[derive] Use #[feature(tracked_env)] to track 'DEBUG_DERIVE'

Also increase precision of the patterns to support matching only
specific items.

This way whenever the environment variable changes you'll crate
will be automatically recompiled!
However, it further increases our reliance on nightly :(

Author: Techcable

libs/derive/src/lib.rs

@@ -1,5 +1,6 @@
 #![feature(
-    proc_macro_diagnostic, // NEEDED for warnings
+    proc_macro_diagnostic, // Used for warnings
+    proc_macro_tracked_env, // Used for `DEBUG_DERIVE`
 )]
 extern crate proc_macro;
 
@@ -313,6 +314,7 @@ pub fn derive_trace(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
         .unwrap_or_else(|e| e.to_compile_error()));
     debug_derive(
         "derive(Trace)",
+        &input.ident,
         &format_args!("#[derive(Trace) for {}", input.ident),
         &res
     );
@@ -854,12 +856,29 @@ fn add_trait_bounds(
     result
 }
 
-fn debug_derive(key: &str, message: &dyn Display, value: &dyn Display) {
+fn debug_derive(key: &str, target: &dyn ToString, message: &dyn Display, value: &dyn Display) {
+    let target = target.to_string();
     // TODO: Use proc_macro::tracked_env::var
-    match ::std::env::var_os("DEBUG_DERIVE") {
-        Some(var) if var == "*" ||
-            var.to_string_lossy().contains(key) => {
-            // Enable this debug
+    match ::proc_macro::tracked_env::var("DEBUG_DERIVE") {
+        Ok(ref var) if var == "*" => {}
+        Ok(var) => {
+            for pattern in var.split_terminator(",") {
+                let parts = pattern.split(":").collect::<Vec<_>>();
+                let (desired_key, desired_target) = match *parts {
+                    [desired_key, desired_target] => (desired_key, Some(desired_target)),
+                    [desired_key] => (desired_key, None),
+                    _ => {
+                        panic!("Invalid pattern for debug derive: {}", pattern)
+                    }
+                };
+                if desired_key != key && desired_key != "*" { return }
+                if let Some(desired_target) = desired_target {
+                   if desired_target != target && desired_target != "*" {
+                       return
+                   }
+                }
+            }
+            // Fallthrough -> enable this debug
         },
         _ => return,
     }