fix: remove error on empty deps (so I can compile 'hello world')

build_defs/rust.build_defs

@@ -182,7 +182,7 @@ def rust_library(name:str, root:str, crate_name:str="", modules:list=None, deps:
     )
 
 
-def rust_binary(name:str, main:str, deps:list=None, edition:str='2021', features:list=None, build_root:str=None, build_deps:list|dict=None, build_compile_env:dict=None, build_run_env:dict=None, target_dir:str="target/debug/deps", visibility:list=None, _os:str=None, _arch:str=None):
+def rust_binary(name:str, main:str, deps:list=[], edition:str='2021', features:list=None, build_root:str=None, build_deps:list|dict=None, build_compile_env:dict=None, build_run_env:dict=None, target_dir:str="target/debug/deps", visibility:list=None, _os:str=None, _arch:str=None):
     """Defines a build rule for a Rust standalone binary.
 
     Args:
@@ -218,15 +218,19 @@ def rust_binary(name:str, main:str, deps:list=None, edition:str='2021', features
 
     # All this really does is ensure that rust_proto_libraries build before the pre_build function.
     # TODO: is there a better way of doing this?
-    collect_deps = genrule(
-        name = f"_{name}#target_deps",
-        cmd = [
-            f"mkdir $OUTS",
-            f"mv $SRCS $OUTS",
-        ],
-        srcs = deps,
-        outs = [target_dir],
-    )
+    if len(deps) > 0:
+        collect_deps = [genrule(
+            name = f"_{name}#target_deps",
+            cmd = [
+                f"mkdir $OUTS",
+                f"mv $SRCS $OUTS",
+            ],
+            srcs = deps,
+            outs = [target_dir],
+        )]
+    else:
+        collect_deps = []
+
     cmd, tools = _rustc_cmds(
         crate_name=name,
         edition=edition,
@@ -240,7 +244,7 @@ def rust_binary(name:str, main:str, deps:list=None, edition:str='2021', features
         binary = True,
         outs = [name],
         cmd = cmd,
-        deps = deps + [collect_deps],
+        deps = deps + collect_deps,
         visibility = visibility,
         requires = ['rust'],
         tools = tools,

examples/hello_world/BUILD

@@ -0,0 +1,8 @@
+subinclude("//build_defs:rust")
+
+rust_binary(
+    name = "hello_world",
+    main = "src/main.rs",
+    edition = "2021",
+)
+

examples/hello_world/src/main.rs

@@ -0,0 +1,4 @@
+fn main() {
+    println!("Hello, world!");
+}
+