add tool to "enforce" semantic line breaks

See rust-lang#1132

.gitignore

@@ -4,6 +4,7 @@ book
 book.toml
 
 ci/date-check/target/
+ci/semantic-line-breaks/target/
 
 # Generated by check-in.sh
 pulls.json

ci/semantic-line-breaks/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "semantic-line-breaks"
+version = "0.0.0"
+edition = "2021"
+
+[dependencies]
+anyhow = "1"
+ignore = "0.4"
+
+[dependencies.regex]
+version = "1"
+features = ["pattern"]

ci/semantic-line-breaks/src/main.rs

@@ -0,0 +1,121 @@
+use std::{env, fs, process};
+
+use anyhow::Result;
+use ignore::Walk;
+use regex::Regex;
+
+fn main() -> Result<()> {
+    let mut args = env::args();
+    if args.len() == 1 {
+        eprintln!("error: expected root Markdown directory as CLI argument");
+        process::exit(1);
+    }
+    let root_dir = args.nth(1).unwrap();
+    for result in Walk::new(root_dir) {
+        let entry = result?;
+        if entry.file_type().expect("no stdin").is_dir() {
+            continue;
+        }
+        let path = entry.path();
+        if let Some(extension) = path.extension() {
+            if extension != "md" {
+                continue;
+            }
+        } else {
+            continue;
+        }
+        let old = fs::read_to_string(path)?;
+        let new = comply(&old)?;
+        if new != old {
+            fs::write(path, new)?;
+        }
+    }
+    Ok(())
+}
+
+fn comply(content: &str) -> Result<String> {
+    let content: Vec<_> = content.lines().map(|line| line.to_owned()).collect();
+    let mut new_content = content.clone();
+    let mut new_n = 0;
+    let mut in_code_block = false;
+    let split_re = Regex::new(r"(\.|\?|;|!)\s+")?;
+    let ignore_re = Regex::new(r"(\d\.|\-|\*|r\?)\s+")?;
+    for (n, line) in content.iter().enumerate() {
+        if n != 0 {
+            new_n += 1;
+        }
+        if ignore_re.is_match(line) {
+            continue;
+        }
+        let line = line.trim_end();
+        if line.is_empty() {
+            continue;
+        }
+        // not eol
+        if line.contains("e.g.") {
+            continue;
+        }
+        // not eol
+        if line.contains("i.e.") {
+            continue;
+        }
+        // tables
+        if line.contains(" | ") {
+            continue;
+        }
+        // code blocks
+        if line.starts_with("```") {
+            if in_code_block {
+                in_code_block = false;
+            } else {
+                in_code_block = true;
+                continue;
+            }
+        }
+        if in_code_block {
+            continue;
+        }
+        if split_re.is_match(line) {
+            let indent = line.find(|ch: char| !ch.is_whitespace()).unwrap();
+            let new_lines: Vec<_> = line
+                .split_inclusive(&split_re)
+                .map(|portion| format!("{:indent$}{}", "", portion.trim()))
+                .collect();
+            new_content.splice(new_n..new_n + 1, new_lines.clone());
+            new_n += new_lines.len() - 1;
+        }
+    }
+    Ok(new_content.join("\n") + "\n")
+}
+
+#[test]
+fn test() {
+    let original = "\
+# some heading
+
+must! be; split? now.
+1. ignore numbered
+ignore | tables
+ignore e.g. and i.e. for realsies
+```
+some code. block
+```
+some more text.
+";
+    let reformatted = "\
+# some heading
+
+must!
+be;
+split?
+now.
+1. ignore numbered
+ignore | tables
+ignore e.g. and i.e. for realsies
+```
+some code. block
+```
+some more text.
+";
+    assert_eq!(comply(original).unwrap(), reformatted);
+}