtures/small/word_array_trailing_comment_expected.rb

@@ -1,4 +1,4 @@
-#c
+# c
 a = %w[
   b
 ]

librubyfmt/src/comment_block.rs

@@ -12,6 +12,15 @@ pub struct CommentBlock {
     comments: Vec<Cow<'static, [u8]>>,
 }
 
+const SPECIAL_CHARS_AFTER_HASH_TO_IGNORE: [u8; 6] = [
+    b'!', // #! 'shebang' comments cannot be modified since they are Unix directives.
+    b'=', // #=== is a common delimiting pattern.
+    b'-', // #--- is a common delimiting pattern.
+    b':', // #: is used for RBS annotations.
+    b'|', // #| is used for RBS annotations.
+    b'*', // #** is used for doxygen comments.
+];
+
 impl CommentBlock {
     pub fn new(span: Range<LineNumber>, comments: Vec<Cow<'static, [u8]>>) -> Self {
         CommentBlock { span, comments }
@@ -54,6 +63,43 @@ impl CommentBlock {
         })
     }
 
+    pub fn enforce_at_least_one_space_after_comment_symbol(mut self) -> Self {
+        for comment in &mut self.comments {
+            // Ignore empty vecs -- these represent blank lines between
+            // groups of comments
+            if comment.is_empty() || comment.starts_with(b"=begin") {
+                continue;
+            }
+            if let Some(start_idx) = comment.iter().position(|&c| c == b'#') {
+                // Allow any amount of '#' after the leading '#'.
+                let collapse = comment[start_idx..]
+                    .iter()
+                    .take_while(|&&b| b == b'#')
+                    .count();
+
+                let anchor = start_idx + collapse;
+                if anchor == comment.len() {
+                    // No point truncating an empty comment.
+                    continue;
+                }
+
+                if comment[anchor].is_ascii_whitespace() {
+                    // Any amount of spaces after `#` is fine.
+                    continue;
+                }
+
+                let next_char = comment[anchor];
+                if SPECIAL_CHARS_AFTER_HASH_TO_IGNORE.contains(&next_char) {
+                    continue;
+                }
+
+                // Add one space
+                comment.to_mut().insert(anchor, b' ');
+            }
+        }
+        self
+    }
+
     /// Set each comment's leading indent to exactly `indent_depth` spaces
     pub fn apply_spaces(mut self, indent_depth: ColNumber) -> Self {
         let target = indent_depth as usize;

librubyfmt/src/parser_state.rs

@@ -382,8 +382,11 @@ impl<'src> ParserState<'src> {
     }
 
     pub(crate) fn insert_comment_collection(&mut self, comments: CommentBlock) {
-        self.comments_to_insert
-            .merge(comments.apply_spaces(self.spaces_after_last_newline));
+        self.comments_to_insert.merge(
+            comments
+                .enforce_at_least_one_space_after_comment_symbol()
+                .apply_spaces(self.spaces_after_last_newline),
+        );
     }
 
     pub(crate) fn emit_op(&mut self, op: &'src [u8]) {
@@ -890,6 +893,7 @@ impl<'src> ParserState<'src> {
                 self.on_line(1);
             }
             Some(comments) => {
+                let comments = comments.enforce_at_least_one_space_after_comment_symbol();
                 let line_count = comments.line_count();
                 for token in comments.into_line_tokens() {
                     self.push_concrete_token(token);