Add --depfile option

Add an option to output a depfile for outside build-systems to learn
the source file dependencies of the bindings.
This can be used by 3rd party build system integrations to only rerun bindgen
when necessary.

src/bindgen/bindings.rs

@@ -29,6 +29,7 @@ pub struct Bindings {
     constants: Vec<Constant>,
     items: Vec<ItemContainer>,
     functions: Vec<Function>,
+    source_files: Vec<path::PathBuf>,
     /// Bindings are generated by a recursive call to cbindgen
     /// and shouldn't do anything when written anywhere.
     noop: bool,
@@ -50,6 +51,7 @@ impl Bindings {
         globals: Vec<Static>,
         items: Vec<ItemContainer>,
         functions: Vec<Function>,
+        source_files: Vec<path::PathBuf>,
         noop: bool,
     ) -> Bindings {
         Bindings {
@@ -61,6 +63,7 @@ impl Bindings {
             constants,
             items,
             functions,
+            source_files,
             noop,
         }
     }
@@ -128,6 +131,53 @@ impl Bindings {
         fields
     }
 
+    pub fn generate_depfile<P: AsRef<path::Path>>(
+        &self,
+        header_path: P,
+        depfile_path: P,
+        config_file: Option<&str>,
+    ) {
+        if let Some(dir) = depfile_path.as_ref().parent() {
+            if !dir.exists() {
+                std::fs::create_dir_all(dir).unwrap()
+            }
+        }
+        let canon_header_path = header_path.as_ref().canonicalize().unwrap();
+        let mut canon_source_files: Vec<_> = self
+            .source_files
+            .iter()
+            .map(|p| p.canonicalize().unwrap())
+            .collect();
+        // Changing the config-file should also re-trigger cbindgen.
+        if let Some(file) = config_file {
+            canon_source_files.push(PathBuf::from(file).canonicalize().unwrap())
+        }
+
+        // When writing the depfile we must escape whitespace in paths to avoid it being interpreted
+        // as a seperator.
+        // It is not clear how to otherwise _correctly_ replace whitespace in a non-unicode
+        // compliant slice, without knowing the encoding, so we lossy convert such cases,
+        // to avoid panics.
+        let mut depfile = File::create(depfile_path).unwrap();
+        write!(
+            &mut depfile,
+            "{}:",
+            canon_header_path.to_string_lossy().replace(' ', "\\ ")
+        )
+        .expect("Writing header name to depfile failed");
+        canon_source_files.into_iter().for_each(|source_file| {
+            // Add line-continue and line-break and then indent with 4 spaces.
+            // This makes the output more human-readable.
+            depfile.write_all(b" \\\n    ").unwrap();
+            let escaped_path = source_file.to_string_lossy().replace(' ', "\\ ");
+            depfile.write_all(escaped_path.as_bytes()).unwrap();
+        });
+
+        writeln!(&mut depfile).unwrap();
+
+        depfile.flush().unwrap();
+    }
+
     pub fn write_to_file<P: AsRef<path::Path>>(&self, path: P) -> bool {
         if self.noop {
             return false;

src/bindgen/builder.rs

@@ -359,6 +359,7 @@ impl Builder {
                 Default::default(),
                 Default::default(),
                 Default::default(),
+                Default::default(),
                 true,
             ));
         }
@@ -391,6 +392,8 @@ impl Builder {
             result.extend_with(&parser::parse_lib(cargo, &self.config)?);
         }
 
+        result.source_files.extend_from_slice(self.srcs.as_slice());
+
         Library::new(
             self.config,
             result.constants,
@@ -401,6 +404,7 @@ impl Builder {
             result.opaque_items,
             result.typedefs,
             result.functions,
+            result.source_files,
         )
         .generate()
     }

src/bindgen/library.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use std::collections::HashMap;
+use std::path::PathBuf;
 
 use crate::bindgen::bindings::Bindings;
 use crate::bindgen::config::{Config, Language, SortKey};
@@ -25,6 +26,7 @@ pub struct Library {
     opaque_items: ItemMap<OpaqueItem>,
     typedefs: ItemMap<Typedef>,
     functions: Vec<Function>,
+    source_files: Vec<PathBuf>,
 }
 
 impl Library {
@@ -39,6 +41,7 @@ impl Library {
         opaque_items: ItemMap<OpaqueItem>,
         typedefs: ItemMap<Typedef>,
         functions: Vec<Function>,
+        source_files: Vec<PathBuf>,
     ) -> Library {
         Library {
             config,
@@ -50,6 +53,7 @@ impl Library {
             opaque_items,
             typedefs,
             functions,
+            source_files,
         }
     }
 
@@ -135,6 +139,7 @@ impl Library {
             globals,
             items,
             functions,
+            self.source_files,
             false,
         ))
     }

src/bindgen/parser.rs

@@ -181,6 +181,8 @@ impl<'a> Parser<'a> {
             }
         }
 
+        self.out.source_files = self.cache_src.keys().map(|k| k.to_owned()).collect();
+
         Ok(())
     }
 
@@ -406,6 +408,7 @@ pub struct Parse {
     pub opaque_items: ItemMap<OpaqueItem>,
     pub typedefs: ItemMap<Typedef>,
     pub functions: Vec<Function>,
+    pub source_files: Vec<FilePathBuf>,
 }
 
 impl Parse {
@@ -419,6 +422,7 @@ impl Parse {
             opaque_items: ItemMap::default(),
             typedefs: ItemMap::default(),
             functions: Vec::new(),
+            source_files: Vec::new(),
         }
     }
 
@@ -466,6 +470,7 @@ impl Parse {
         self.opaque_items.extend_with(&other.opaque_items);
         self.typedefs.extend_with(&other.typedefs);
         self.functions.extend_from_slice(&other.functions);
+        self.source_files.extend_from_slice(&other.source_files);
     }
 
     fn load_syn_crate_mod<'a>(

src/main.rs

@@ -262,6 +262,20 @@ fn main() {
                 .help("Report errors only (overrides verbosity options).")
                 .required(false),
         )
+        .arg(
+            Arg::new("depfile")
+                .value_name("PATH")
+                .long("depfile")
+                .takes_value(true)
+                .min_values(1)
+                .max_values(1)
+                .required(false)
+                .help("Generate a depfile at the given Path listing the source files \
+                    cbindgen traversed when generating the bindings. Useful when \
+                    integrating cbindgen into 3rd party build-systems. \
+                    This option is ignored if `--out` is missing."
+                )
+        )
         .get_matches();
 
     if !matches.is_present("out") && matches.is_present("verify") {
@@ -306,6 +320,9 @@ fn main() {
                 error!("Bindings changed: {}", file);
                 std::process::exit(2);
             }
+            if let Some(depfile) = matches.value_of("depfile") {
+                bindings.generate_depfile(file, depfile, matches.value_of("config"))
+            }
         }
         _ => {
             bindings.write(io::stdout());