fix(toolchain/distributable): refine handling of known targets with no prebuilt artifacts

Cargo.toml

@@ -66,6 +66,7 @@ openssl = { version = "0.10", optional = true }
 opentelemetry = { version = "0.30", optional = true }
 opentelemetry-otlp = { version = "0.30", features = ["grpc-tonic"], optional = true }
 opentelemetry_sdk = { version = "0.30", features = ["rt-tokio"], optional = true }
+platforms = "3.4"
 pulldown-cmark = { version = "0.13", default-features = false }
 rand = "0.9"
 regex = "1"
@@ -135,7 +136,6 @@ enum-map = "2.5.0"
 http-body-util = "0.1.0"
 hyper = { version = "1.0", default-features = false, features = ["server", "http1"] }
 hyper-util = { version = "0.1.1", features = ["tokio"] }
-platforms = "3.4"
 proptest = "1.1.0"
 trycmd = "0.15.0"
 

src/errors.rs

@@ -7,6 +7,7 @@ use std::io;
 use std::io::Write;
 use std::path::PathBuf;
 
+use platforms::Platform;
 use thiserror::Error as ThisError;
 use url::Url;
 
@@ -125,6 +126,15 @@ pub enum RustupError {
         component: String,
         suggestion: Option<String>,
     },
+    #[error(
+        "toolchain '{desc}' has no prebuilt artifacts available for target '{platform}'\n\
+        note: this may happen to a low-tier target as per https://doc.rust-lang.org/nightly/rustc/platform-support.html\n\
+        note: you can find instructions on that page to build the target support from source"
+    )]
+    UnavailableTarget {
+        desc: ToolchainDesc,
+        platform: &'static Platform,
+    },
     #[error("toolchain '{}' does not support target '{}'{}\n\
     note: you can see a list of supported targets with `rustc --print=target-list`\n\
     note: if you are adding support for a new target to rustc itself, see https://rustc-dev-guide.rust-lang.org/building/new-target.html", .desc, .target,

src/toolchain/distributable.rs

@@ -5,6 +5,7 @@ use std::{convert::Infallible, env::consts::EXE_SUFFIX, ffi::OsStr, path::Path,
 #[cfg(windows)]
 use anyhow::Context;
 use anyhow::anyhow;
+use platforms::Platform;
 
 use crate::{
     RustupError, component_for_bin,
@@ -76,21 +77,26 @@ impl<'a> DistributableToolchain<'a> {
                 let config = manifestation.read_config()?.unwrap_or_default();
                 let suggestion =
                     self.get_component_suggestion(&component, &config, &manifest, false);
+                let desc = self.desc.clone();
                 // Check if the target is supported.
                 if !targ_pkg
                     .components
                     .iter()
                     .any(|c| c.target() == component.target())
                 {
+                    let target = component.target.expect("component target should be known");
+                    if let Some(platform) = Platform::find(&target) {
+                        return Err(RustupError::UnavailableTarget { desc, platform }.into());
+                    };
                     return Err(RustupError::UnknownTarget {
-                        desc: self.desc.clone(),
-                        target: component.target.expect("component target should be known"),
+                        desc,
+                        target,
                         suggestion,
                     }
                     .into());
                 }
                 return Err(RustupError::UnknownComponent {
-                    desc: self.desc.clone(),
+                    desc,
                     component: component.description(&manifest),
                     suggestion,
                 }

tests/suite/cli_v2.rs

@@ -1509,6 +1509,26 @@ note: if you are adding support for a new target to rustc itself, see https://ru
         .is_err();
 }
 
+#[tokio::test]
+async fn add_target_unavailable() {
+    let cx = CliTestContext::new(Scenario::SimpleV2).await;
+    cx.config
+        .expect(["rustup", "default", "nightly"])
+        .await
+        .is_ok();
+    cx.config
+        .expect(["rustup", "target", "add", "mipsel-sony-psp"])
+        .await
+        .with_stderr(snapbox::str![[r#"
+...
+error: toolchain 'nightly-[HOST_TRIPLE]' has no prebuilt artifacts available for target 'mipsel-sony-psp'
+note: this may happen to a low-tier target as per https://doc.rust-lang.org/nightly/rustc/platform-support.html
+note: you can find instructions on that page to build the target support from source
+
+"#]])
+        .is_err();
+}
+
 #[tokio::test]
 async fn add_target_v1_toolchain() {
     let cx = CliTestContext::new(Scenario::SimpleV1).await;