drop confstr implementation

Added in libc 0.2.163.

The constants for _CS_PATH are not implemented for some of the BSDs yet
(rust-lang/cmake#3612), so we need to keep our linking of this via the C
compiler for now.

src/env/environment.rs

@@ -27,7 +27,7 @@ use crate::wcstringutil::join_strings;
 use crate::wutil::{fish_wcstol, wgetcwd, wgettext};
 use std::sync::atomic::Ordering;
 
-use libc::{c_int, uid_t, STDOUT_FILENO, _IONBF};
+use libc::{c_int, confstr, uid_t, STDOUT_FILENO, _IONBF};
 use once_cell::sync::OnceCell;
 use std::collections::HashMap;
 use std::ffi::CStr;
@@ -568,7 +568,7 @@ fn setup_user(vars: &EnvStack) {
 
 /// Make sure the PATH variable contains something.
 fn setup_path() {
-    use crate::libc::{confstr, _CS_PATH};
+    use crate::libc::_CS_PATH;
 
     let vars = EnvStack::globals();
     let path = vars.get_unless_empty(L!("PATH"));

src/libc.c

@@ -19,21 +19,6 @@ uint64_t C_ST_LOCAL() {
 #endif
 }
 
-// confstr + _CS_PATH is only available on macOS with rust's libc
-// we could just declare extern "C" confstr directly in Rust
-// that would panic if it failed to link, which C++ did not
-// therefore we define a backup, which just returns an error
-// which for confstr is 0
-#if defined(_CS_PATH)
-#else
-size_t confstr(int name, char* buf, size_t size) {
-    UNUSED(name);
-    UNUSED(buf);
-    UNUSED(size);
-    return 0;
-}
-#endif
-
 int C_CS_PATH() {
 #if defined(_CS_PATH)
     return _CS_PATH;

src/libc.rs

@@ -14,11 +14,6 @@ extern "C" {
 }
 
 extern "C" {
-    pub(crate) fn confstr(
-        name: libc::c_int,
-        buf: *mut libc::c_char,
-        len: libc::size_t,
-    ) -> libc::size_t;
     pub fn stdout_stream() -> *mut libc::FILE;
     pub fn setlinebuf(stream: *mut libc::FILE);
 }