set: Don't treat toplevel scope the same as global

When using the `set` command with the -l flag, if we're at the top
level, create a temporary local scope. This makes query/assignment
behavior be consistent with the value-printing behavior.

This works by marking the current block as needing to pop the
environment if a local scope was pushed. I assume this is safe to do. I
also assume the current block is the right one to modify, rather than
trying to walk up the stack to the root.

builtin_set.cpp

@@ -559,6 +559,17 @@ static int builtin_set(parser_t &parser, wchar_t **argv)
     */
     scope = (local ? ENV_LOCAL : 0) | (global ? ENV_GLOBAL : 0) | (exportv ? ENV_EXPORT : 0) | (unexport ? ENV_UNEXPORT : 0) | (universal ? ENV_UNIVERSAL:0) | ENV_USER;
 
+    /*
+      If we're interacting with the local scope at all, ensure we actually have
+      one that's distinct from the global scope. If we don't have one yet,
+      create one and modify the current block to pop it.
+    */
+    if ((scope & ENV_LOCAL) && env_ensure_local_scope())
+    {
+        block_t *block = parser.current_block();
+        block->wants_pop_env = true;
+    }
+
     if (query)
     {
         /*

env.cpp

@@ -1159,6 +1159,16 @@ void env_pop()
     }
 }
 
+bool env_ensure_local_scope()
+{
+    if (top == global_env)
+    {
+        env_push(false);
+        return true;
+    }
+    return false;
+}
+
 /**
    Function used with to insert keys of one table into a set::set<wcstring>
 */

env.h

@@ -199,6 +199,12 @@ void env_push(bool new_scope);
 */
 void env_pop();
 
+/**
+  Push the variable stack if the current scope is the global scope. Returns
+  whether it pushed a scope.
+*/
+bool env_ensure_local_scope();
+
 /** Synchronizes all universal variable changes: writes everything out, reads stuff in */
 void env_universal_barrier();