feat(cli): emit error when add is given a toolchain name instead of a crate name

src/bin/cargo/commands/add.rs

@@ -242,7 +242,20 @@ fn parse_dependencies(config: &Config, matches: &ArgMatches) -> CargoResult<Vec<
         .flatten()
         .map(|c| (Some(c.clone()), None))
         .collect::<IndexMap<_, _>>();
+
     let mut infer_crate_name = false;
+
+    for (crate_name, _) in crates.iter() {
+        let crate_name = crate_name.as_ref().unwrap();
+
+        if let Some(toolchain) = crate_name.strip_prefix("+") {
+            anyhow::bail!(
+                "invalid character `+` in dependency name: `+{toolchain}`
+    Use `cargo +{toolchain} add` if you meant to use the `{toolchain}` toolchain."
+            );
+        }
+    }
+
     if crates.is_empty() {
         if path.is_some() || git.is_some() {
             crates.insert(None, None);

tests/testsuite/cargo_add/add_toolchain/in

@@ -0,0 +1 @@
+../add-basic.in

tests/testsuite/cargo_add/add_toolchain/mod.rs

@@ -0,0 +1,23 @@
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::prelude::*;
+use cargo_test_support::Project;
+
+use cargo_test_support::curr_dir;
+
+#[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("add")
+        .arg_line("+nightly")
+        .current_dir(cwd)
+        .assert()
+        .failure()
+        .stdout_matches_path(curr_dir!().join("stdout.log"))
+        .stderr_matches_path(curr_dir!().join("stderr.log"));
+
+    assert_ui().subset_matches(curr_dir!().join("out"), &project_root);
+}

tests/testsuite/cargo_add/add_toolchain/out/Cargo.toml

@@ -0,0 +1,5 @@
+[workspace]
+
+[package]
+name = "cargo-list-test-fixture"
+version = "0.0.0"

tests/testsuite/cargo_add/add_toolchain/stderr.log

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

tests/testsuite/cargo_add/mod.rs

@@ -1,6 +1,7 @@
 mod add_basic;
 mod add_multiple;
 mod add_normalized_name_external;
+mod add_toolchain;
 mod build;
 mod build_prefer_existing_version;
 mod change_rename_target;