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.

src/bin/cargo/cli.rs

@@ -1,4 +1,4 @@
-use anyhow::anyhow;
+use anyhow::{anyhow, Context as _};
 use cargo::core::{features, CliUnstable};
 use cargo::{self, drop_print, drop_println, CliResult, Config};
 use clap::{
@@ -55,6 +55,13 @@ pub fn main(config: &mut Config) -> CliResult {
         }
     };
 
+    if let Some(new_cwd) = args.get_one::<std::path::PathBuf>("directory") {
+        // Change the configured directory to work in, before any config files are read
+        config
+            .set_cwd(new_cwd.as_path())
+            .with_context(|| "could not change to requested directory")?;
+    }
+
     // Global args need to be extracted before expanding aliases because the
     // clap code for extracting a subcommand discards global options
     // (appearing before the subcommand).
@@ -473,6 +480,13 @@ 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))

src/cargo/util/config/mod.rs

@@ -483,6 +483,23 @@ impl Config {
         Ok(())
     }
 
+    /// Updates the application's notion of current working directory, both in
+    /// the process environment as well as the configuration data
+    pub fn set_cwd(&mut self, new_cwd: &Path) -> CargoResult<()> {
+        // Update the process-level notion of cwd
+        std::env::set_current_dir(&new_cwd)?;
+        // Update struct members derived from the process-wide cwd
+        // processing occurs
+        self.cwd = new_cwd.to_path_buf();
+        self.home_path = Filesystem::new(homedir(new_cwd).ok_or_else(|| {
+            anyhow!(
+                "Cargo couldn't find your home directory. \
+                     This probably means that $HOME was not set."
+            )
+        })?);
+        Ok(())
+    }
+
     /// The current working directory.
     pub fn cwd(&self) -> &Path {
         &self.cwd