[build] Replace mentions of `gen.py` with direct `gn gen`

Change-Id: I2204a3d7ca413019213d6cd8c3afe7218b709ebc

development/build/overview.md

@@ -68,14 +68,14 @@ packages to incorporate:
 # fuchsia_base is typically "default".
 # my_stuff is a possibly-empty list of extra packages to include.
 
-$ build/gn/gen.py --packages build/gn/fuchsia_base,build/gn/my_stuff
+$ buildtools/gn gen out/x64 --args='fuchsia_packages="build/gn/fuchsia_base,build/gn/my_stuff"'
 ```
-This will create an `out/debug-<arch>` directory containing Ninja files.
+This will create an `out/x64` directory containing Ninja files.
 
 This is what gets run under the hood by `fx set`.
 
-For a list of all `gen.py` options, run `gen.py --help`.
-For documentation on the `--variants` flag, see [Variants](variants.md).
+For a list of all GN build arguments, run `buildtools/gn args out/x64 --list`.
+For documentation on the `select_variant` argument, see [Variants](variants.md).
 
 ### C
 

development/build/variants.md

@@ -13,21 +13,18 @@ in which variants.  It applies automatically to every `executable`,
 mechanism in which you give a list of matching rules to apply to each
 target to decide which variant to use (if any).  To support this
 flexibility, the value for `select_variant` uses a detailed GN syntax.
+For simple cases, this can just be a list of strings.
 
-Some shorthands for common cases are provided by the `gen.py` script that
-is the usual way to run `gn gen` in the Fuchsia build, via `--variant`
-switches.  Each `--variant` switch adds one matching rule.
-
-Here's an example running `gen.py` directly:
+Here's an example running `gn gen` directly:
 
 ```sh
-./build/gn/gen.py --variant=host_asan --variant=asan=cat,ledger
+./buildtools/gn gen out/x64 --args='select_variant=["host_asan", "asan/cat", "asan/ledger"]'
 ```
 
 This does the same thing using the `fx set` tool:
 
 ```sh
-fx set x64 --variant host_asan --variant asan=cat,ledger
+fx set x64 --variant={host_asan,asan/cat,asan/ledger}
 ```
 
  1. The first switch applies the `host_asan` matching rule, which enables
@@ -36,71 +33,43 @@ fx set x64 --variant host_asan --variant asan=cat,ledger
 
  2. The second switch applies the `asan` matching rule, which enables
     AddressSanitizer for executables built to run on the target (i.e. the
-    Fuchsia device).  The `=...` suffix constrains this matching rule only
-    to the binaries named `cat` and `ledger`.
+    Fuchsia device).  The `/cat` suffix constrains this matching rule only
+    to the binary named `cat`.
 
-The switch `--variant=help` will make `gen.py` show the list of recognized
-shorthands, and which ones can work with the `=...` suffix.
+ 3. The third switch is like the second, but matches the binary named `ledger`.
 
 The GN code supports much more flexible matching rules than just the binary
 name, but there are no shorthands for those.  To do something more complex,
 set the `select_variant` GN build argument directly.
 
- * You can do this via the `--args` switch to `gen.py` once you have the
+ * You can do this via the `--args` switch to `gn gen` once you have the
    syntax down.
 
  * The easiest way to experiment is to start with some `--variant` switches that
-   approximate what you want and then edit the `select_variant` value `gen.py`
+   approximate what you want and then edit the `select_variant` value `fx set`
    produces:
    * You can just edit the `args.gn` file in the GN output directory
-     (e.g. `out/debug-x64/args.gn`) and the next `ninja` run (aka `fx build`)
+     (e.g. `out/x64/args.gn`) and the next `ninja` run (aka `fx build`)
      will re-run `gn gen` with those changes.
-   * You can use the command `./buildtools gn args out/debug-x64`, which
+   * You can use the command `./buildtools gn args out/x64`, which
      will run your `$EDITOR` on the `args.gn` file and then do `gn gen`
      immediately so you can see any errors in your GN syntax.
 
 Like all GN build arguments, `select_variant` is documented in comments in
 the `.gn` source file that defines the argument, and these can be displayed
-using `./buildtools/gn args --list`.  The `--help-args` switch to `gen.py`
+using `./buildtools/gn args --list`.  The `--help-args` switch to `fx set`
 also runs GN this way:
 
 ```sh
-./build/gn/gen.py -p topaz/packages/default --help-args=select_variant
+./buildtools/gn args out/x64 --list=select_variant
 ```
 
-You can also use the `fx set` tool to run `gen.py` for you:
+You can also use the `fx set` tool to run `gn args` for you:
 
 ```sh
-fx set x64 --help-args select_variant
-```
-
-If you combine this with one or more `--variant` switches, GN will show you
-the actual value of `select_variant` that `gen.py` chose.  For example:
-
-```sh
-./build/gn/gen.py -p topaz/packages/default --help-args=select_variant --variant=asan=cat,ledger
-```
-
-or:
-
-```sh
-fx set x64 --help-args select_variant --variant asan=cat,ledger
-```
-
-prints out:
-
-```
-select_variant
-    Current value = [{
-  output_name = ["cat", "ledger"]
-  variant = "asan"
-  host = false
-}]
-      From :1
-    Overridden from the default = []
-...
+fx set x64 --help-args=select_variant
 ```
 
 To see the list of variants available and learn more about how to define
-knew ones, use `--help-args known_variants` with `gen.py` or `fx set`
+knew ones, use `--help-args=known_variants` or `fx set`
 (or `gn args out/... --list=known_variants`).