Rollup merge of rust-lang#116888 - tbu-:pr_unsafe_env, r=Amanieu

Add discussion that concurrent access to the environment is unsafe

The bug report rust-lang#27970 has existed for 8 years, the actual bug dates back to Rust pre-1.0. I documented it since it's in the interest of the user to be aware of it. The note can be removed once rust-lang#27970 is fixed.

Author: matthiaskrgr

library/std/src/env.rs

@@ -313,17 +313,32 @@ impl Error for VarError {
 /// Sets the environment variable `key` to the value `value` for the currently running
 /// process.
 ///
-/// Note that while concurrent access to environment variables is safe in Rust,
-/// some platforms only expose inherently unsafe non-threadsafe APIs for
-/// inspecting the environment. As a result, extra care needs to be taken when
-/// auditing calls to unsafe external FFI functions to ensure that any external
-/// environment accesses are properly synchronized with accesses in Rust.
+/// # Safety
+///
+/// Even though this function is currently not marked as `unsafe`, it needs to
+/// be because invoking it can cause undefined behaviour. The function will be
+/// marked `unsafe` in a future version of Rust. This is tracked in
+/// [rust#27970](https://github.com/rust-lang/rust/issues/27970).
+///
+/// This function is safe to call in a single-threaded program.
+///
+/// In multi-threaded programs, you must ensure that are no other threads
+/// concurrently writing or *reading*(!) from the environment through functions
+/// other than the ones in this module. You are responsible for figuring out
+/// how to achieve this, but we strongly suggest not using `set_var` or
+/// `remove_var` in multi-threaded programs at all.
+///
+/// Most C libraries, including libc itself do not advertise which functions
+/// read from the environment. Even functions from the Rust standard library do
+/// that, e.g. for DNS lookups from [`std::net::ToSocketAddrs`].
 ///
 /// Discussion of this unsafety on Unix may be found in:
 ///
 ///  - [Austin Group Bugzilla](https://austingroupbugs.net/view.php?id=188)
 ///  - [GNU C library Bugzilla](https://sourceware.org/bugzilla/show_bug.cgi?id=15607#c2)
 ///
+/// [`std::net::ToSocketAddrs`]: crate::net::ToSocketAddrs
+///
 /// # Panics
 ///
 /// This function may panic if `key` is empty, contains an ASCII equals sign `'='`
@@ -351,17 +366,32 @@ fn _set_var(key: &OsStr, value: &OsStr) {
 
 /// Removes an environment variable from the environment of the currently running process.
 ///
-/// Note that while concurrent access to environment variables is safe in Rust,
-/// some platforms only expose inherently unsafe non-threadsafe APIs for
-/// inspecting the environment. As a result extra care needs to be taken when
-/// auditing calls to unsafe external FFI functions to ensure that any external
-/// environment accesses are properly synchronized with accesses in Rust.
+/// # Safety
+///
+/// Even though this function is currently not marked as `unsafe`, it needs to
+/// be because invoking it can cause undefined behaviour. The function will be
+/// marked `unsafe` in a future version of Rust. This is tracked in
+/// [rust#27970](https://github.com/rust-lang/rust/issues/27970).
+///
+/// This function is safe to call in a single-threaded program.
+///
+/// In multi-threaded programs, you must ensure that are no other threads
+/// concurrently writing or *reading*(!) from the environment through functions
+/// other than the ones in this module. You are responsible for figuring out
+/// how to achieve this, but we strongly suggest not using `set_var` or
+/// `remove_var` in multi-threaded programs at all.
+///
+/// Most C libraries, including libc itself do not advertise which functions
+/// read from the environment. Even functions from the Rust standard library do
+/// that, e.g. for DNS lookups from [`std::net::ToSocketAddrs`].
 ///
 /// Discussion of this unsafety on Unix may be found in:
 ///
 ///  - [Austin Group Bugzilla](https://austingroupbugs.net/view.php?id=188)
 ///  - [GNU C library Bugzilla](https://sourceware.org/bugzilla/show_bug.cgi?id=15607#c2)
 ///
+/// [`std::net::ToSocketAddrs`]: crate::net::ToSocketAddrs
+///
 /// # Panics
 ///
 /// This function may panic if `key` is empty, contains an ASCII equals sign