Auto merge of #28256 - petrochenkov:conv, r=alexcrichton

This patch transforms functions of the form
```
fn f<Generic: AsRef<Concrete>>(arg: Generic) {
	let arg: &Concrete = arg.as_ref();
	// Code using arg
}
```
to the next form:
```
#[inline]
fn f<Generic: AsRef<Concrete>>(arg: Generic) {
	fn f_inner(arg: &Concrete) {
		// Code using arg
	}
	
	f_inner(arg.as_ref());
}
```

Therefore, most of the code is concrete and not duplicated during monomorphisation (unless inlined)
and only the tiny bit of conversion code is duplicated. This method was mentioned by @aturon in the
Conversion Traits RFC (https://github.com/rust-lang/rfcs/blame/master/text/0529-conversion-traits.md#L249) and similar techniques are not uncommon in C++ template libraries.

This patch goes to the extremes and applies the transformation even to smaller functions<sup>1</sup>
for purity of the experiment. *Some of them can be rolled back* if considered too ridiculous.

<sup>1</sup> However who knows how small are these functions are after inlining and everything.

The functions in question are mostly `fs`/`os` functions and not used especially often with variety
of argument types, so the code size reduction is rather small (but consistent). Here are the sizes
of stage2 artifacts before and after the patch:
https://gist.github.com/petrochenkov/e76a6b280f382da13c5d
https://gist.github.com/petrochenkov/6cc28727d5256dbdfed0

Note:
All the `inner` functions are concrete and unavailable for cross-crate inlining, some of them may
need `#[inline]` annotations in the future.

r? @aturon

7 files changed, 101 insertions, 28 deletions

diff --git a/src/librustc_back/tempdir.rs b/src/librustc_back/tempdir.rs
index 077a0feebd4..04739c7418c 100644
--- a/src/librustc_back/tempdir.rs
+++ b/src/librustc_back/tempdir.rs
@@ -38,8 +38,12 @@ impl TempDir {
     #[allow(deprecated)] // rand usage
     pub fn new_in<P: AsRef<Path>>(tmpdir: P, prefix: &str)
                                   -> io::Result<TempDir> {
+        Self::_new_in(tmpdir.as_ref(), prefix)
+    }
+
+    fn _new_in(tmpdir: &Path, prefix: &str) -> io::Result<TempDir> {
         let storage;
-        let mut tmpdir = tmpdir.as_ref();
+        let mut tmpdir = tmpdir;
         if !tmpdir.is_absolute() {
             let cur_dir = try!(env::current_dir());
             storage = cur_dir.join(tmpdir);
diff --git a/src/libstd/env.rs b/src/libstd/env.rs
index d80a222d8d2..3447d1683a8 100644
--- a/src/libstd/env.rs
+++ b/src/libstd/env.rs
@@ -174,6 +174,10 @@ impl Iterator for VarsOs {
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
 pub fn var<K: AsRef<OsStr>>(key: K) -> Result<String, VarError> {
+    _var(key.as_ref())
+}
+
+fn _var(key: &OsStr) -> Result<String, VarError> {
     match var_os(key) {
         Some(s) => s.into_string().map_err(VarError::NotUnicode),
         None => Err(VarError::NotPresent)
@@ -196,8 +200,12 @@ pub fn var<K: AsRef<OsStr>>(key: K) -> Result<String, VarError> {
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
 pub fn var_os<K: AsRef<OsStr>>(key: K) -> Option<OsString> {
+    _var_os(key.as_ref())
+}
+
+fn _var_os(key: &OsStr) -> Option<OsString> {
     let _g = ENV_LOCK.lock();
-    os_imp::getenv(key.as_ref())
+    os_imp::getenv(key)
 }
 
 /// Possible errors from the `env::var` method.
@@ -263,8 +271,12 @@ impl Error for VarError {
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
 pub fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(k: K, v: V) {
+    _set_var(k.as_ref(), v.as_ref())
+}
+
+fn _set_var(k: &OsStr, v: &OsStr) {
     let _g = ENV_LOCK.lock();
-    os_imp::setenv(k.as_ref(), v.as_ref())
+    os_imp::setenv(k, v)
 }
 
 /// Removes an environment variable from the environment of the currently running process.
@@ -294,8 +306,12 @@ pub fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(k: K, v: V) {
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
 pub fn remove_var<K: AsRef<OsStr>>(k: K) {
+    _remove_var(k.as_ref())
+}
+
+fn _remove_var(k: &OsStr) {
     let _g = ENV_LOCK.lock();
-    os_imp::unsetenv(k.as_ref())
+    os_imp::unsetenv(k)
 }
 
