fix(toolchain): forbid toolchain names starting with +

src/toolchain/names.rs

@@ -67,6 +67,8 @@ pub enum InvalidName {
     ToolchainPath(String),
     #[error("invalid toolchain name '{0}'")]
     ToolchainName(String),
+    #[error("invalid toolchain name '+{0}'; valid toolchain names do not start with '+'")]
+    PlusPrefix(String),
 }
 
 macro_rules! from_variant {
@@ -117,6 +119,9 @@ macro_rules! try_from_str {
 
 /// Common validate rules for all sorts of toolchain names
 fn validate(candidate: &str) -> Result<&str, InvalidName> {
+    if let Some(without_plus) = candidate.strip_prefix('+') {
+        return Err(InvalidName::PlusPrefix(without_plus.to_string()));
+    }
     let normalized_name = candidate.trim_end_matches('/');
     if normalized_name.is_empty() {
         Err(InvalidName::ToolchainName(candidate.into()))
@@ -501,8 +506,9 @@ mod tests {
 
     prop_compose! {
         fn arb_custom_name()
-            (s in r"[^\\/]+") -> String {
+            (s in r"[^\\/+][^\\/]*") -> String {
                 // perhaps need to filter 'none' and partial toolchains - but they won't typically be generated anyway.
+                // Also filter '+' prefix as that's reserved for +toolchain syntax.
                 s
         }
     }

tests/suite/cli_rustup.rs

@@ -2222,6 +2222,25 @@ error:[..] relative path toolchain '[..]/toolchains/nightly-[HOST_TRIPLE]'
         .is_err();
 }
 
+#[tokio::test]
+async fn run_with_plus_prefix_fails_with_helpful_message() {
+    let cx = CliTestContext::new(Scenario::SimpleV2).await;
+    cx.config
+        .expect(["rustup", "default", "stable"])
+        .await
+        .is_ok();
+
+    cx.config
+        .expect(["rustup", "run", "+stable", "cargo", "check"])
+        .await
+        .with_stderr(snapbox::str![[r#"
+...
+error:[..] invalid toolchain name '+stable'; valid toolchain names do not start with '+'
+...
+"#]])
+        .is_err();
+}
+
 #[tokio::test]
 async fn plus_override_abspath_is_supported() {
     let cx = CliTestContext::new(Scenario::SimpleV2).await;