tidy: some minor improvements.

- Tweak some comments.
- No need to do the `concat!` trick on `START_MARKER` because it's
  immediately followed by `END_MARKER`.
- Fix an off-by-one error in the line number for an error message.
- When a second start marker is found without an intervening end marker,
  after giving an error, treat it as though it ends the section. It's
  hard to know exactly what to do in this case, but it makes unit
  testing this case a little simpler (in the next commit).
- If an end marker occurs without a preceding start marker, issue an
  error.

src/tools/tidy/src/alphabetical.rs

@@ -14,11 +14,13 @@
 //! - Lines that are indented with more or less spaces than the first line
 //! - Lines starting with `//`, `#` (except those starting with `#!`), `)`, `]`, `}` if the comment
 //!   has the same indentation as the first line
+//! - Lines starting with a closing delimiter (`)`, `[`, `}`) are ignored.
 //!
-//! If a line ends with an opening bracket, the line is ignored and the next line will have
-//! its extra indentation ignored.
+//! If a line ends with an opening delimiter, we effectively join the following line to it before
+//! checking it. E.g. `foo(\nbar)` is treated like `foo(bar)`.
 
-use std::{fmt::Display, path::Path};
+use std::fmt::Display;
+use std::path::Path;
 
 use crate::walk::{filter_dirs, walk};
 
@@ -30,8 +32,7 @@ fn is_close_bracket(c: char) -> bool {
     matches!(c, ')' | ']' | '}')
 }
 
-// Don't let tidy check this here :D
-const START_MARKER: &str = concat!("tidy-alphabetical", "-start");
+const START_MARKER: &str = "tidy-alphabetical-start";
 const END_MARKER: &str = "tidy-alphabetical-end";
 
 fn check_section<'a>(
@@ -43,13 +44,14 @@ fn check_section<'a>(
     let mut first_indent = None;
     let mut in_split_line = None;
 
-    for (line_idx, line) in lines {
+    for (idx, line) in lines {
         if line.is_empty() {
             continue;
         }
 
         if line.contains(START_MARKER) {
-            tidy_error!(bad, "{file}:{} found `{START_MARKER}` expecting `{END_MARKER}`", line_idx)
+            tidy_error!(bad, "{file}:{} found `{START_MARKER}` expecting `{END_MARKER}`", idx + 1);
+            return;
         }
 
         if line.contains(END_MARKER) {
@@ -63,6 +65,7 @@ fn check_section<'a>(
         });
 
         let line = if let Some(prev_split_line) = in_split_line {
+            // Join the split lines.
             in_split_line = None;
             format!("{prev_split_line}{}", line.trim_start())
         } else {
@@ -90,7 +93,7 @@ fn check_section<'a>(
         let prev_line_trimmed_lowercase = prev_line.trim_start_matches(' ').to_lowercase();
 
         if trimmed_line.to_lowercase() < prev_line_trimmed_lowercase {
-            tidy_error!(bad, "{file}:{}: line not in alphabetical order", line_idx + 1,);
+            tidy_error!(bad, "{file}:{}: line not in alphabetical order", idx + 1);
         }
 
         prev_line = line;
@@ -104,7 +107,15 @@ pub fn check(path: &Path, bad: &mut bool) {
         let file = &entry.path().display();
 
         let mut lines = contents.lines().enumerate();
-        while let Some((_, line)) = lines.next() {
+        while let Some((idx, line)) = lines.next() {
+            if line.contains(END_MARKER) {
+                tidy_error!(
+                    bad,
+                    "{file}:{} found `{END_MARKER}` expecting `{START_MARKER}`",
+                    idx + 1
+                )
+            }
+
             if line.contains(START_MARKER) {
                 check_section(file, &mut lines, bad);
             }