Fixes custom toolchain support without rustup.

Avoids checking for installed toolchains, and also prevents renaming the
sysroot based on the toolchain full name, since this may be in a
directory completely different than the target name.

.changes/1085.json

@@ -0,0 +1,4 @@
+{
+    "type": "added",
+    "description": "support custom toolchains without rustup."
+}

src/docker/shared.rs

@@ -1625,6 +1625,7 @@ mod tests {
                 &None,
                 &image_platform,
                 &sysroot,
+                false,
             ))
         }
 

src/lib.rs

@@ -578,11 +578,10 @@ To override the toolchain mounted in the image, set `target.{}.image.toolchain =
             // set the sysroot explicitly to the toolchain
             let mut is_nightly = toolchain.channel.contains("nightly");
 
-            let installed_toolchains = rustup::installed_toolchains(msg_info)?;
-
-            if !installed_toolchains
-                .into_iter()
-                .any(|t| t == toolchain.to_string())
+            if !toolchain.is_custom
+                && !rustup::installed_toolchains(msg_info)?
+                    .into_iter()
+                    .any(|t| t == toolchain.to_string())
             {
                 rustup::install_toolchain(&toolchain, msg_info)?;
             }

src/rustc.rs

@@ -92,27 +92,35 @@ pub struct QualifiedToolchain {
 }
 
 impl QualifiedToolchain {
-    pub fn new(channel: &str, date: &Option<String>, host: &ImagePlatform, sysroot: &Path) -> Self {
+    pub fn new(
+        channel: &str,
+        date: &Option<String>,
+        host: &ImagePlatform,
+        sysroot: &Path,
+        is_custom: bool,
+    ) -> Self {
         let mut this = Self {
             channel: channel.to_owned(),
             date: date.clone(),
             host: host.clone(),
-            is_custom: false,
+            is_custom,
             full: if let Some(date) = date {
                 format!("{}-{}-{}", channel, date, host.target)
             } else {
                 format!("{}-{}", channel, host.target)
             },
             sysroot: sysroot.to_owned(),
         };
-        this.sysroot.set_file_name(&this.full);
+        if !is_custom {
+            this.sysroot.set_file_name(&this.full);
+        }
         this
     }
 
     /// Replace the host, does nothing if ran on a custom toolchain
     pub fn replace_host(&mut self, host: &ImagePlatform) -> &mut Self {
         if !self.is_custom {
-            *self = Self::new(&self.channel, &self.date, host, &self.sysroot);
+            *self = Self::new(&self.channel, &self.date, host, &self.sysroot, false);
             self.sysroot.set_file_name(&self.full);
         }
         self
@@ -168,8 +176,8 @@ impl QualifiedToolchain {
                 &build_date,
                 &ImagePlatform::from_target(host.into())?,
                 sysroot,
+                true,
             );
-            toolchain.is_custom = true;
             toolchain.full = name.to_owned();
             return Ok(toolchain);
         }
@@ -217,6 +225,7 @@ impl QualifiedToolchain {
             &date,
             &host,
             &self.sysroot,
+            false,
         ))
     }
 
@@ -256,7 +265,11 @@ impl QualifiedToolchain {
             Ok(_) | Err(_) if config.custom_toolchain() => {
                 QualifiedToolchain::custom(toolchain, &sysroot, config, msg_info)
             }
-            Ok(_) => eyre::bail!("toolchain is not fully qualified"),
+            Ok(_) => return Err(eyre::eyre!("toolchain is not fully qualified")
+                .with_note(|| "cross expects the toolchain to be a rustup installed toolchain")
+                .with_suggestion(|| {
+                    "if you're using a custom toolchain try setting `CROSS_CUSTOM_TOOLCHAIN=1` or install rust via rustup"
+            })),
             Err(e) => Err(e),
         }
     }

src/tests.rs

@@ -86,6 +86,7 @@ release: {version}
                     &None,
                     &ImagePlatform::from_const_target(TargetTriple::X86_64UnknownLinuxGnu),
                     Path::new("/toolchains/xxxx-x86_64-unknown-linux-gnu"),
+                    false,
                 ),
                 &target_meta.0,
                 &target_meta.1,