Move the target-guessing code from #676 to its own function.

Seems nicer.

src/bindgen/cargo/cargo_metadata.rs

@@ -176,6 +176,41 @@ impl PartialEq for Dependency {
 
 impl Eq for Dependency {}
 
+fn discover_target(manifest_path: &Path) -> Option<String> {
+    if let Ok(target) = std::env::var("TARGET") {
+        return Some(target);
+    }
+
+    // We must be running as a standalone script, not under cargo.
+    // Let's use the host platform instead.
+    // We figure out the host platform through rustc and use that.
+    // We unfortunatelly cannot go through cargo, since cargo rustc _also_ builds.
+    // If `rustc` fails to run, we just fall back to not passing --filter-platforms.
+    //
+    // NOTE: We set the current directory in case of rustup shenanigans.
+    let rustc = env::var("RUSTC").unwrap_or_else(|_| String::from("rustc"));
+    debug!("Discovering host platform by {:?}", rustc);
+
+    let rustc_output = Command::new(rustc)
+        .current_dir(manifest_path.parent().unwrap())
+        .arg("-vV")
+        .output();
+    let rustc_output = match rustc_output {
+        Ok(ref out) => String::from_utf8_lossy(&out.stdout),
+        Err(..) => return None,
+    };
+
+    let field = "host: ";
+    rustc_output.lines().find_map(|l| {
+        // XXX l.strip_prefix(field) re-implemented to preserve MSRV
+        if l.starts_with(field) {
+            Some(l[field.len()..].into())
+        } else {
+            None
+        }
+    })
+}
+
 /// The main entry point to obtaining metadata
 pub fn metadata(
     manifest_path: &Path,
@@ -186,49 +221,18 @@ pub fn metadata(
     let metadata = match existing_metadata_file {
         Some(path) => Cow::Owned(std::fs::read_to_string(path)?),
         None => {
-            let mut target = None;
-
-            if only_target {
-                target = std::env::var_os("TARGET");
-            }
-
-            if only_target && target.is_none() {
-                // We must be running as a standalone script, not under cargo.
-                // Let's use the host platform instead.
-                // We figure out the host platform through rustc and use that.
-                // We unfortunatelly cannot go through cargo, since cargo rustc _also_ builds.
-                // If `rustc` fails to run, we just fall back to not passing --filter-platforms.
-                //
-                // NOTE: We set the current directory in case of rustup shenanigans.
-                let rustc = env::var("RUSTC").unwrap_or_else(|_| String::from("rustc"));
-                let rustc = Command::new(rustc)
-                    .current_dir(manifest_path.parent().unwrap())
-                    .arg("-vV")
-                    .output();
-                log::debug!("Discovering host platform by {:?}", rustc);
-                if let Ok(out) = rustc {
-                    if let Ok(stdout) = std::str::from_utf8(&out.stdout) {
-                        let field = "host: ";
-                        let value = stdout.lines().find_map(|l| {
-                            // XXX l.strip_prefix(field) re-implemented to preserve MSRV
-                            if l.starts_with(field) {
-                                Some(&l[field.len()..])
-                            } else {
-                                None
-                            }
-                        });
-                        if let Some(value) = value {
-                            target = Some(value.into());
-                        }
-                    }
-                }
+            let target = if only_target {
+                let target = discover_target(manifest_path);
                 if target.is_none() {
-                    log::warn!(
+                    warn!(
                         "Failed to discover host platform for cargo metadata; \
                         will fetch dependencies for all platforms."
                     );
                 }
-            }
+                target
+            } else {
+                None
+            };
 
             let cargo = env::var("CARGO").unwrap_or_else(|_| String::from("cargo"));
             let mut cmd = Command::new(cargo);

src/bindgen/config.rs

@@ -968,6 +968,10 @@ pub struct Config {
     /// If this value is instead to `true`, cbindgen will _not_ download the `windows` dependency
     /// if it's not compiling for Windows, but will also fail to generate a Windows binding for
     /// `Error` as it does not know the definition for `ErrorCode`.
+    ///
+    /// The target can be chosen via the `TARGET` environment variable (if used
+    /// via the CLI, when ran from a build script cargo sets this variable
+    /// appropriately).
     pub only_target_dependencies: bool,
     /// Configuration options specific to Cython.
     pub cython: CythonConfig,