This patch revert 3d169bd3f4cdc2dc3dd09eadbbc17c19214d69f3 (Add track_env_var to the proc macro server).

diff --git a/crates/proc_macro_srv/src/proc_macro/bridge/client.rs b/crates/proc_macro_srv/src/proc_macro/bridge/client.rs
index 55d6330cc..cb4b3bdb0 100644
--- a/crates/proc_macro_srv/src/proc_macro/bridge/client.rs
+++ b/crates/proc_macro_srv/src/proc_macro/bridge/client.rs
@@ -160,7 +160,6 @@ macro_rules! define_handles {
 }
 define_handles! {
     'owned:
-    FreeFunctions,
     TokenStream,
     TokenStreamBuilder,
     TokenStreamIter,
diff --git a/crates/proc_macro_srv/src/proc_macro/bridge/mod.rs b/crates/proc_macro_srv/src/proc_macro/bridge/mod.rs
index b97886eb9..aeb05aad4 100644
--- a/crates/proc_macro_srv/src/proc_macro/bridge/mod.rs
+++ b/crates/proc_macro_srv/src/proc_macro/bridge/mod.rs
@@ -57,10 +57,6 @@ 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;
diff --git a/crates/proc_macro_srv/src/proc_macro/bridge/server.rs b/crates/proc_macro_srv/src/proc_macro/bridge/server.rs
index 3acb239af..45d41ac02 100644
--- a/crates/proc_macro_srv/src/proc_macro/bridge/server.rs
+++ b/crates/proc_macro_srv/src/proc_macro/bridge/server.rs
@@ -11,8 +11,6 @@ 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) =>
diff --git a/crates/proc_macro_srv/src/proc_macro/mod.rs b/crates/proc_macro_srv/src/proc_macro/mod.rs
index fc6e7344f..ee0dc9722 100644
--- a/crates/proc_macro_srv/src/proc_macro/mod.rs
+++ b/crates/proc_macro_srv/src/proc_macro/mod.rs
@@ -924,25 +924,3 @@ 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
-    }
-}
diff --git a/crates/proc_macro_srv/src/rustc_server.rs b/crates/proc_macro_srv/src/rustc_server.rs
index c5fe3591e..7d1695c86 100644
--- a/crates/proc_macro_srv/src/rustc_server.rs
+++ b/crates/proc_macro_srv/src/rustc_server.rs
@@ -242,8 +242,6 @@ impl TokenStreamBuilder {
     }
 }

-pub struct FreeFunctions;
-
 #[derive(Clone)]
 pub struct TokenStreamIter {
     trees: IntoIter<TokenTree>,
@@ -256,7 +254,6 @@ pub struct Rustc {
 }

 impl server::Types for Rustc {
-    type FreeFunctions = FreeFunctions;
     type TokenStream = TokenStream;
     type TokenStreamBuilder = TokenStreamBuilder;
     type TokenStreamIter = TokenStreamIter;
@@ -270,13 +267,6 @@ impl server::Types for Rustc {
     type MultiSpan = Vec<Span>;
 }

-impl server::FreeFunctions for Rustc {
-    fn track_env_var(&mut self, _var: &str, _value: Option<&str>) {
-        // FIXME: track env var accesses
-        // https://github.com/rust-lang/rust/pull/71858
-    }
-}
-
 impl server::TokenStream for Rustc {
     fn new(&mut self) -> Self::TokenStream {
         Self::TokenStream::new()
diff --git a/xtask/src/install.rs b/xtask/src/install.rs
index fcc4f05e4..d829790d7 100644
--- a/xtask/src/install.rs
+++ b/xtask/src/install.rs
@@ -7,7 +7,7 @@ use anyhow::{bail, format_err, Context, Result};
 use crate::not_bash::{pushd, run};

 // Latest stable, feel free to send a PR if this lags behind.
-const REQUIRED_RUST_VERSION: u32 = 47;
+const REQUIRED_RUST_VERSION: u32 = 46;

 pub struct InstallCmd {
     pub client: Option<ClientOpt>,