improve error message for no such subcommand

src/bin/cargo/main.rs

@@ -168,7 +168,17 @@ fn execute_external_subcommand(config: &Config, cmd: &str, args: &[&str]) -> Cli
         None => {
             let suggestions = list_commands(config);
             let did_you_mean = closest_msg(cmd, suggestions.keys(), |c| c);
-            let err = anyhow::format_err!("no such subcommand: `{}`{}", cmd, did_you_mean);
+
+            let err = if cmd.contains('+') {
+                anyhow::format_err!("no such subcommand: `{}`{}\n\n\tCargo does not handle `+toolchain` directives itself.\n\tDid you mean to run `cargo` through `rustup` instead?", cmd, did_you_mean)
+            } else {
+                if did_you_mean.is_empty() {
+                    anyhow::format_err!("no such subcommand: `{}`\n\n\tA similar Cargo subcommand could not be found\n\tView a list of installed Cargo subcommands using `cargo --list`", cmd)
+                } else {
+                    anyhow::format_err!("no such subcommand: `{}`{}", cmd, did_you_mean)
+                }
+            };
+
             return Err(CliError::new(err, 101));
         }
     };

tests/testsuite/cargo_command.rs

@@ -273,7 +273,7 @@ fn find_closest_dont_correct_nonsense() {
 fn displays_subcommand_on_error() {
     cargo_process("invalid-command")
         .with_status(101)
-        .with_stderr("[ERROR] no such subcommand: `invalid-command`\n")
+        .with_stderr("[ERROR] no such subcommand: `invalid-command`\n\n<tab>A similar Cargo subcommand could not be found\n<tab>View a list of installed Cargo subcommands using `cargo --list`")
         .run();
 }