Make std::env::{set_var, remove_var} unsafe in edition 2024

Allow calling these functions without `unsafe` blocks in editions up
until 2021, but don't trigger the `unused_unsafe` lint for `unsafe`
blocks containing these functions.

Fixes #27970.
Fixes #90308.
CC #124866.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -578,6 +578,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "rustc_allowed_through_unstable_modules special cases accidental stabilizations of stable items \
         through unstable paths"
     ),
+    rustc_attr!(
+        rustc_deprecated_safe_2024, Normal, template!(Word), WarnFollowing,
+        EncodeCrossCrate::Yes,
+        "rustc_deprecated_safe_2024 is supposed to be used in libstd only",
+    ),
+
 
     // ==========================================================================
     // Internal attributes: Type system related:

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -110,14 +110,19 @@ impl<'tcx> UnsafetyVisitor<'_, 'tcx> {
                 );
                 self.suggest_unsafe_block = false;
             }
-            SafetyContext::Safe => {
-                kind.emit_requires_unsafe_err(
+            SafetyContext::Safe => match kind {
+                // Allow calls to deprecated-safe unsafe functions if the
+                // caller is from an edition before 2024.
+                UnsafeOpKind::CallToUnsafeFunction(Some(id))
+                    if !span.at_least_rust_2024()
+                        && self.tcx.has_attr(id, sym::rustc_deprecated_safe_2024) => {}
+                _ => kind.emit_requires_unsafe_err(
                     self.tcx,
                     span,
                     self.hir_context,
                     unsafe_op_in_unsafe_fn_allowed,
-                );
-            }
+                ),
+            },
         }
     }
 

compiler/rustc_span/src/symbol.rs

@@ -1575,6 +1575,7 @@ symbols! {
         rustc_def_path,
         rustc_default_body_unstable,
         rustc_deny_explicit_impl,
+        rustc_deprecated_safe_2024,
         rustc_diagnostic_item,
         rustc_diagnostic_macros,
         rustc_dirty,

library/std/src/env.rs

@@ -318,11 +318,6 @@ impl Error for VarError {
 ///
 /// # 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
@@ -331,7 +326,7 @@ impl Error for VarError {
 /// 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
+/// 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`].
 ///
@@ -353,15 +348,26 @@ impl Error for VarError {
 /// use std::env;
 ///
 /// let key = "KEY";
-/// env::set_var(key, "VALUE");
+/// unsafe {
+///     env::set_var(key, "VALUE");
+/// }
 /// assert_eq!(env::var(key), Ok("VALUE".to_string()));
 /// ```
+#[cfg(not(bootstrap))]
+#[rustc_deprecated_safe_2024]
 #[stable(feature = "env", since = "1.0.0")]
-pub fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(key: K, value: V) {
+pub unsafe fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(key: K, value: V) {
     _set_var(key.as_ref(), value.as_ref())
 }
 
-fn _set_var(key: &OsStr, value: &OsStr) {
+#[cfg(bootstrap)]
+#[allow(missing_docs)]
+#[stable(feature = "env", since = "1.0.0")]
+pub fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(key: K, value: V) {
+    unsafe { _set_var(key.as_ref(), value.as_ref()) }
+}
+
+unsafe fn _set_var(key: &OsStr, value: &OsStr) {
     os_imp::setenv(key, value).unwrap_or_else(|e| {
         panic!("failed to set environment variable `{key:?}` to `{value:?}`: {e}")
     })
@@ -371,11 +377,6 @@ fn _set_var(key: &OsStr, value: &OsStr) {
 ///
 /// # 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
@@ -384,7 +385,7 @@ fn _set_var(key: &OsStr, value: &OsStr) {
 /// 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
+/// 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`].
 ///
@@ -403,22 +404,35 @@ fn _set_var(key: &OsStr, value: &OsStr) {
 ///
 /// # Examples
 ///
-/// ```
+/// ```no_run
 /// use std::env;
 ///
 /// let key = "KEY";
-/// env::set_var(key, "VALUE");
+/// unsafe {
+///     env::set_var(key, "VALUE");
+/// }
 /// assert_eq!(env::var(key), Ok("VALUE".to_string()));
 ///
-/// env::remove_var(key);
+/// unsafe {
+///     env::remove_var(key);
+/// }
 /// assert!(env::var(key).is_err());
 /// ```
+#[cfg(not(bootstrap))]
+#[rustc_deprecated_safe_2024]
 #[stable(feature = "env", since = "1.0.0")]
-pub fn remove_var<K: AsRef<OsStr>>(key: K) {
+pub unsafe fn remove_var<K: AsRef<OsStr>>(key: K) {
     _remove_var(key.as_ref())
 }
 
