feat: improve error message for rustup which

Also adds more tests for `rustup which`. Includes:
- ask binary included by uninstalled component
- ask binary included by uninstalled component with non-default
  toolchain
- ask binary not known to be included by what toolchain

Author: Bogay

src/cli/rustup_mode.rs

@@ -19,7 +19,6 @@ use itertools::Itertools;
 use tracing::{info, trace, warn};
 use tracing_subscriber::{EnvFilter, Registry, reload::Handle};
 
-use crate::dist::AutoInstallMode;
 use crate::{
     cli::{
         common::{self, PackageUpdate, update_console_filter},
@@ -28,10 +27,10 @@ use crate::{
         self_update::{self, SelfUpdateMode, check_rustup_update},
         topical_doc,
     },
-    command,
+    command, component_for_bin,
     config::{ActiveReason, Cfg},
     dist::{
-        PartialToolchainDesc, Profile, TargetTriple,
+        AutoInstallMode, PartialToolchainDesc, Profile, TargetTriple,
         manifest::{Component, ComponentStatus},
     },
     errors::RustupError,
@@ -1034,12 +1033,28 @@ async fn which(
     binary: &str,
     toolchain: Option<ResolvableToolchainName>,
 ) -> Result<utils::ExitCode> {
-    let binary_path = cfg.resolve_toolchain(toolchain).await?.binary_file(binary);
+    let toolchain = cfg.resolve_toolchain(toolchain).await?;
+    let binary_path = toolchain.binary_file(binary);
+    if utils::is_file(&binary_path) {
+        writeln!(cfg.process.stdout().lock(), "{}", binary_path.display())?;
+        return Ok(utils::ExitCode(0));
+    }
+
+    let toolchain_name = toolchain.name();
+    let Some(component_name) = component_for_bin(binary) else {
+        return Err(anyhow!(
+            "unknown binary '{binary}' in toolchain '{toolchain_name}'",
+        ));
+    };
 
-    utils::assert_is_file(&binary_path)?;
+    let selector = match cfg.active_toolchain() {
+        Ok(Some((t, _))) if &t == toolchain_name => String::new(),
+        _ => format!("--toolchain {} ", toolchain.name()),
+    };
 
-    writeln!(cfg.process.stdout().lock(), "{}", binary_path.display())?;
-    Ok(utils::ExitCode(0))
+    Err(anyhow!(
+        "'{binary}' is not installed for the toolchain '{toolchain_name}'.\nTo install, run `rustup component add {selector}{component_name}`"
+    ))
 }
 
 #[tracing::instrument(level = "trace", skip_all)]

tests/suite/cli_misc.rs

@@ -1384,6 +1384,70 @@ async fn which_asking_uninstalled_toolchain() {
         .is_ok();
 }
 
+#[tokio::test]
+async fn which_asking_uninstalled_components() {
+    let cx = CliTestContext::new(Scenario::SimpleV2).await;
+
+    let path_1 = cx.config.customdir.join("custom-1");
+    let path_1 = path_1.to_string_lossy();
+    cx.config
+        .expect(["rustup", "toolchain", "link", "custom-1", &path_1])
+        .await
+        .is_ok();
+    cx.config
+        .expect(["rustup", "default", "custom-1"])
+        .await
+        .is_ok();
+    cx.config
+        .expect(["rustup", "which", "rustfmt"])
+        .await
+        .with_stderr(snapbox::str![[r#"
+error: 'rustfmt' is not installed for the toolchain 'custom-1'.
+[..]`rustup component add rustfmt`
+
+"#]])
+        .is_err();
+
+    let path_2 = cx.config.customdir.join("custom-2");
+    let path_2 = path_2.to_string_lossy();
+    cx.config
+        .expect(["rustup", "toolchain", "link", "custom-2", &path_2])
+        .await
+        .is_ok();
+    cx.config
+        .expect(["rustup", "which", "--toolchain=custom-2", "rustfmt"])
+        .await
+        .with_stderr(snapbox::str![[r#"
+[..]'rustfmt' is not installed for the toolchain 'custom-2'.
+[..]`rustup component add --toolchain custom-2 rustfmt`
+
+"#]])
+        .is_err();
+}
+
+#[tokio::test]
+async fn which_unknown_binary() {
+    let cx = CliTestContext::new(Scenario::SimpleV2).await;
+    let path_1 = cx.config.customdir.join("custom-1");
+    let path_1 = path_1.to_string_lossy();
+    cx.config
+        .expect(["rustup", "toolchain", "link", "custom-1", &path_1])
+        .await
+        .is_ok();
+    cx.config
+        .expect(["rustup", "default", "custom-1"])
+        .await
+        .is_ok();
+    cx.config
+        .expect(["rustup", "which", "ls"])
+        .await
+        .with_stderr(snapbox::str![[r#"
+[..]unknown binary 'ls' in toolchain 'custom-1'
+
+"#]])
+        .is_err();
+}
+
 #[tokio::test]
 async fn override_by_toolchain_on_the_command_line() {
     let cx = CliTestContext::new(Scenario::SimpleV2).await;