Support emitting Makefile-syntax depfiles like gcc/clang/rustc.

Needed to auto-bindgen with a ninja build without the build graph
going stale.

Cargo.lock

@@ -50,6 +50,7 @@ dependencies = [
  "regex",
  "rustc-hash",
  "shlex",
+ "tempfile",
  "which",
 ]
 
@@ -125,6 +126,17 @@ dependencies = [
  "termcolor",
 ]
 
+[[package]]
+name = "getrandom"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c9495705279e7140bf035dde1f6e750c162df8b625267cd52cc44e0b156732c8"
+dependencies = [
+ "cfg-if 1.0.0",
+ "libc",
+ "wasi",
+]
+
 [[package]]
 name = "glob"
 version = "0.3.0"
@@ -205,6 +217,12 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "19b17cddbe7ec3f8bc800887bab5e717348c95ea2ca0b1bf0837fb964dc67099"
 
+[[package]]
+name = "ppv-lite86"
+version = "0.2.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ac74c624d6b2d21f425f752262f42188365d7b8ff1aff74c82e45136510a4857"
+
 [[package]]
 name = "proc-macro2"
 version = "1.0.24"
@@ -223,6 +241,55 @@ dependencies = [
  "proc-macro2",
 ]
 
+[[package]]
+name = "rand"
+version = "0.8.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0ef9e7e66b4468674bfcb0c81af8b7fa0bb154fa9f28eb840da5c447baeb8d7e"
+dependencies = [
+ "libc",
+ "rand_chacha",
+ "rand_core",
+ "rand_hc",
+]
+
+[[package]]
+name = "rand_chacha"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e12735cf05c9e10bf21534da50a147b924d555dc7a547c42e6bb2d5b6017ae0d"
+dependencies = [
+ "ppv-lite86",
+ "rand_core",
+]
+
+[[package]]
+name = "rand_core"
+version = "0.6.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "34cf66eb183df1c5876e2dcf6b13d57340741e8dc255b48e40a26de954d06ae7"
+dependencies = [
+ "getrandom",
+]
+
+[[package]]
+name = "rand_hc"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3190ef7066a446f2e7f42e239d161e905420ccab01eb967c9eb27d21b2322a73"
+dependencies = [
+ "rand_core",
+]
+
+[[package]]
+name = "redox_syscall"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "94341e4e44e24f6b591b59e47a8a027df12e008d73fd5672dbea9cc22f4507d9"
+dependencies = [
+ "bitflags",
+]
+
 [[package]]
 name = "regex"
 version = "1.4.2"
@@ -241,6 +308,15 @@ version = "0.6.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3b181ba2dcf07aaccad5448e8ead58db5b742cf85dfe035e2227f137a539a189"
 
+[[package]]
+name = "remove_dir_all"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3acd125665422973a33ac9d3dd2df85edad0f4ae9b00dafb1a05e43a9f5ef8e7"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "rustc-hash"
 version = "1.1.0"
@@ -259,6 +335,20 @@ version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8ea5119cdb4c55b55d432abb513a0429384878c15dde60cc77b1c99de1a95a6a"
 
+[[package]]
+name = "tempfile"
+version = "3.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dac1c663cfc93810f88aed9b8941d48cabf856a1b111c29a40439018d870eb22"
+dependencies = [
+ "cfg-if 1.0.0",
+ "libc",
+ "rand",
+ "redox_syscall",
+ "remove_dir_all",
+ "winapi",
+]
+
 [[package]]
 name = "termcolor"
 version = "1.1.0"
@@ -310,6 +400,12 @@ version = "0.9.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b5a972e5669d67ba988ce3dc826706fb0a8b01471c088cb0b6110b805cc36aed"
 
+[[package]]
+name = "wasi"
+version = "0.10.2+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fd6fbd9a79829dd1ad0cc20627bf1ed606756a7f77edff7b66b7064f9cb327c6"
+
 [[package]]
 name = "which"
 version = "3.1.1"

Cargo.toml

@@ -43,6 +43,7 @@ required-features = ["clap"]
 diff = "0.1"
 clap = "2"
 shlex = "1"
+tempfile = "3"
 