-fn _remove_var(key: &OsStr) {
+#[cfg(bootstrap)]
+#[allow(missing_docs)]
+#[stable(feature = "env", since = "1.0.0")]
+pub fn remove_var<K: AsRef<OsStr>>(key: K) {
+    unsafe { _remove_var(key.as_ref()) }
+}
+
+unsafe fn _remove_var(key: &OsStr) {
     os_imp::unsetenv(key)
         .unwrap_or_else(|e| panic!("failed to remove environment variable `{key:?}`: {e}"))
 }

library/std/src/sys/pal/hermit/os.rs

@@ -172,18 +172,14 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     unsafe { ENV.as_ref().unwrap().lock().unwrap().get_mut(k).cloned() }
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
-    unsafe {
-        let (k, v) = (k.to_owned(), v.to_owned());
-        ENV.as_ref().unwrap().lock().unwrap().insert(k, v);
-    }
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+    let (k, v) = (k.to_owned(), v.to_owned());
+    ENV.as_ref().unwrap().lock().unwrap().insert(k, v);
     Ok(())
 }
 
-pub fn unsetenv(k: &OsStr) -> io::Result<()> {
-    unsafe {
-        ENV.as_ref().unwrap().lock().unwrap().remove(k);
-    }
+pub unsafe fn unsetenv(k: &OsStr) -> io::Result<()> {
+    ENV.as_ref().unwrap().lock().unwrap().remove(k);
     Ok(())
 }
 

library/std/src/sys/pal/sgx/os.rs

@@ -157,13 +157,13 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     get_env_store().and_then(|s| s.lock().unwrap().get(k).cloned())
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     let (k, v) = (k.to_owned(), v.to_owned());
     create_env_store().lock().unwrap().insert(k, v);
     Ok(())
 }
 
-pub fn unsetenv(k: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(k: &OsStr) -> io::Result<()> {
     if let Some(env) = get_env_store() {
         env.lock().unwrap().remove(k);
     }

library/std/src/sys/pal/solid/os.rs

@@ -191,7 +191,7 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     .flatten()
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     run_with_cstr(k.as_bytes(), &|k| {
         run_with_cstr(v.as_bytes(), &|v| {
             let _guard = ENV_LOCK.write();
@@ -200,7 +200,7 @@ pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     })
 }
 
-pub fn unsetenv(n: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(n: &OsStr) -> io::Result<()> {
     run_with_cstr(n.as_bytes(), &|nbuf| {
         let _guard = ENV_LOCK.write();
         cvt_env(unsafe { libc::unsetenv(nbuf.as_ptr()) }).map(drop)

library/std/src/sys/pal/teeos/os.rs

@@ -109,11 +109,11 @@ pub fn getenv(_: &OsStr) -> Option<OsString> {
     None
 }
 
-pub fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
     Err(io::Error::new(io::ErrorKind::Unsupported, "cannot set env vars on this platform"))
 }
 
-pub fn unsetenv(_: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(_: &OsStr) -> io::Result<()> {
     Err(io::Error::new(io::ErrorKind::Unsupported, "cannot unset env vars on this platform"))
 }
 

library/std/src/sys/pal/uefi/os.rs

@@ -203,11 +203,11 @@ pub fn getenv(_: &OsStr) -> Option<OsString> {
     None
 }
 
-pub fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
     Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot set env vars on this platform"))
 }
 
-pub fn unsetenv(_: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(_: &OsStr) -> io::Result<()> {
     Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot unset env vars on this platform"))
 }
 

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

@@ -675,19 +675,19 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     .flatten()
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     run_with_cstr(k.as_bytes(), &|k| {
         run_with_cstr(v.as_bytes(), &|v| {
             let _guard = ENV_LOCK.write();
-            cvt(unsafe { libc::setenv(k.as_ptr(), v.as_ptr(), 1) }).map(drop)
+            cvt(libc::setenv(k.as_ptr(), v.as_ptr(), 1)).map(drop)
         })
     })
 }
 
-pub fn unsetenv(n: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(n: &OsStr) -> io::Result<()> {
     run_with_cstr(n.as_bytes(), &|nbuf| {
         let _guard = ENV_LOCK.write();
-        cvt(unsafe { libc::unsetenv(nbuf.as_ptr()) }).map(drop)
+        cvt(libc::unsetenv(nbuf.as_ptr())).map(drop)
     })
 }
 

library/std/src/sys/pal/unsupported/os.rs

@@ -96,11 +96,11 @@ pub fn getenv(_: &OsStr) -> Option<OsString> {
     None
 }
 
-pub fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
     Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot set env vars on this platform"))
 }
 
-pub fn unsetenv(_: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(_: &OsStr) -> io::Result<()> {
     Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot unset env vars on this platform"))
 }
 

library/std/src/sys/pal/wasi/os.rs

@@ -244,7 +244,7 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     .flatten()
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     run_with_cstr(k.as_bytes(), &|k| {
         run_with_cstr(v.as_bytes(), &|v| unsafe {
             let _guard = env_write_lock();
@@ -253,7 +253,7 @@ pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     })
 }
 
