Basic implementation of cargo clippy --all

This implements workspace support for `cargo clippy` by running clippy
over all packages in the workspace (in serial).

This should probably be parallelised in future (as `cargo build --all`).

src/main.rs

@@ -152,6 +152,7 @@ Common options:
     -h, --help               Print this message
     --features               Features to compile for the package
     -V, --version            Print version info and exit
+    --all                    @@@ something sensible here (copy from cargo-edit?)
 
 Other options are the same as `cargo rustc`.
 
@@ -217,6 +218,9 @@ pub fn main() {
                 .expect("manifest path could not be canonicalized")
         });
 
+        let packages = if std::env::args().any(|a| a == "--all" ) {
+            metadata.packages
+        } else {
             let package_index = {
                 if let Some(manifest_path) = manifest_path {
                     metadata.packages.iter().position(|package| {
@@ -265,9 +269,16 @@ pub fn main() {
                 }
             }.expect("could not find matching package");
 
-        let package = metadata.packages.remove(package_index);
+            vec![metadata.packages.remove(package_index)]
+        };
+
+        for package in packages {
+            let manifest_path = package.manifest_path;
+
             for target in package.targets {
-            let args = std::env::args().skip(2);
+                let args = std::env::args().skip(2).filter(|a| a != "--all" && !a.starts_with("--manifest-path="));
+
+                let args = std::iter::once(format!("--manifest-path={}", manifest_path)).chain(args);
                 if let Some(first) = target.kind.get(0) {
                     if target.kind.len() > 1 || first.ends_with("lib") {
                         if let Err(code) = process(std::iter::once("--lib".to_owned()).chain(args)) {
@@ -287,6 +298,7 @@ pub fn main() {
                     panic!("badly formatted cargo metadata: target::kind is an empty array");
                 }
             }
+        }
     } else {
         // this arm is executed when cargo-clippy runs `cargo rustc` with the `RUSTC`
         // env var set to itself