Optimize local linkchecker program

I noticed on a [recent build][1] that the linkchecker stage of CI took a
whopping 15 minutes of CI time for something that should be near
instantaneous. Some local profiling showed some very hot functions and
clones which were pretty easy to remove, and now instead of running in
minutes locally it runs in seconds.

[1]: https://ci.appveyor.com/project/rust-lang/rust/build/job/kptifw1kb1nm4xuu

src/tools/linkchecker/main.rs

@@ -24,12 +24,12 @@
 //! A few whitelisted exceptions are allowed as there's known bugs in rustdoc,
 //! but this should catch the majority of "broken link" cases.
 
+use std::collections::hash_map::Entry;
+use std::collections::{HashMap, HashSet};
 use std::env;
-use std::fs::File;
-use std::io::prelude::*;
+use std::fs;
 use std::path::{Path, PathBuf, Component};
-use std::collections::{HashMap, HashSet};
-use std::collections::hash_map::Entry;
+use std::rc::Rc;
 
 use Redirect::*;
 
@@ -63,7 +63,7 @@ enum Redirect {
 }
 
 struct FileEntry {
-    source: String,
+    source: Rc<String>,
     ids: HashSet<String>,
 }
 
@@ -113,7 +113,7 @@ fn walk(cache: &mut Cache, root: &Path, dir: &Path, errors: &mut bool) {
                 let entry = cache.get_mut(&pretty_path).unwrap();
                 // we don't need the source anymore,
                 // so drop to reduce memory-usage
-                entry.source = String::new();
+                entry.source = Rc::new(String::new());
             }
         }
     }
@@ -287,24 +287,24 @@ fn load_file(cache: &mut Cache,
              root: &Path,
              file: &Path,
              redirect: Redirect)
-             -> Result<(PathBuf, String), LoadError> {
-    let mut contents = String::new();
+             -> Result<(PathBuf, Rc<String>), LoadError> {
     let pretty_file = PathBuf::from(file.strip_prefix(root).unwrap_or(&file));
 
-    let maybe_redirect = match cache.entry(pretty_file.clone()) {
+    let (maybe_redirect, contents) = match cache.entry(pretty_file.clone()) {
         Entry::Occupied(entry) => {
-            contents = entry.get().source.clone();
-            None
+            (None, entry.get().source.clone())
         }
         Entry::Vacant(entry) => {
-            let mut fp = File::open(file).map_err(|err| {
-                if let FromRedirect(true) = redirect {
-                    LoadError::BrokenRedirect(file.to_path_buf(), err)
-                } else {
-                    LoadError::IOError(err)
+            let contents = match fs::read_to_string(file) {
+                Ok(s) => Rc::new(s),
+                Err(err) => {
+                    return Err(if let FromRedirect(true) = redirect {
+                        LoadError::BrokenRedirect(file.to_path_buf(), err)
+                    } else {
+                        LoadError::IOError(err)
+                    })
                 }
-            })?;
-            fp.read_to_string(&mut contents).map_err(|err| LoadError::IOError(err))?;
+            };
 
             let maybe = maybe_redirect(&contents);
             if maybe.is_some() {
@@ -317,7 +317,7 @@ fn load_file(cache: &mut Cache,
                     ids: HashSet::new(),
                 });
             }
-            maybe
+            (maybe, contents)
         }
     };
     match maybe_redirect.map(|url| file.parent().unwrap().join(url)) {