Don't modify the actual environment in RUSTC_BOOTSTRAP unit tests

Modifying process-global state in unit tests is bad news, because it can affect
other unit tests running in parallel in the same process.

Author: Zalathar

compiler/rustc_feature/src/lib.rs

@@ -26,6 +26,7 @@ mod unstable;
 #[cfg(test)]
 mod tests;
 
+use std::env;
 use std::num::NonZero;
 
 use rustc_span::symbol::Symbol;
@@ -68,14 +69,22 @@ impl UnstableFeatures {
     /// If `krate` is [`Some`], then setting `RUSTC_BOOTSTRAP=krate` will enable the nightly
     /// features. Otherwise, only `RUSTC_BOOTSTRAP=1` will work.
     pub fn from_environment(krate: Option<&str>) -> Self {
+        Self::from_environment_inner(krate, |name| env::var(name))
+    }
+
+    /// Unit tests can pass a mock `std::env::var` instead of modifying the real environment.
+    fn from_environment_inner(
+        krate: Option<&str>,
+        env_var: impl Fn(&str) -> Result<String, env::VarError>, // std::env::var
+    ) -> Self {
         // `true` if this is a feature-staged build, i.e., on the beta or stable channel.
         let disable_unstable_features =
             option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some_and(|s| s != "0");
         // Returns whether `krate` should be counted as unstable
         let is_unstable_crate =
             |var: &str| krate.is_some_and(|name| var.split(',').any(|new_krate| new_krate == name));
 
-        let bootstrap = std::env::var("RUSTC_BOOTSTRAP").ok();
+        let bootstrap = env_var("RUSTC_BOOTSTRAP").ok();
         if let Some(val) = bootstrap.as_deref() {
             match val {
                 val if val == "1" || is_unstable_crate(val) => return UnstableFeatures::Cheat,

compiler/rustc_feature/src/tests.rs

@@ -1,10 +1,18 @@
+use std::env;
+
 use super::UnstableFeatures;
 
+fn unstable_features(rustc_bootstrap: &str, crate_name: Option<&str>) -> UnstableFeatures {
+    UnstableFeatures::from_environment_inner(crate_name, |name| match name {
+        "RUSTC_BOOTSTRAP" => Ok(rustc_bootstrap.to_owned()),
+        _ => Err(env::VarError::NotPresent),
+    })
+}
+
 #[test]
 fn rustc_bootstrap_parsing() {
-    let is_bootstrap = |env, krate| {
-        std::env::set_var("RUSTC_BOOTSTRAP", env);
-        matches!(UnstableFeatures::from_environment(krate), UnstableFeatures::Cheat)
+    let is_bootstrap = |rustc_bootstrap, crate_name| {
+        matches!(unstable_features(rustc_bootstrap, crate_name), UnstableFeatures::Cheat)
     };
     assert!(is_bootstrap("1", None));
     assert!(is_bootstrap("1", Some("x")));
@@ -22,10 +30,8 @@ fn rustc_bootstrap_parsing() {
     assert!(!is_bootstrap("0", None));
 
     // `RUSTC_BOOTSTRAP=-1` is force-stable, no unstable features allowed.
-    let is_force_stable = |krate| {
-        std::env::set_var("RUSTC_BOOTSTRAP", "-1");
-        matches!(UnstableFeatures::from_environment(krate), UnstableFeatures::Disallow)
-    };
+    let is_force_stable =
+        |crate_name| matches!(unstable_features("-1", crate_name), UnstableFeatures::Disallow);
     assert!(is_force_stable(None));
     // Does not support specifying any crate.
     assert!(is_force_stable(Some("x")));