Use str::strip* in bootstrap

This commit replaces the use of `trim_start_matches`
because in `rustc -Vv` output there are no lines
starting with multiple "release:".

src/bootstrap/compile.rs

@@ -963,10 +963,11 @@ pub fn run_cargo(
         .collect::<Vec<_>>();
     for (prefix, extension, expected_len) in toplevel {
         let candidates = contents.iter().filter(|&&(_, ref filename, ref meta)| {
-            filename.starts_with(&prefix[..])
-                && filename[prefix.len()..].starts_with('-')
-                && filename.ends_with(&extension[..])
-                && meta.len() == expected_len
+            meta.len() == expected_len
+                && filename
+                    .strip_prefix(&prefix[..])
+                    .map(|s| s.starts_with('-') && s.ends_with(&extension[..]))
+                    .unwrap_or(false)
         });
         let max = candidates
             .max_by_key(|&&(_, _, ref metadata)| FileTime::from_last_modification_time(metadata));

src/bootstrap/lib.rs

@@ -436,10 +436,9 @@ pub fn new(config: Config) -> Build {
             output(Command::new(&build.initial_rustc).arg("--version").arg("--verbose"));
         let local_release = local_version_verbose
             .lines()
-            .filter(|x| x.starts_with("release:"))
+            .filter_map(|x| x.strip_prefix("release:"))
             .next()
             .unwrap()
-            .trim_start_matches("release:")
             .trim();
         let my_version = channel::CFG_RELEASE_NUM;
         if local_release.split('.').take(2).eq(my_version.split('.').take(2)) {
@@ -1089,10 +1088,10 @@ fn release_num(&self, package: &str) -> String {
         let toml_file_name = self.src.join(&format!("src/tools/{}/Cargo.toml", package));
         let toml = t!(fs::read_to_string(&toml_file_name));
         for line in toml.lines() {
-            let prefix = "version = \"";
-            let suffix = "\"";
-            if line.starts_with(prefix) && line.ends_with(suffix) {
-                return line[prefix.len()..line.len() - suffix.len()].to_string();
+            if let Some(stripped) =
+                line.strip_prefix("version = \"").and_then(|s| s.strip_suffix("\""))
+            {
+                return stripped.to_owned();
             }
         }