Generate recursive meta language dependencies.

Cargo doesn't scan a directory for changed dependencies recursively, so do that
as part of the build.py script.

cranelift/src/libcretonne/build.rs

@@ -32,9 +32,6 @@ fn main() {
     let meta_dir = top_dir.join("meta");
     let build_script = meta_dir.join("build.py");
 
-    // Let Cargo known that this script should be rerun if anything changes in the meta directory.
-    println!("cargo:rerun-if-changed={}", meta_dir.display());
-
     // Launch build script with Python. We'll just find python in the path.
     let status = process::Command::new("python")
                      .current_dir(top_dir)

meta/build.py

@@ -5,6 +5,7 @@
 import argparse
 import target
 import gen_instr
+import gen_build_deps
 
 parser = argparse.ArgumentParser(description='Generate sources for Cretonne.')
 parser.add_argument('--out-dir', help='set output directory')
@@ -15,3 +16,4 @@ out_dir = args.out_dir
 targets = target.all_targets()
 
 gen_instr.generate(targets, out_dir)
+gen_build_deps.generate()

meta/gen_build_deps.py

@@ -0,0 +1,36 @@
+"""
+Generate build dependencies for Cargo.
+
+The `build.py` script is invoked by cargo when building libcretonne to
+generate Rust code from the instruction descriptions. Cargo needs to know when
+it is necessary to rerun the build script.
+
+If the build script outputs lines of the form:
+
+    cargo:rerun-if-changed=/path/to/file
+
+cargo will rerun the build script when those files have changed since the last
+build.
+"""
+
+import os
+from os.path import dirname, abspath, join
+
+
+def source_files(top):
+    """
+    Recursively find all interesting source files and directories in the
+    directory tree starting at top. Yield a path to each file.
+    """
+    for (dirpath, dirnames, filenames) in os.walk(top):
+        yield dirpath
+        for f in filenames:
+            if f.endswith('.py'):
+                yield join(dirpath, f)
+
+
+def generate():
+    print "Dependencies from meta language directory:"
+    meta = dirname(abspath(__file__))
+    for path in source_files(meta):
+        print "cargo:rerun-if-changed=" + path