Make find_attr_val a little bit more precise

`find_attr_val(&line, "since")` returns `Some(", issue = ")` when
`line` is set to the following line:

```
[unstable(feature = "checked_duration_since", issue = "58402")]
```

Make `find_attr_val` use regex that is a little bit more
precise (requires `=` after key name).

It still does not handle all cases (e.g., extra leading chars in key
name, or escaped quotes in value), but is good enough for now.

Cargo.lock

@@ -3565,6 +3565,7 @@ dependencies = [
 name = "tidy"
 version = "0.1.0"
 dependencies = [
+ "regex 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 1.0.82 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 1.0.81 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_json 1.0.33 (registry+https://github.com/rust-lang/crates.io-index)",

src/tools/tidy/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.1.0"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 
 [dependencies]
+regex = "1"
 serde = "1.0.8"
 serde_derive = "1.0.8"
 serde_json = "1.0.2"

src/tools/tidy/src/features.rs

@@ -14,6 +14,8 @@ use std::fs::{self, File};
 use std::io::prelude::*;
 use std::path::Path;
 
+use regex::{Regex, escape};
+
 #[derive(Debug, PartialEq, Clone)]
 pub enum Status {
     Stable,
@@ -151,10 +153,19 @@ pub fn check(path: &Path, bad: &mut bool, quiet: bool) {
 }
 
 fn find_attr_val<'a>(line: &'a str, attr: &str) -> Option<&'a str> {
-    line.find(attr)
-        .and_then(|i| line[i..].find('"').map(|j| i + j + 1))
-        .and_then(|i| line[i..].find('"').map(|j| (i, i + j)))
-        .map(|(i, j)| &line[i..j])
+    let r = Regex::new(&format!(r#"{} *= *"([^"]*)""#, escape(attr)))
+        .expect("malformed regex for find_attr_val");
+    r.captures(line)
+        .and_then(|c| c.get(1))
+        .map(|m| m.as_str())
+}
+
+#[test]
+fn test_find_attr_val() {
+    let s = r#"#[unstable(feature = "checked_duration_since", issue = "58402")]"#;
+    assert_eq!(find_attr_val(s, "feature"), Some("checked_duration_since"));
+    assert_eq!(find_attr_val(s, "issue"), Some("58402"));
+    assert_eq!(find_attr_val(s, "since"), None);
 }
 
 fn test_filen_gate(filen_underscore: &str, features: &mut Features) -> bool {

src/tools/tidy/src/lib.rs

@@ -5,6 +5,7 @@
 
 #![deny(rust_2018_idioms)]
 
+extern crate regex;
 extern crate serde_json;
 #[macro_use]
 extern crate serde_derive;