16/n ifdef: Add is_form_active_with_env for cross-file context

Summary:
- Add `is_form_active_with_env` method to `FormList` that accepts a caller-provided `MacroEnvironment` instead of building one from `project_macro_environment`
- This is needed for included files where the caller has accumulated macro state (e.g., include guards defined by a previous inclusion)
- Add test `include_guard_with_env` verifying that an include guard header is active on first inclusion (no guard defined) and inactive on second inclusion (guard predefined in env)

Reviewed By: TD5

Differential Revision: D94370681

fbshipit-source-id: 9ea57a973cccbfb5cd2d09d76e79392e577bdb77

Author: alanz

crates/hir/src/form_list.rs

@@ -65,6 +65,7 @@ use la_arena::Idx;
 use la_arena::IdxRange;
 
 use crate::Diagnostic;
+use crate::MacroEnvironment;
 use crate::MacroName;
 use crate::Name;
 use crate::NameArity;
@@ -276,6 +277,30 @@ impl FormList {
             }
         }
     }
+
+    /// Evaluates whether a form is active using a caller-provided macro environment.
+    ///
+    /// Unlike `is_form_active`, this does not use `project_macro_environment` to build
+    /// the environment. This is needed for included files where the caller has accumulated
+    /// macro state (e.g., include guards defined by a previous inclusion).
+    pub fn is_form_active_with_env(
+        &self,
+        db: &dyn DefDatabase,
+        file_id: FileId,
+        pp_ctx: &FormPPContext,
+        env: Arc<MacroEnvironment>,
+    ) -> PPConditionResult {
+        match pp_ctx.condition {
+            None => PPConditionResult::Active,
+            Some(cond_id) => {
+                if !env.new_ifdef {
+                    return PPConditionResult::Active;
+                }
+                let analysis = db.file_preprocessor_analysis(file_id, env);
+                analysis.is_condition_active(cond_id)
+            }
+        }
+    }
 }
 
 #[derive(Debug, Default, Eq, PartialEq)]

crates/hir/src/form_list/tests.rs

@@ -8,10 +8,15 @@
  * above-listed licenses.
  */
 
+use std::sync::Arc;
+
 use elp_base_db::fixture::WithFixture;
 use expect_test::Expect;
 use expect_test::expect;
 
+use crate::MacroEnvironment;
+use crate::MacroName;
+use crate::Name;
 use crate::PPDirective;
 use crate::db::DefDatabase;
 use crate::form_list::PPConditionResult;
@@ -892,6 +897,45 @@ in_inner_else() -> ok.
     }
 }
 
+#[test]
+fn include_guard_with_env() {
+    // Header with an include guard pattern.
+    // is_form_active (no GUARD in project env) → Active (first inclusion)
+    // is_form_active_with_env (GUARD predefined) → Inactive (second inclusion)
+    let (db, files, _) = TestDB::with_many_files(
+        r#"
+//- /src/main.erl
+-module(main).
+//- /src/guard.hrl
+-ifndef(GUARD).
+-define(GUARD, 1).
+foo() -> ok.
+-endif.
+"#,
+    );
+
+    let header_file_id = files[1];
+    let form_list = db.file_form_list(header_file_id);
+
+    // First inclusion: GUARD not defined, ifndef is active
+    for (_idx, clause) in form_list.function_clauses() {
+        let result = form_list.is_form_active(&db, header_file_id, &clause.pp_ctx, None);
+        assert_eq!(result, PPConditionResult::Active);
+    }
+
+    // Second inclusion: GUARD predefined in env, ifndef is inactive
+    let mut env = MacroEnvironment::new();
+    env.set_new_ifdef(true);
+    env.define(MacroName::new(Name::from_erlang_service("GUARD"), None));
+    let env = Arc::new(env);
+
+    for (_idx, clause) in form_list.function_clauses() {
+        let result =
+            form_list.is_form_active_with_env(&db, header_file_id, &clause.pp_ctx, env.clone());
+        assert_eq!(result, PPConditionResult::Inactive);
+    }
+}
+
 #[test]
 fn resolved_include_populated() {
     // Test that ConditionEnv.resolved_include is populated for include directives