Optimize away some fs::metadata calls.

This also eliminates a use of a `Path` convenience function, in support
of #80741, refactoring `std::path` to focus on pure data structures and
algorithms.

compiler/rustc_incremental/src/persist/file_format.rs

@@ -52,11 +52,11 @@ pub fn read_file(
     path: &Path,
     nightly_build: bool,
 ) -> io::Result<Option<(Vec<u8>, usize)>> {
-    if !path.exists() {
-        return Ok(None);
-    }
-
-    let data = fs::read(path)?;
+    let data = match fs::read(path) {
+        Ok(data) => data,
+        Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(None),
+        Err(err) => return Err(err),
+    };
 
     let mut file = io::Cursor::new(data);
 

compiler/rustc_incremental/src/persist/fs.rs

@@ -922,22 +922,24 @@ fn all_except_most_recent(
 /// before passing it to std::fs::remove_dir_all(). This will convert the path
 /// into the '\\?\' format, which supports much longer paths.
 fn safe_remove_dir_all(p: &Path) -> io::Result<()> {
-    if p.exists() {
-        let canonicalized = p.canonicalize()?;
+    let canonicalized = match std_fs::canonicalize(p) {
+        Ok(canonicalized) => canonicalized,
+        Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(()),
+        Err(err) => return Err(err),
+    };
+
     std_fs::remove_dir_all(canonicalized)
-    } else {
-        Ok(())
-    }
 }
 
 fn safe_remove_file(p: &Path) -> io::Result<()> {
-    if p.exists() {
-        let canonicalized = p.canonicalize()?;
+    let canonicalized = match std_fs::canonicalize(p) {
+        Ok(canonicalized) => canonicalized,
+        Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(()),
+        Err(err) => return Err(err),
+    };
+
     match std_fs::remove_file(canonicalized) {
-            Err(ref err) if err.kind() == io::ErrorKind::NotFound => Ok(()),
+        Err(err) if err.kind() == io::ErrorKind::NotFound => Ok(()),
         result => result,
     }
-    } else {
-        Ok(())
-    }
 }

compiler/rustc_incremental/src/persist/save.rs

@@ -6,6 +6,7 @@ use rustc_serialize::opaque::Encoder;
 use rustc_serialize::Encodable as RustcEncodable;
 use rustc_session::Session;
 use std::fs;
+use std::io;
 use std::path::PathBuf;
 
 use super::data::*;
@@ -101,11 +102,11 @@ where
     // Note: It's important that we actually delete the old file and not just
     // truncate and overwrite it, since it might be a shared hard-link, the
     // underlying data of which we don't want to modify
-    if path_buf.exists() {
     match fs::remove_file(&path_buf) {
         Ok(()) => {
             debug!("save: remove old file");
         }
+        Err(err) if err.kind() == io::ErrorKind::NotFound => (),
         Err(err) => {
             sess.err(&format!(
                 "unable to delete old dep-graph at `{}`: {}",
@@ -115,7 +116,6 @@ where
             return;
         }
     }
-    }
 
     // generate the data in a memory buffer
     let mut encoder = Encoder::new(Vec::new());