Add var_guard and modify anchor_cli_lib to use it

Author: smoelius

src/anchor_cli_lib/mod.rs

@@ -343,11 +343,9 @@ fn generate_idl(
     // smoelius: If `RUSTUP_TOOLCHAIN` is not cleared, `Span::unwrap` might not be found in
     // `proc-macro2`. The affected code is in `anchor-syn`:
     // https://github.com/solana-foundation/anchor/blob/38f2b554b2dc6632376241f94048639eaf33d7bd/lang/syn/src/idl/defined.rs#L498-L502
-    let rustup_toolchain = std::env::var_os("RUSTUP_TOOLCHAIN");
-    if rustup_toolchain.is_some() {
-        unsafe {
-            std::env::remove_var("RUSTUP_TOOLCHAIN");
-        }
+    let _guard: crate::util::var_guard::VarGuard;
+    if std::env::var_os("RUSTUP_TOOLCHAIN").is_some() {
+        _guard = crate::util::var_guard::VarGuard::set("RUSTUP_TOOLCHAIN", None);
     }
 
     let result = anchor_lang_idl::build::IdlBuilder::new()
@@ -357,12 +355,6 @@ fn generate_idl(
         .cargo_args(cargo_args.into())
         .build();
 
-    if let Some(rustup_toolchain) = rustup_toolchain {
-        unsafe {
-            std::env::set_var("RUSTUP_TOOLCHAIN", rustup_toolchain);
-        }
-    }
-
     result
 }
 

src/util/mod.rs

@@ -6,6 +6,8 @@ use std::{
     path::{Path, PathBuf},
 };
 
+pub mod var_guard;
+
 pub fn files_with_extension(dir: impl AsRef<Path>, extension: &str) -> Result<Vec<PathBuf>> {
     let mut pcs_paths = Vec::new();
     for result in read_dir(dir)? {

src/util/var_guard.rs

@@ -0,0 +1,43 @@
+// smoelius: `VarGuard` is based on a struct of the same name from Clippy's source code. The `set`
+// method was modified to accept an `Option` so that variables could be cleared. Also, `unsafe`
+// annotations were added to make the struct compile with Rust Edition 2024. See:
+// https://github.com/rust-lang/rust-clippy/blob/c51472b4b09d22bdbb46027f08be54c4b285a725/tests/compile-test.rs#L267-L289
+
+use std::{
+    env::{remove_var, set_var, var_os},
+    ffi::{OsStr, OsString},
+    marker::PhantomData,
+};
+
+/// Restores an env var on drop
+#[must_use]
+pub struct VarGuard<T = OsString> {
+    key: &'static str,
+    value: Option<OsString>,
+    _phantom: PhantomData<T>,
+}
+
+impl<T: AsRef<OsStr>> VarGuard<T> {
+    pub fn set(key: &'static str, val: Option<T>) -> Self {
+        let value = var_os(key);
+        if let Some(val) = val {
+            unsafe { set_var(key, val) };
+        } else {
+            unsafe { remove_var(key) };
+        }
+        Self {
+            key,
+            value,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+impl<T> Drop for VarGuard<T> {
+    fn drop(&mut self) {
+        match self.value.as_deref() {
+            None => unsafe { remove_var(self.key) },
+            Some(value) => unsafe { set_var(self.key, value) },
+        }
+    }
+}