feat(rustup-mode): add no_update flag to rustup toolchain install

src/cli/rustup_mode.rs

@@ -378,6 +378,10 @@ struct UpdateOpts {
     #[arg(long)]
     no_self_update: bool,
 
+    /// Don't try to update the installed toolchain
+    #[arg(long)]
+    no_update: bool,
+
     /// Force an update, even if some components are missing
     #[arg(long)]
     force: bool,
@@ -967,8 +971,12 @@ async fn update(
             let profile = cfg.get_profile()?;
             let status = match DistributableToolchain::new(cfg, desc.clone()) {
                 Ok(mut d) => {
-                    d.update_extra(&components, &targets, profile, force, allow_downgrade)
-                        .await?
+                    if !opts.no_update {
+                        d.update_extra(&components, &targets, profile, force, allow_downgrade)
+                            .await?
+                    } else {
+                        UpdateStatus::Unchanged
+                    }
                 }
                 Err(RustupError::ToolchainNotInstalled { .. }) => {
                     DistributableToolchain::install(

tests/suite/cli_rustup.rs

@@ -1737,6 +1737,32 @@ info: it's active because: overridden by '[TOOLCHAIN_FILE]'
         .is_ok();
 }
 
+#[tokio::test]
+async fn toolchain_install_no_change_with_no_update() {
+    let mut cx = CliTestContext::new(Scenario::None).await;
+
+    {
+        let cx = cx.with_dist_dir(Scenario::ArchivesV2_2015_01_01);
+        cx.config
+            .expect(["rustup", "toolchain", "add", "stable"])
+            .await
+            .is_ok();
+    }
+
+    let cx = cx.with_dist_dir(Scenario::SimpleV2);
+    cx.config
+        .expect(["rustup", "install", "--no-update", "stable"])
+        .await
+        .with_stdout(snapbox::str![[r#"
+
+  stable-[HOST_TRIPLE] unchanged - 1.0.0 (hash-stable-1.0.0)
+
+
+"#]])
+        .with_stderr(snapbox::str![[""]])
+        .is_ok();
+}
+
 #[tokio::test]
 async fn toolchain_update_is_like_update() {
     let cx = CliTestContext::new(Scenario::SimpleV2).await;