Introduce --incompatible_disallow_toplevel_if_statement to forbid top-level if statements.

RELNOTES:
  In .bzl files, top-level `if` statements are deprecated and will be forbidden
  in the future. Move them in a function body instead (or use a conditional
  expression instead: `x if condition else y`).
PiperOrigin-RevId: 158276986

src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
index f25ec69..6ce4801 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
@@ -86,4 +86,14 @@
             + "statement."
   )
   public boolean incompatibleBzlDisallowLoadAfterStatement;
+
+  @Option(
+    name = "incompatible_disallow_toplevel_if_statement",
+    defaultValue = "false",
+    category = "incompatible changes",
+    help =
+        "If set to true, 'if' statements are forbidden at the top-level "
+            + "(outside a function definition)"
+  )
+  public boolean incompatibleDisallowToplevelIfStatement;
 }

src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java b/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
index 84a7cbb..b66e0d6 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
@@ -180,6 +180,20 @@
     }
   }
 
+  /** Throws EvalException if a `if` statement appears at the top level. */
+  private void checkToplevelIfStatement(List<Statement> statements) throws EvalException {
+    for (Statement statement : statements) {
+      if (statement instanceof IfStatement) {
+        throw new EvalException(
+            statement.getLocation(),
+            "if statements are not allowed at the top level. You may move it inside a function "
+                + "or use an if expression (x if condition else y). "
+                + "Use --incompatible_disallow_toplevel_if_statement to temporarily disable "
+                + "this check.");
+      }
+    }
+  }
+
   /**
    * Validates the AST and runs static checks.
    */
@@ -189,6 +203,11 @@
       checkLoadAfterStatement(statements);
     }
 
+    // Check that load() statements are on top.
+    if (semantics.incompatibleDisallowToplevelIfStatement) {
+      checkToplevelIfStatement(statements);
+    }
+
     // Add every function in the environment before validating. This is
     // necessary because functions may call other functions defined
     // later in the file.

src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java b/src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java
index 0ad0297..10c80c7 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java
@@ -60,6 +60,18 @@
   }
 
   @Test
+  public void testForbiddenToplevelIfStatement() throws Exception {
+    env = newEnvironmentWithSkylarkOptions("--incompatible_disallow_toplevel_if_statement=true");
+    checkError("if statements are not allowed at the top level", "if True: a = 2");
+  }
+
+  @Test
+  public void testAllowedToplevelIfStatement() throws Exception {
+    env = newEnvironmentWithSkylarkOptions("--incompatible_disallow_toplevel_if_statement=false");
+    parse("if True: a = 5");
+  }
+
+  @Test
   public void testTwoFunctionsWithTheSameName() throws Exception {
     checkError(
         "Variable foo is read only", "def foo():", "  return 1", "def foo(x, y):", "  return 1");