Add '--directory,-C' flag for changing current dir before build

This implements the suggestion in rust-lang#10098 to make cargo change cwd before
completing config processing and starting the build. It is also an
alternative to --manifest-path that resolves the issue described
in rust-lang#2930.

Author: William Page

src/bin/cargo/cli.rs

@@ -1,4 +1,4 @@
-use anyhow::anyhow;
+use anyhow::{anyhow, Context as _};
 use cargo::core::shell::Shell;
 use cargo::core::{features, CliUnstable};
 use cargo::{self, drop_print, drop_println, CliResult, Config};
@@ -27,6 +27,12 @@ lazy_static::lazy_static! {
 pub fn main(config: &mut LazyConfig) -> CliResult {
     let args = cli().try_get_matches()?;
 
+    // Update the process-level notion of cwd
+    if let Some(new_cwd) = args.get_one::<std::path::PathBuf>("directory") {
+        std::env::set_current_dir(&new_cwd)
+            .with_context(|| "could not change to requested directory")?;
+    }
+
     // CAUTION: Be careful with using `config` until it is configured below.
     // In general, try to avoid loading config values unless necessary (like
     // the [alias] table).
@@ -467,6 +473,13 @@ See 'cargo help <command>' for more information on a specific command.\n",
                 .value_name("WHEN")
                 .global(true),
         )
+        .arg(
+            opt("directory", "Change to DIRECTORY before doing anything")
+                .short('C')
+                .value_name("DIRECTORY")
+                .value_hint(clap::ValueHint::DirPath)
+                .value_parser(clap::builder::ValueParser::path_buf()),
+        )
         .arg(flag("frozen", "Require Cargo.lock and cache are up to date").global(true))
         .arg(flag("locked", "Require Cargo.lock is up to date").global(true))
         .arg(flag("offline", "Run without accessing the network").global(true))