Add track_env_var to the proc macro server

crates/proc_macro_srv/src/proc_macro/bridge/client.rs

@@ -160,6 +160,7 @@ macro_rules! define_handles {
 }
 define_handles! {
     'owned:
+    FreeFunctions,
     TokenStream,
     TokenStreamBuilder,
     TokenStreamIter,

crates/proc_macro_srv/src/proc_macro/bridge/mod.rs

@@ -57,6 +57,10 @@ use std::thread;
 macro_rules! with_api {
     ($S:ident, $self:ident, $m:ident) => {
         $m! {
+            FreeFunctions {
+                fn drop($self: $S::FreeFunctions);
+                fn track_env_var(var: &str, value: Option<&str>);
+            },
             TokenStream {
                 fn drop($self: $S::TokenStream);
                 fn clone($self: &$S::TokenStream) -> $S::TokenStream;

crates/proc_macro_srv/src/proc_macro/bridge/server.rs

@@ -11,6 +11,8 @@ use super::client::HandleStore;
 /// Declare an associated item of one of the traits below, optionally
 /// adjusting it (i.e., adding bounds to types and default bodies to methods).
 macro_rules! associated_item {
+    (type FreeFunctions) =>
+        (type FreeFunctions: 'static;);
     (type TokenStream) =>
         (type TokenStream: 'static + Clone;);
     (type TokenStreamBuilder) =>

crates/proc_macro_srv/src/proc_macro/mod.rs

@@ -924,3 +924,25 @@ impl fmt::Debug for Literal {
         self.0.fmt(f)
     }
 }
+
+pub mod tracked_env {
+    use std::env::{self, VarError};
+    use std::ffi::OsStr;
+
+    /// Retrieve an environment variable and add it to build dependency info.
+    /// Build system executing the compiler will know that the variable was accessed during
+    /// compilation, and will be able to rerun the build when the value of that variable changes.
+    /// Besides the dependency tracking this function should be equivalent to `env::var` from the
+    /// standard library, except that the argument must be UTF-8.
+    pub fn var<K: AsRef<OsStr> + AsRef<str>>(key: K) -> Result<String, VarError> {
+        use std::ops::Deref;
+
+        let key: &str = key.as_ref();
+        let value = env::var(key);
+        super::bridge::client::FreeFunctions::track_env_var(
+            key,
+            value.as_ref().map(|t| t.deref()).ok(),
+        );
+        value
+    }
+}