 /// An iterator over `Path` instances for parsing an environment variable
diff --git a/src/libstd/ffi/c_str.rs b/src/libstd/ffi/c_str.rs
index 3a1853a7a6c..7c9c2066ae9 100644
--- a/src/libstd/ffi/c_str.rs
+++ b/src/libstd/ffi/c_str.rs
@@ -181,7 +181,10 @@ impl CString {
     /// the position of the nul byte.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new<T: Into<Vec<u8>>>(t: T) -> Result<CString, NulError> {
-        let bytes = t.into();
+        Self::_new(t.into())
+    }
+
+    fn _new(bytes: Vec<u8>) -> Result<CString, NulError> {
         match bytes.iter().position(|x| *x == 0) {
             Some(i) => Err(NulError(i, bytes)),
             None => Ok(unsafe { CString::from_vec_unchecked(bytes) }),
diff --git a/src/libstd/ffi/os_str.rs b/src/libstd/ffi/os_str.rs
index 751c76b9960..7409d9b45d2 100644
--- a/src/libstd/ffi/os_str.rs
+++ b/src/libstd/ffi/os_str.rs
@@ -73,18 +73,18 @@ impl OsString {
     /// convert; non UTF-8 data will produce `None`.
     #[unstable(feature = "convert", reason = "recently added", issue = "27704")]
     pub fn from_bytes<B>(bytes: B) -> Option<OsString> where B: Into<Vec<u8>> {
-        #[cfg(unix)]
-        fn from_bytes_inner(vec: Vec<u8>) -> Option<OsString> {
-            use os::unix::ffi::OsStringExt;
-            Some(OsString::from_vec(vec))
-        }
+        Self::_from_bytes(bytes.into())
+    }
 
-        #[cfg(windows)]
-        fn from_bytes_inner(vec: Vec<u8>) -> Option<OsString> {
-            String::from_utf8(vec).ok().map(OsString::from)
-        }
+    #[cfg(unix)]
+    fn _from_bytes(vec: Vec<u8>) -> Option<OsString> {
+        use os::unix::ffi::OsStringExt;
+        Some(OsString::from_vec(vec))
+    }
 
-        from_bytes_inner(bytes.into())
+    #[cfg(windows)]
+    fn _from_bytes(vec: Vec<u8>) -> Option<OsString> {
+        String::from_utf8(vec).ok().map(OsString::from)
     }
 
     /// Converts to an `OsStr` slice.
diff --git a/src/libstd/fs.rs b/src/libstd/fs.rs
index 0014391670a..a3d509ba0f1 100644
--- a/src/libstd/fs.rs
+++ b/src/libstd/fs.rs
@@ -184,7 +184,7 @@ impl File {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn open<P: AsRef<Path>>(path: P) -> io::Result<File> {
-        OpenOptions::new().read(true).open(path)
+        OpenOptions::new().read(true).open(path.as_ref())
     }
 
     /// Opens a file in write-only mode.
@@ -206,7 +206,7 @@ impl File {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn create<P: AsRef<Path>>(path: P) -> io::Result<File> {
-        OpenOptions::new().write(true).create(true).truncate(true).open(path)
+        OpenOptions::new().write(true).create(true).truncate(true).open(path.as_ref())
     }
 
     /// Attempts to sync all OS-internal metadata to disk.
@@ -494,7 +494,10 @@ impl OpenOptions {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn open<P: AsRef<Path>>(&self, path: P) -> io::Result<File> {
-        let path = path.as_ref();
+        self._open(path.as_ref())
+    }
+
+    fn _open(&self, path: &Path) -> io::Result<File> {
         let inner = try!(fs_imp::File::open(path, &self.0));
         Ok(File { inner: inner })
     }
@@ -1048,7 +1051,10 @@ pub fn remove_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn remove_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
-    let path = path.as_ref();
+    _remove_dir_all(path.as_ref())
+}
+
+fn _remove_dir_all(path: &Path) -> io::Result<()> {
     for child in try!(read_dir(path)) {
         let child = try!(child).path();
         let stat = try!(symlink_metadata(&*child));
@@ -1113,6 +1119,10 @@ pub fn read_dir<P: AsRef<Path>>(path: P) -> io::Result<ReadDir> {
                      as symlinks differently",
            issue = "27707")]
 pub fn walk_dir<P: AsRef<Path>>(path: P) -> io::Result<WalkDir> {
+    _walk_dir(path.as_ref())
+}
+
+fn _walk_dir(path: &Path) -> io::Result<WalkDir> {
     let start = try!(read_dir(path));
     Ok(WalkDir { cur: Some(start), stack: Vec::new() })
 }
@@ -1272,7 +1282,10 @@ impl DirBuilder {
     /// Create the specified directory with the options configured in this
     /// builder.
     pub fn create<P: AsRef<Path>>(&self, path: P) -> io::Result<()> {
-        let path = path.as_ref();
+        self._create(path.as_ref())
+    }
+
+    fn _create(&self, path: &Path) -> io::Result<()> {
         if self.recursive {
             self.create_dir_all(path)
         } else {
diff --git a/src/libstd/io/error.rs b/src/libstd/io/error.rs
index 576d9b92156..773f9ac6a12 100644
--- a/src/libstd/io/error.rs
+++ b/src/libstd/io/error.rs
@@ -191,10 +191,14 @@ impl Error {
     pub fn new<E>(kind: ErrorKind, error: E) -> Error
         where E: Into<Box<error::Error+Send+Sync>>
     {
+        Self::_new(kind, error.into())
+    }
+
+    fn _new(kind: ErrorKind, error: Box<error::Error+Send+Sync>) -> Error {
         Error {
             repr: Repr::Custom(Box::new(Custom {
                 kind: kind,
-                error: error.into(),
+                error: error,
             }))
         }
     }
diff --git a/src/libstd/path.rs b/src/libstd/path.rs
index 66893ffd330..8eb5d1f2726 100644
--- a/src/libstd/path.rs
+++ b/src/libstd/path.rs
@@ -965,8 +965,10 @@ impl PathBuf {
     /// * if `path` has a prefix but no root, it replaces `self`.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn push<P: AsRef<Path>>(&mut self, path: P) {
-        let path = path.as_ref();
+        self._push(path.as_ref())
+    }
 