-pub fn unsetenv(n: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(n: &OsStr) -> io::Result<()> {
     run_with_cstr(n.as_bytes(), &|nbuf| unsafe {
         let _guard = env_write_lock();
         cvt(libc::unsetenv(nbuf.as_ptr())).map(drop)

library/std/src/sys/pal/windows/os.rs

@@ -302,16 +302,16 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     .ok()
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     let k = to_u16s(k)?;
     let v = to_u16s(v)?;
 
-    cvt(unsafe { c::SetEnvironmentVariableW(k.as_ptr(), v.as_ptr()) }).map(drop)
+    cvt(c::SetEnvironmentVariableW(k.as_ptr(), v.as_ptr())).map(drop)
 }
 
-pub fn unsetenv(n: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(n: &OsStr) -> io::Result<()> {
     let v = to_u16s(n)?;
-    cvt(unsafe { c::SetEnvironmentVariableW(v.as_ptr(), ptr::null()) }).map(drop)
+    cvt(c::SetEnvironmentVariableW(v.as_ptr(), ptr::null())).map(drop)
 }
 
 pub fn temp_dir() -> PathBuf {

library/std/src/sys/pal/xous/os.rs

@@ -149,11 +149,11 @@ pub fn getenv(_: &OsStr) -> Option<OsString> {
     None
 }
 
-pub fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
     Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot set env vars on this platform"))
 }
 
-pub fn unsetenv(_: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(_: &OsStr) -> io::Result<()> {
     Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot unset env vars on this platform"))
 }
 

library/std/src/sys/pal/zkvm/os.rs

@@ -115,11 +115,11 @@ pub fn getenv(varname: &OsStr) -> Option<OsString> {
     Some(OsString::from_inner(os_str::Buf { inner: u8s.to_vec() }))
 }
 
-pub fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
     Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot set env vars on this platform"))
 }
 
-pub fn unsetenv(_: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(_: &OsStr) -> io::Result<()> {
     Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot unset env vars on this platform"))
 }
 

tests/ui/rust-2024/unsafe-env.e2021.stderr

@@ -0,0 +1,23 @@
+error[E0133]: call to unsafe function `unsafe_fn` is unsafe and requires unsafe function or block
+  --> $DIR/unsafe-env.rs:24:5
+   |
+LL |     unsafe_fn();
+   |     ^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error: unnecessary `unsafe` block
+  --> $DIR/unsafe-env.rs:27:5
+   |
+LL |     unsafe {
+   |     ^^^^^^ unnecessary `unsafe` block
+   |
+note: the lint level is defined here
+  --> $DIR/unsafe-env.rs:12:8
+   |
+LL | #[deny(unused_unsafe)]
+   |        ^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0133`.

tests/ui/rust-2024/unsafe-env.e2024.stderr

@@ -0,0 +1,39 @@
+error[E0133]: call to unsafe function `set_var` is unsafe and requires unsafe block
+  --> $DIR/unsafe-env.rs:14:5
+   |
+LL |     env::set_var("FOO", "BAR");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error[E0133]: call to unsafe function `remove_var` is unsafe and requires unsafe block
+  --> $DIR/unsafe-env.rs:16:5
+   |
+LL |     env::remove_var("FOO");
+   |     ^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error[E0133]: call to unsafe function `unsafe_fn` is unsafe and requires unsafe block
+  --> $DIR/unsafe-env.rs:24:5
+   |
+LL |     unsafe_fn();
+   |     ^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error: unnecessary `unsafe` block
+  --> $DIR/unsafe-env.rs:27:5
+   |
+LL |     unsafe {
+   |     ^^^^^^ unnecessary `unsafe` block
+   |
+note: the lint level is defined here
+  --> $DIR/unsafe-env.rs:12:8
+   |
+LL | #[deny(unused_unsafe)]
+   |        ^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0133`.

tests/ui/rust-2024/unsafe-env.rs

@@ -0,0 +1,31 @@
+//@ revisions: e2021 e2024
+//@[e2021] edition: 2021
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+
+use std::env;
+use std::mem;
+
+unsafe fn unsafe_fn() {}
+fn safe_fn() {}
+
+#[deny(unused_unsafe)]
+fn main() {
+    env::set_var("FOO", "BAR");
+    //[e2024]~^ ERROR call to unsafe function `set_var` is unsafe
+    env::remove_var("FOO");
+    //[e2024]~^ ERROR call to unsafe function `remove_var` is unsafe
+
+    unsafe {
+        env::set_var("FOO", "BAR");
+        env::remove_var("FOO");
+    }
+
+    unsafe_fn();
+    //~^ ERROR call to unsafe function `unsafe_fn` is unsafe
+
+    unsafe {
+        //~^ ERROR unnecessary `unsafe` block
+        safe_fn();
+    }
+}