For cargo completion script, output line to source real script

This allows the cargo script for the current default toolchain to be called,
and if the script is ever updated the user won't have to maually update their
completion script after updating Rust.

Signed-off-by: Gabriel Smith <[email protected]>

src/rustup-cli/rustup_mode.rs

@@ -1097,26 +1097,19 @@ fn output_completion_script(shell: Shell, command: CompletionCommand) -> Result<
             cli().gen_completions_to("rustup", shell, &mut io::stdout());
         }
         CompletionCommand::Cargo => {
-            let prefix = "$(rustc --print sysroot)";
-            match shell {
-                Shell::Bash => {
-                    writeln!(
-                        &mut io::stdout(),
-                        "{}{}",
-                        prefix,
-                        "/etc/bash_completion.d/cargo"
-                    )?;
-                }
-                Shell::Zsh => {
-                    writeln!(
-                        &mut io::stdout(),
-                        "{}{}",
-                        prefix,
-                        "/share/zsh/site-functions/_cargo"
-                    )?;
-                }
-                _ => {}
+            let script = match shell {
+                Shell::Bash => Some("/etc/bash_completion.d/cargo"),
+                Shell::Zsh => Some("/share/zsh/site-functions/_cargo"),
+                _ => None,
             };
+
+            if let Some(script) = script {
+                writeln!(
+                    &mut io::stdout(),
+                    "source $(rustc --print sysroot){}",
+                    script,
+                )?;
+            }
         }
     }