+    fn _push(&mut self, path: &Path) {
         // in general, a separator is needed if the rightmost byte is not a separator
         let mut need_sep = self.as_mut_vec().last().map(|c| !is_sep_byte(*c)).unwrap_or(false);
 
@@ -1033,11 +1035,15 @@ impl PathBuf {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn set_file_name<S: AsRef<OsStr>>(&mut self, file_name: S) {
+        self._set_file_name(file_name.as_ref())
+    }
+
+    fn _set_file_name(&mut self, file_name: &OsStr) {
         if self.file_name().is_some() {
             let popped = self.pop();
             debug_assert!(popped);
         }
-        self.push(file_name.as_ref());
+        self.push(file_name);
     }
 
     /// Updates `self.extension()` to `extension`.
@@ -1048,6 +1054,10 @@ impl PathBuf {
     /// is added; otherwise it is replaced.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn set_extension<S: AsRef<OsStr>>(&mut self, extension: S) -> bool {
+        self._set_extension(extension.as_ref())
+    }
+
+    fn _set_extension(&mut self, extension: &OsStr) -> bool {
         if self.file_name().is_none() { return false; }
 
         let mut stem = match self.file_stem() {
@@ -1055,7 +1065,6 @@ impl PathBuf {
             None => OsString::new(),
         };
 
-        let extension = extension.as_ref();
         if !os_str_as_u8_slice(extension).is_empty() {
             stem.push(".");
             stem.push(extension);
@@ -1106,7 +1115,7 @@ impl<P: AsRef<Path>> iter::FromIterator<P> for PathBuf {
 impl<P: AsRef<Path>> iter::Extend<P> for PathBuf {
     fn extend<I: IntoIterator<Item = P>>(&mut self, iter: I) {
         for p in iter {
-            self.push(p)
+            self.push(p.as_ref())
         }
     }
 }
@@ -1452,7 +1461,11 @@ impl Path {
                issue = "23284")]
     pub fn relative_from<'a, P: ?Sized + AsRef<Path>>(&'a self, base: &'a P) -> Option<&Path>
     {
-        iter_after(self.components(), base.as_ref().components()).map(|c| c.as_path())
+        self._relative_from(base.as_ref())
+    }
+
+    fn _relative_from<'a>(&'a self, base: &'a Path) -> Option<&'a Path> {
+        iter_after(self.components(), base.components()).map(|c| c.as_path())
     }
 
     /// Determines whether `base` is a prefix of `self`.
@@ -1472,7 +1485,11 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn starts_with<P: AsRef<Path>>(&self, base: P) -> bool {
-        iter_after(self.components(), base.as_ref().components()).is_some()
+        self._starts_with(base.as_ref())
+    }
+
+    fn _starts_with(&self, base: &Path) -> bool {
+        iter_after(self.components(), base.components()).is_some()
     }
 
     /// Determines whether `child` is a suffix of `self`.
@@ -1490,7 +1507,11 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn ends_with<P: AsRef<Path>>(&self, child: P) -> bool {
-        iter_after(self.components().rev(), child.as_ref().components().rev()).is_some()
+        self._ends_with(child.as_ref())
+    }
+
+    fn _ends_with(&self, child: &Path) -> bool {
+        iter_after(self.components().rev(), child.components().rev()).is_some()
     }
 
     /// Extracts the stem (non-extension) portion of `self.file_name()`.
@@ -1552,6 +1573,10 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn join<P: AsRef<Path>>(&self, path: P) -> PathBuf {
+        self._join(path.as_ref())
+    }
+
+    fn _join(&self, path: &Path) -> PathBuf {
         let mut buf = self.to_path_buf();
         buf.push(path);
         buf
@@ -1571,6 +1596,10 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn with_file_name<S: AsRef<OsStr>>(&self, file_name: S) -> PathBuf {
+        self._with_file_name(file_name.as_ref())
+    }
+
+    fn _with_file_name(&self, file_name: &OsStr) -> PathBuf {
         let mut buf = self.to_path_buf();
         buf.set_file_name(file_name);
         buf
@@ -1590,6 +1619,10 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn with_extension<S: AsRef<OsStr>>(&self, extension: S) -> PathBuf {
+        self._with_extension(extension.as_ref())
+    }
+
+    fn _with_extension(&self, extension: &OsStr) -> PathBuf {
         let mut buf = self.to_path_buf();
         buf.set_extension(extension);
         buf