When using rustup, override RUSTUP_TOOLCHAIN

tools/cargos/src/main.rs

@@ -27,7 +27,7 @@
 use std::{
     env, fmt,
     io::{self, Read as _},
-    process::{self, Command},
+    process::{self, Command, Output},
 };
 
 use serde_json::{Map, Value};
@@ -92,11 +92,10 @@ fn get_toolchain_versions() -> Versions {
     // use Cargo with that toolchain, `pkg-meta` would return `null` when asked
     // to retrieve the `rust-version` field. This also requires `RUSTFLAGS=''`
     // to override any unstable `RUSTFLAGS` set by the caller.
-    let output = Command::new("rustup")
-        .args(["run", "stable", "cargo", "metadata", "--format-version", "1"])
-        .env("CARGO_TARGET_DIR", "target/cargos")
-        .output()
-        .unwrap();
+    let output = rustup(
+        ["run", "stable", "cargo", "metadata", "--format-version", "1"],
+        Some(("CARGO_TARGET_DIR", "target/cargos")),
+    );
 
     let json = serde_json::from_slice::<Value>(&output.stdout).unwrap();
     let packages = json.as_object().unwrap().get("packages").unwrap();
@@ -124,14 +123,10 @@ fn get_toolchain_versions() -> Versions {
 }
 
 fn is_toolchain_installed(versions: &Versions, name: &str) -> Result<bool, Error> {
-    let output = Command::new("rustup")
-        .args(["run", versions.get(name)?, "cargo", "version"])
-        .output()
-        .unwrap();
-
+    let version = versions.get(name)?;
+    let output = rustup(["run", version, "cargo", "version"], None);
     if output.status.success() {
-        let output = Command::new("rustup").args(["component", "list"]).output().unwrap();
-
+        let output = rustup([&format!("+{version}"), "component", "list"], None);
         let stdout = String::from_utf8(output.stdout).unwrap();
         Ok(stdout.contains("rust-src (installed)"))
     } else {
@@ -147,9 +142,8 @@ fn install_toolchain_or_exit(versions: &Versions, name: &str) -> Result<(), Erro
         process::exit(1);
     }
 
-    eprintln!("[cargos] would you like to install toolchain '{name}' and component 'rust-src' via 'rustup'? yn");
-
     loop {
+        eprint!("[cargos] would you like to install toolchain '{name}' and component 'rust-src' via 'rustup' (y/n)? ");
         let mut input = [0];
         io::stdin().read_exact(&mut input).unwrap();
         match input[0] as char {
@@ -176,6 +170,18 @@ fn get_rustflags(name: &str) -> &'static str {
     }
 }
 
+fn rustup<'a>(args: impl IntoIterator<Item = &'a str>, env: Option<(&str, &str)>) -> Output {
+    let mut cmd = Command::new("rustup");
+    // It's important to set `RUSTUP_TOOLCHAIN` to override any value set while
+    // running this program. That variable overrides any `+<version>` CLI
+    // argument.
+    cmd.args(args).env("RUSTUP_TOOLCHAIN", "");
+    if let Some((name, val)) = env {
+        cmd.env(name, val);
+    }
+    cmd.output().unwrap()
+}
+
 fn delegate_cargo() -> Result<(), Error> {
     let mut args = env::args();
     let this = args.next().unwrap();