 [dependencies]
 bitflags = "1.0.3"

src/codegen/mod.rs

@@ -4207,6 +4207,16 @@ pub(crate) fn codegen(
             }
         }
 
+        if let Some(spec) = context.options().depfile.as_ref() {
+            match spec.write(context.deps()) {
+                Ok(()) => info!(
+                    "Your depfile was generated successfully into: {}",
+                    spec.depfile_path.display()
+                ),
+                Err(e) => warn!("{}", e),
+            }
+        }
+
         context.resolve_item(context.root_module()).codegen(
             context,
             &mut result,

src/deps.rs

@@ -0,0 +1,20 @@
+/// Generating build depfiles from parsed bindings.
+use std::{collections::BTreeSet, path::PathBuf};
+
+#[derive(Debug)]
+pub(crate) struct DepfileSpec {
+    pub output_module: String,
+    pub depfile_path: PathBuf,
+}
+
+impl DepfileSpec {
+    pub fn write(&self, deps: &BTreeSet<String>) -> std::io::Result<()> {
+        let mut buf = format!("{}:", self.output_module);
+
+        for file in deps {
+            buf = format!("{} {}", buf, file);
+        }
+
+        std::fs::write(&self.depfile_path, &buf)
+    }
+}

src/ir/context.rs

@@ -29,7 +29,7 @@ use clang_sys;
 use proc_macro2::{Ident, Span};
 use std::borrow::Cow;
 use std::cell::{Cell, RefCell};
-use std::collections::HashMap as StdHashMap;
+use std::collections::{BTreeSet, HashMap as StdHashMap};
 use std::iter::IntoIterator;
 use std::mem;
 
@@ -354,6 +354,9 @@ pub struct BindgenContext {
     /// This needs to be an std::HashMap because the cexpr API requires it.
     parsed_macros: StdHashMap<Vec<u8>, cexpr::expr::EvalResult>,
 
+    /// A set of all the included filenames.
+    deps: BTreeSet<String>,
+
     /// The active replacements collected from replaces="xxx" annotations.
     replacements: HashMap<Vec<String>, ItemId>,
 
@@ -545,8 +548,16 @@ If you encounter an error missing from this list, please file an issue or a PR!"
         let root_module = Self::build_root_module(ItemId(0));
         let root_module_id = root_module.id().as_module_id_unchecked();
 
+        // depfiles need to include the explicitly listed headers too
+        let mut deps = BTreeSet::default();
+        if let Some(filename) = &options.input_header {
+            deps.insert(filename.clone());
+        }
+        deps.extend(options.extra_input_headers.iter().cloned());
+
         BindgenContext {
             items: vec![Some(root_module)],
+            deps,
             types: Default::default(),
             type_params: Default::default(),
             modules: Default::default(),
@@ -632,6 +643,19 @@ If you encounter an error missing from this list, please file an issue or a PR!"
         self.options().parse_callbacks.as_ref().map(|t| &**t)
     }
 
+    /// Add another path to the set of included files.
+    pub fn include_file(&mut self, filename: String) {
+        if let Some(cbs) = self.parse_callbacks() {
+            cbs.include_file(&filename);
+        }
+        self.deps.insert(filename);
+    }
+
+    /// Get any included files.
+    pub fn deps(&self) -> &BTreeSet<String> {
+        &self.deps
+    }
+
     /// Define a new item.
     ///
     /// This inserts it into the internal items set, and its type into the

src/ir/item.rs

@@ -1415,9 +1415,7 @@ impl ClangItemParser for Item {
                             );
                         }
                         Some(filename) => {
-                            if let Some(cb) = ctx.parse_callbacks() {
-                                cb.include_file(&filename)
-                            }
+                            ctx.include_file(filename);
                         }
                     }
                 }

src/lib.rs

@@ -51,6 +51,7 @@ macro_rules! doc_mod {
 
 mod clang;
 mod codegen;
+mod deps;
 mod features;
 mod ir;
 mod parse;
@@ -604,6 +605,19 @@ impl Builder {
         self
     }
 
+    /// Add a depfile output which will be written alongside the generated bindings.
+    pub fn depfile<H: Into<String>, D: Into<PathBuf>>(
+        mut self,
+        output_module: H,
+        depfile: D,
+    ) -> Builder {
+        self.options.depfile = Some(deps::DepfileSpec {
+            output_module: output_module.into(),
+            depfile_path: depfile.into(),
+        });
+        self
+    }
+
     /// Add `contents` as an input C/C++ header named `name`.
     ///
     /// The file `name` will be added to the clang arguments.
@@ -1417,11 +1431,13 @@ impl Builder {
 
