Auto merge of rust-lang#118830 - GuillaumeGomez:env-tracked_env, r=Nilstrieb

Add support for `--env` on `tracked_env::var`

Follow-up of rust-lang#118368.
Part of Part of rust-lang#80792.

It adds support of the `--env` option for proc-macros through `tracked_env::var`.

r? `@Nilstrieb`

Author: bors

proc_macro/src/bridge/mod.rs

@@ -55,6 +55,7 @@ macro_rules! with_api {
         $m! {
             FreeFunctions {
                 fn drop($self: $S::FreeFunctions);
+                fn injected_env_var(var: &str) -> Option<String>;
                 fn track_env_var(var: &str, value: Option<&str>);
                 fn track_path(path: &str);
                 fn literal_from_str(s: &str) -> Result<Literal<$S::Span, $S::Symbol>, ()>;

proc_macro/src/lib.rs

@@ -1503,7 +1503,8 @@ pub mod tracked_env {
     #[unstable(feature = "proc_macro_tracked_env", issue = "99515")]
     pub fn var<K: AsRef<OsStr> + AsRef<str>>(key: K) -> Result<String, VarError> {
         let key: &str = key.as_ref();
-        let value = env::var(key);
+        let value = crate::bridge::client::FreeFunctions::injected_env_var(key)
+            .map_or_else(|| env::var(key), Ok);
         crate::bridge::client::FreeFunctions::track_env_var(key, value.as_deref().ok());
         value
     }