fix(dist): add missing host toolchain to nightly backtracking flow

src/dist/manifest.rs

@@ -23,7 +23,7 @@ use anyhow::{Context, anyhow, bail};
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    dist::{Profile, TargetTuple, ToolchainDesc, config::Config},
+    dist::{DistError, Profile, TargetTuple, ToolchainDesc, config::Config},
     errors::RustupError,
     toolchain::DistributableToolchain,
 };
@@ -513,8 +513,7 @@ impl Package {
                 if let Some(t) = target {
                     tpkgs
                         .get(t)
-                        .ok_or_else(|| anyhow!(format!("target '{t}' not found in channel.  \
-                        Perhaps check https://doc.rust-lang.org/nightly/rustc/platform-support.html for available targets")))
+                        .ok_or_else(|| DistError::HostTupleUnsupported(t.clone()).into())
                 } else {
                     Err(anyhow!("no target specified"))
                 }

src/dist/mod.rs

@@ -123,6 +123,12 @@ pub enum DistError {
     MissingReleaseForToolchain(String),
     #[error("invalid toolchain name: '{0}'")]
     InvalidOfficialName(String),
+    #[error(
+        "host tuple '{0}' not found in channel manifest
+help: this could mean that it has been demoted and no longer has a host toolchain
+help: see <https://doc.rust-lang.org/nightly/rustc/platform-support.html> for available targets"
+    )]
+    HostTupleUnsupported(TargetTuple),
 }
 
 #[derive(Debug, PartialEq)]
@@ -1074,14 +1080,16 @@ impl<'cfg, 'a> DistOptions<'cfg, 'a> {
                     backtrack_limit = backtrack_limit.map(|n| n - 1);
                 }
 
-                Some(DistError::MissingReleaseForToolchain(..)) => {
-                    // no need to even print anything for missing nightlies,
-                    // since we don't really "skip" them
-                }
-                _ => {
-                    // All other errors break the loop
-                    break Err(e);
+                Some(e @ DistError::HostTupleUnsupported(_)) => {
+                    info!("skipping nightly with unsupported host: {e:#}")
                 }
+
+                // no need to even print anything for missing nightlies,
+                // since we don't really "skip" them
+                Some(DistError::MissingReleaseForToolchain(..)) => (),
+
+                // All other errors break the loop
+                _ => break Err(e),
             };
 
             if let Some(backtrack_limit) = backtrack_limit

tests/suite/cli_rustup.rs

@@ -4114,10 +4114,12 @@ async fn check_host_goes_away() {
         .await
         .with_stderr(snapbox::str![[r#"
 ...
-error: target '[HOST_TUPLE]' not found in channel[..]
+info: skipping nightly with unsupported host: host tuple '[HOST_TUPLE]' not found in channel manifest
+help: this could mean that it has been demoted and no longer has a host toolchain
+help: see <https://doc.rust-lang.org/nightly/rustc/platform-support.html> for available targets
 ...
 "#]])
-        .is_err();
+        .is_ok();
 }
 
 #[cfg(unix)]