fix(update): Improve error on bad argument order

This is inspired by `cargo install`

src/bin/cargo/commands/update.rs

@@ -1,5 +1,6 @@
 use crate::command_prelude::*;
 
+use anyhow::anyhow;
 use cargo::ops::{self, UpdateOptions};
 use cargo::util::print_available_packages;
 
@@ -58,11 +59,21 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     } else {
         "package2"
     };
+    let to_update = values(args, to_update);
+    for crate_name in to_update.iter() {
+        if let Some(toolchain) = crate_name.strip_prefix("+") {
+            return Err(anyhow!(
+                "invalid character `+` in package name: `+{toolchain}`
+    Use `cargo +{toolchain} update` if you meant to use the `{toolchain}` toolchain."
+            )
+            .into());
+        }
+    }
 
     let update_opts = UpdateOptions {
         aggressive: args.flag("aggressive"),
         precise: args.get_one::<String>("precise").map(String::as_str),
-        to_update: values(args, to_update),
+        to_update,
         dry_run: args.dry_run(),
         workspace: args.flag("workspace"),
         config,

tests/testsuite/cargo_update/mod.rs

@@ -1 +1,2 @@
 mod help;
+mod toolchain_pkgname;

tests/testsuite/cargo_update/toolchain_pkgname/in/Cargo.toml

@@ -0,0 +1,5 @@
+[package]
+name = "test"
+version = "0.1.0"
+
+[dependencies]

tests/testsuite/cargo_update/toolchain_pkgname/in/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, world!");
+}

tests/testsuite/cargo_update/toolchain_pkgname/mod.rs

@@ -0,0 +1,19 @@
+use cargo_test_support::curr_dir;
+use cargo_test_support::prelude::*;
+use cargo_test_support::Project;
+
+#[cargo_test]
+fn case() {
+    let project = Project::from_template(curr_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("update")
+        .arg("+stable")
+        .current_dir(cwd)
+        .assert()
+        .code(101)
+        .stdout_matches_path(curr_dir!().join("stdout.log"))
+        .stderr_matches_path(curr_dir!().join("stderr.log"));
+}

tests/testsuite/cargo_update/toolchain_pkgname/stderr.log

@@ -0,0 +1,2 @@
+error: invalid character `+` in package name: `+stable`
+    Use `cargo +stable update` if you meant to use the `stable` toolchain.