feat(cli): emit error when install is given a toolchain name instead …

…of crate name

src/bin/cargo/commands/install.rs

@@ -1,5 +1,6 @@
 use crate::command_prelude::*;
 
+use anyhow::anyhow;
 use cargo::core::{GitReference, SourceId, Workspace};
 use cargo::ops;
 use cargo::util::IntoUrl;
@@ -108,6 +109,16 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
         .map(|k| resolve_crate(k, version))
         .collect::<crate::CargoResult<Vec<_>>>()?;
 
+    for (crate_name, _) in krates.iter() {
+        if let Some(toolchain) = crate_name.strip_prefix("+") {
+            return Err(anyhow!(
+                "invalid character `+` in dependency name: \"+{toolchain}\"
+    Use `cargo +{toolchain} install` if you meant to use the `{toolchain}` toolchain."
+            )
+            .into());
+        }
+    }
+
     let mut from_cwd = false;
 
     let source = if let Some(url) = args.get_one::<String>("git") {

tests/testsuite/install.rs

@@ -57,6 +57,20 @@ fn simple() {
     assert_has_not_installed_exe(cargo_home(), "foo");
 }
 
+#[cargo_test]
+fn toolchain() {
+    pkg("foo", "0.0.1");
+
+    cargo_process("install +nightly")
+        .with_status(101)
+        .with_stderr(
+            "\
+[ERROR] invalid character `+` in dependency name: \"+nightly\"
+    Use `cargo +nightly install` if you meant to use the `nightly` toolchain.",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn simple_with_message_format() {
     pkg("foo", "0.0.1");