perf: simplify export enumeration (#363)

brush-core/src/commands.rs

@@ -212,9 +212,7 @@ pub(crate) fn compose_std_command<S: AsRef<OsStr>>(
     cmd.arg0(argv0);
 
     // Pass through args.
-    for arg in args {
-        cmd.arg(arg);
-    }
+    cmd.args(args);
 
     // Use the shell's current working dir.
     cmd.current_dir(shell.working_dir.as_path());
@@ -224,11 +222,8 @@ pub(crate) fn compose_std_command<S: AsRef<OsStr>>(
 
     // Add in exported variables.
     if !empty_env {
-        for (name, var) in shell.env.iter() {
-            if var.is_exported() {
-                let value_as_str = var.value().to_cow_str(shell);
-                cmd.env(name, value_as_str.as_ref());
-            }
+        for (k, v) in shell.env.iter_exported() {
+            cmd.env(k.as_str(), v.value().to_cow_str(shell).as_ref());
         }
     }
 

brush-core/src/env.rs

@@ -83,6 +83,25 @@ impl ShellEnvironment {
     // Iterators/Getters
     //
 
+    /// Returns an iterator over all exported variables defined in the variable.
+    pub fn iter_exported(&self) -> impl Iterator<Item = (&String, &ShellVariable)> {
+        // We won't actually need to store all entries, but we expect it should be
+        // within the same order.
+        let mut visible_vars: HashMap<&String, &ShellVariable> =
+            HashMap::with_capacity(self.entry_count);
+
+        for (_, var_map) in self.scopes.iter().rev() {
+            for (name, var) in var_map.iter().filter(|(_, v)| v.is_exported()) {
+                // Only insert the variable if it hasn't been seen yet.
+                if let hash_map::Entry::Vacant(entry) = visible_vars.entry(name) {
+                    entry.insert(var);
+                }
+            }
+        }
+
+        visible_vars.into_iter()
+    }
+
     /// Returns an iterator over all the variables defined in the environment.
     pub fn iter(&self) -> impl Iterator<Item = (&String, &ShellVariable)> {
         self.iter_using_policy(EnvironmentLookup::Anywhere)