Index: tools/gn/functions.cc |
diff --git a/tools/gn/functions.cc b/tools/gn/functions.cc |
index 86043f0a73ab930c5c00fd28832e434b7a208331..01d0aeb027b04b2ee36467ff99b2577392ffb360 100644 |
--- a/tools/gn/functions.cc |
+++ b/tools/gn/functions.cc |
@@ -344,7 +344,46 @@ const char kDeclareArgs_Help[] = |
"\n" |
" See also \"gn help buildargs\" for an overview.\n" |
"\n" |
- "Example:\n" |
+ " The precise behavior of declare args is:\n" |
+ "\n" |
+ " 1. The declare_arg block executes. Any variables in the enclosing\n" |
+ " scope are available for reading.\n" |
+ "\n" |
+ " 2. At the end of executing the block, any variables set within that\n" |
+ " scope are saved globally as build arguments, with their current\n" |
+ " values being saved as the \"default value\" for that argument.\n" |
+ "\n" |
+ " 3. User-defined overrides are applied. Anything set in \"gn args\"\n" |
+ " now overrides any default values. The resulting set of variables\n" |
+ " is promoted to be readable from the following code in the file.\n" |
+ "\n" |
+ " This has some ramifications that may not be obvious:\n" |
+ "\n" |
+ " - You should not perform difficult work inside a declare_args block\n" |
+ " since this only sets a default value that may be discarded. In\n" |
+ " particular, don't use the result of exec_script() to set the\n" |
+ " default value. If you want to have a script-defined default, set\n" |
+ " some default \"undefined\" value like [], \"\", or -1, and after\n" |
+ " the declare_args block, call exec_script if the value is unset by\n" |
+ " the user.\n" |
+ "\n" |
+ " - Any code inside of the declare_args block will see the default\n" |
+ " values of previous variables defined in the block rather than\n" |
+ " the user-overridden value. This can be surprising because you will\n" |
+ " be used to seeing the overridden value. If you need to make the\n" |
+ " default value of one arg dependent on the possibly-overridden\n" |
+ " value of another, write two separate declare_args blocks:\n" |
+ "\n" |
+ " declare_args() {\n" |
+ " enable_foo = true\n" |
+ " }\n" |
+ " declare_args() {\n" |
+ " # Bar defaults to same user-overridden state as foo.\n" |
+ " enable_bar = enable_foo\n" |
+ " }\n" |
+ "\n" |
+ "Example\n" |
+ "\n" |
" declare_args() {\n" |
" enable_teleporter = true\n" |
" enable_doom_melon = false\n" |