Parse the version and date from verbose info

The plain `rustc --version` string is not well structured, and in
particular the git commit info is not necessarily present when rustc was
built out of tree, like distro builds. Furthermore, rust-lang/rust#79115
made it possible to have completely custom information in that version's
parenthesized block, which may not look like git info at all.

Adding `--verbose` outputs each field on its own line, in particular
"release: ..." for the version number and "commit-date: ..." for the git
info, although the latter is just "unknown" for out-of-tree builds. This
still works all the way back to Rust 1.0.0.

src/lib.rs

@@ -85,19 +85,24 @@ use std::process::Command;
 
 /// Parses (version, date) as available from rustc version string.
 fn version_and_date_from_rustc_version(s: &str) -> (Option<String>, Option<String>) {
-    let last_line = s.lines().last().unwrap_or(s);
-    let mut components = last_line.trim().split(" ");
-    let version = components.nth(1);
-    let date = components.filter(|c| c.ends_with(')')).next()
-        .map(|s| s.trim_right().trim_right_matches(")").trim_left().trim_left_matches('('));
-    (version.map(|s| s.to_string()), date.map(|s| s.to_string()))
+    let split = |s: &str| s.splitn(2, ": ").nth(1).map(str::to_string);
+    let mut version = None;
+    let mut date = None;
+    for line in s.lines() {
+        if line.starts_with("release:") {
+            version = split(line);
+        }
+        if line.starts_with("commit-date:") {
+            date = split(line);
+        }
+    }
+    (version, date)
 }
 
 /// Returns (version, date) as available from `rustc --version`.
 fn get_version_and_date() -> Option<(Option<String>, Option<String>)> {
-    env::var("RUSTC").ok()
-        .and_then(|rustc| Command::new(rustc).arg("--version").output().ok())
-        .or_else(|| Command::new("rustc").arg("--version").output().ok())
+    let rustc = env::var("RUSTC").unwrap_or_else(|_| "rustc".to_string());
+    Command::new(rustc).arg("--version").arg("--verbose").output().ok()
         .and_then(|output| String::from_utf8(output.stdout).ok())
         .map(|s| version_and_date_from_rustc_version(&s))
 }