Rollup merge of rust-lang#96193 - djkoloski:fuchsia_current_exe, r=tmandry

[fuchsia] Add implementation for `current_exe`

This implementation returns a best attempt at the current exe path. On
fuchsia, fdio will always use `argv[0]` as the process name and if it is
not set then an error will be returned. Because this is not guaranteed
to be the case, this implementation returns an error if `argv` does not
contain any elements.

Author: Dylan-DPC

library/std/src/sys/unix/os.rs

@@ -427,7 +427,7 @@ pub fn current_exe() -> io::Result<PathBuf> {
     crate::fs::read_to_string("sys:exe").map(PathBuf::from)
 }
 
-#[cfg(any(target_os = "fuchsia", target_os = "l4re"))]
+#[cfg(target_os = "l4re")]
 pub fn current_exe() -> io::Result<PathBuf> {
     use crate::io::ErrorKind;
     Err(io::const_io_error!(ErrorKind::Unsupported, "Not yet implemented!"))
@@ -451,6 +451,26 @@ pub fn current_exe() -> io::Result<PathBuf> {
     super::unsupported::unsupported()
 }
 
+#[cfg(target_os = "fuchsia")]
+pub fn current_exe() -> io::Result<PathBuf> {
+    use crate::io::ErrorKind;
+
+    #[cfg(test)]
+    use realstd::env;
+
+    #[cfg(not(test))]
+    use crate::env;
+
+    let exe_path = env::args().next().ok_or(io::const_io_error!(
+        ErrorKind::Uncategorized,
+        "an executable path was not found because no arguments were provided through argv"
+    ))?;
+    let path = PathBuf::from(exe_path);
+
+    // Prepend the current working directory to the path if it's not absolute.
+    if !path.is_absolute() { getcwd().map(|cwd| cwd.join(path)) } else { Ok(path) }
+}
+
 pub struct Env {
     iter: vec::IntoIter<(OsString, OsString)>,
 }