Use Cargo.lock from workspace root

cbindgen currently assumes that the `Cargo.lock` is in directly in the
crate directory as a sibling to `Cargo.toml`; however that is usually
not the case inside a workspace.

Instead, this PR extracts the workspace root from the output of `cargo
metadata` [1] (already used to get a list of packages) and uses the
`Cargo.lock` from there.

 1. This is available since Rust 1.24; see rust-lang/cargo#4940

src/bindgen/cargo/cargo.rs

@@ -46,10 +46,13 @@ impl Cargo {
         clean: bool,
     ) -> Result<Cargo, Error> {
         let toml_path = crate_dir.join("Cargo.toml");
-        let lock_path = match lock_file {
-            Some(v) => PathBuf::from(v),
-            None => crate_dir.join("Cargo.lock"),
-        };
+        let metadata = cargo_metadata::metadata(&toml_path)
+            .map_err(|x| Error::CargoMetadata(toml_path.to_str().unwrap().to_owned(), x))?;
+        let lock_path = lock_file
+            .map(PathBuf::from)
+            .unwrap_or_else(|| {
+                Path::new(&metadata.workspace_root).join("Cargo.lock")
+            });
 
         let lock = if use_cargo_lock {
             match cargo_lock::lock(&lock_path) {
@@ -62,8 +65,6 @@ impl Cargo {
         } else {
             None
         };
-        let metadata = cargo_metadata::metadata(&toml_path)
-            .map_err(|x| Error::CargoMetadata(toml_path.to_str().unwrap().to_owned(), x))?;
 
         // Use the specified binding crate name or infer it from the manifest
         let manifest = cargo_toml::manifest(&toml_path)

src/bindgen/cargo/cargo_metadata.rs

@@ -24,6 +24,8 @@ pub struct Metadata {
     /// A list of all crates referenced by this crate (and the crate itself)
     pub packages: Vec<Package>,
     version: usize,
+    /// path to the workspace containing the `Cargo.lock`
+    pub workspace_root: String,
 }
 
 #[derive(Clone, Deserialize, Debug)]