xtask-specific binaries in GRAPEVINE_XTASK_PATH

Some possible alternatives to this:

 - Keep putting them in PATH.
 - Make xtask a nix derivation. We would lose out on incremental
   compilation this way, and would end up recompiling xtask from scratch
   whenever something in the main package changed.
 - Have xtask call `nix build --inputs-from $toplevel nixpkgs#go` and
   such. Slow and tedious.

xtask/src/main.rs

@@ -1,8 +1,10 @@
 mod complement;
 
-use std::process::ExitCode;
+use std::{env, ffi::OsString, process::ExitCode};
 
 use clap::{Parser, Subcommand};
+use miette::{miette, IntoDiagnostic, Result, WrapErr};
+use xshell::Shell;
 
 #[derive(Parser)]
 struct Args {
@@ -16,11 +18,7 @@ enum Command {
 }
 
 fn main() -> ExitCode {
-    let args = Args::parse();
-    let result = match args.command {
-        Command::Complement(args) => complement::main(args),
-    };
-    let Err(e) = result else {
+    let Err(e) = try_main() else {
         return ExitCode::SUCCESS;
     };
     // Include a leading newline because sometimes an error will occur in
@@ -28,3 +26,49 @@ fn main() -> ExitCode {
     eprintln!("\n{e:?}");
     ExitCode::FAILURE
 }
+
+fn try_main() -> Result<()> {
+    let args = Args::parse();
+    let sh = new_shell()?;
+    match args.command {
+        Command::Complement(args) => complement::main(args, &sh),
+    }
+}
+
+fn new_shell() -> Result<Shell> {
+    let path = get_shell_path()?;
+    let sh = Shell::new()
+        .into_diagnostic()
+        .wrap_err("failed to initialize internal xshell::Shell wrapper")?;
+    sh.set_var("PATH", path);
+    Ok(sh)
+}
+
+/// Returns the value to set the `PATH` environment variable to in
+/// [`xshell::Shell`] instances.
+///
+/// This function appends the paths from the `GRAPEVINE_XTASK_PATH` environment
+/// variable to the existing value of `PATH` set in the xtask process.
+///
+/// Executable dependencies that are only called by commands in xtask should be
+/// added to `GRAPEVINE_XTASK_PATH` instead of `PATH` in the devshell, to avoid
+/// polluting the devshell path with extra entries.
+fn get_shell_path() -> Result<OsString> {
+    let xtask_path = env::var_os("GRAPEVINE_XTASK_PATH").ok_or(miette!(
+        help = "This tool must be run from inside the Grapevine devshell. \
+                Make sure you didn't interrupt direnv or something similar.",
+        "GRAPEVINE_XTASK_PATH environment variable is unset"
+    ))?;
+    if let Some(path) = env::var_os("PATH") {
+        let old_paths = env::split_paths(&path);
+        let xtask_paths = env::split_paths(&xtask_path);
+        env::join_paths(old_paths.chain(xtask_paths))
+            .into_diagnostic()
+            .wrap_err(
+                "error constructing new PATH value to include the paths from \
+                 GRAPEVINE_XTASK_PATH",
+            )
+    } else {
+        Ok(xtask_path)
+    }
+}