         // Transform input headers to arguments on the clang command line.
         self.options.input_header = self.input_headers.pop();
-        self.options
-            .clang_args
-            .extend(self.input_headers.drain(..).flat_map(|header| {
-                iter::once("-include".into()).chain(iter::once(header))
-            }));
+        self.options.extra_input_headers = self.input_headers;
+        self.options.clang_args.extend(
+            self.options.extra_input_headers.iter().flat_map(|header| {
+                iter::once("-include".into())
+                    .chain(iter::once(header.to_string()))
+            }),
+        );
 
         self.options.input_unsaved_files.extend(
             self.input_header_contents
@@ -1624,6 +1640,9 @@ struct BindgenOptions {
     /// The explicit rustfmt path.
     rustfmt_path: Option<PathBuf>,
 
+    /// The path to which we should write a Makefile-syntax depfile (if any).
+    depfile: Option<deps::DepfileSpec>,
+
     /// The set of types that we should have bindings for in the generated
     /// code.
     ///
@@ -1785,6 +1804,9 @@ struct BindgenOptions {
     /// The input header file.
     input_header: Option<String>,
 
+    /// Any additional input header files.
+    extra_input_headers: Vec<String>,
+
     /// Unsaved files for input.
     input_unsaved_files: Vec<clang::UnsavedFile>,
 
@@ -1963,6 +1985,7 @@ impl Default for BindgenOptions {
             blocklisted_items: Default::default(),
             opaque_types: Default::default(),
             rustfmt_path: Default::default(),
+            depfile: Default::default(),
             allowlisted_types: Default::default(),
             allowlisted_functions: Default::default(),
             allowlisted_vars: Default::default(),
@@ -2008,6 +2031,7 @@ impl Default for BindgenOptions {
             module_lines: HashMap::default(),
             clang_args: vec![],
             input_header: None,
+            extra_input_headers: vec![],
             input_unsaved_files: vec![],
             parse_callbacks: None,
             codegen_config: CodegenConfig::all(),

src/options.rs

@@ -30,6 +30,10 @@ where
             Arg::with_name("header")
                 .help("C or C++ header file")
                 .required(true),
+            Arg::with_name("depfile")
+                .long("depfile")
+                .takes_value(true)
+                .help("Path to write depfile to"),
             Arg::with_name("default-enum-style")
                 .long("default-enum-style")
                 .help("The default style of code used to generate enums.")
@@ -848,8 +852,14 @@ where
 
     let output = if let Some(path) = matches.value_of("output") {
         let file = File::create(path)?;
+        if let Some(depfile) = matches.value_of("depfile") {
+            builder = builder.depfile(path, depfile);
+        }
         Box::new(io::BufWriter::new(file)) as Box<dyn io::Write>
     } else {
+        if let Some(depfile) = matches.value_of("depfile") {
+            builder = builder.depfile("-", depfile);
+        }
         Box::new(io::BufWriter::new(io::stdout())) as Box<dyn io::Write>
     };
 

tests/expectations/tests/enum-default-rust.d

@@ -0,0 +1 @@
+tests/expectations/tests/enum-default-rust.rs: tests/headers/enum-default-rust.h tests/headers/enum.h

tests/tests.rs

@@ -578,6 +578,32 @@ fn no_system_header_includes() {
         .success());
 }
 
+#[test]
+fn emit_depfile() {
+    let header = PathBuf::from("tests/headers/enum-default-rust.h");
+    let expected_depfile = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .join("tests")
+        .join("expectations")
+        .join("tests")
+        .join("enum-default-rust.d");
+    let observed_depfile = tempfile::NamedTempFile::new().unwrap();
+    let mut builder = create_bindgen_builder(&header).unwrap();
+    builder.builder = builder.builder.depfile(
+        "tests/expectations/tests/enum-default-rust.rs",
+        observed_depfile.path(),
+    );
+
+    let check_roundtrip =
+        env::var_os("BINDGEN_DISABLE_ROUNDTRIP_TEST").is_none();
+    let (builder, _roundtrip_builder) =
+        builder.into_builder(check_roundtrip).unwrap();
+    let _bindings = builder.generate().unwrap();
+
+    let observed = std::fs::read_to_string(observed_depfile).unwrap();
+    let expected = std::fs::read_to_string(expected_depfile).unwrap();
+    assert_eq!(observed.trim(), expected.trim());
+}
+
 #[test]
 fn dump_preprocessed_input() {
     let arg_keyword =