Switch from .init_array constructors to /proc/self/auxv.

Cargo.toml

@@ -100,7 +100,13 @@ targets = [
 ]
 
 [features]
-default = ["std"]
+
+# By default, use `std` and use libc for aux values.
+#
+# It turns out to be bizarrely awkward to obtain the aux values reliably and
+# efficiently on Linux from anywhere other than libc. We can do it, but most
+# users are better served by just using libc for this.
+default = ["std", "use-libc-auxv"]
 
 # This enables use of std. Disabling this enables `#![no_std], and requires
 # nightly Rust.
@@ -170,6 +176,11 @@ all-apis = [
     "time",
 ]
 
+# When using the linux_raw backend, and not using Mustang, should we use libc
+# for reading the aux vectors, instead of reading them ourselves from
+# /proc/self/auxv?
+use-libc-auxv = ["libc"]
+
 # Expose io-lifetimes' features for third-party crate impls.
 async-std = ["io-lifetimes/async-std"]
 tokio = ["io-lifetimes/tokio"]

src/backend/libc/param/auxv.rs

@@ -52,15 +52,3 @@ pub(crate) fn linux_execfn() -> &'static CStr {
         cstr!("")
     }
 }
-
-/// Initialize process-wide state.
-#[cfg(any(
-    target_vendor = "mustang",
-    not(any(target_env = "gnu", target_env = "musl")),
-))]
-#[inline]
-#[doc(hidden)]
-pub(crate) unsafe fn init(_envp: *mut *mut u8) {
-    // Nothing to do. This is the libc backend, and libc does the
-    // initialization for us.
-}

src/backend/linux_raw/elf.rs

@@ -1,6 +1,10 @@
 //! The ELF ABI.
 
 #![allow(non_snake_case)]
+#![cfg_attr(
+    all(not(target_vendor = "mustang"), feature = "use-libc-auxv"),
+    allow(dead_code)
+)]
 
 pub(super) const SELFMAG: usize = 4;
 pub(super) const ELFMAG: [u8; SELFMAG] = [0x7f, b'E', b'L', b'F'];