Rollup merge of rust-lang#56525 - udoprog:linux-current-exe, r=alexcr…

…ichton

Avoid extra copy and syscall in std::env::current_exe

src/libstd/sys/unix/os.rs

@@ -283,11 +283,14 @@ pub fn current_exe() -> io::Result<PathBuf> {
 
 #[cfg(any(target_os = "linux", target_os = "android", target_os = "emscripten"))]
 pub fn current_exe() -> io::Result<PathBuf> {
-    let selfexe = PathBuf::from("/proc/self/exe");
-    if selfexe.exists() {
-        ::fs::read_link(selfexe)
-    } else {
-        Err(io::Error::new(io::ErrorKind::Other, "no /proc/self/exe available. Is /proc mounted?"))
+    match ::fs::read_link("/proc/self/exe") {
+        Err(ref e) if e.kind() == io::ErrorKind::NotFound => {
+            Err(io::Error::new(
+                io::ErrorKind::Other,
+                "no /proc/self/exe available. Is /proc mounted?"
+            ))
+        },
+        other => other,
     }
 }