Error message for transitive artifact dependencies with targets the package doesn't directly interact with

src/cargo/core/compiler/build_context/target_info.rs

@@ -938,10 +938,22 @@ impl<'cfg> RustcTargetData<'cfg> {
     }
 
     /// Information about the given target platform, learned by querying rustc.
+    ///
+    /// # Panics
+    ///
+    /// Panics, if the target platform described by `kind` can't be found.
+    /// See [`get_info`](Self::get_info) for a non-panicking alternative.
     pub fn info(&self, kind: CompileKind) -> &TargetInfo {
+        self.get_info(kind).unwrap()
+    }
+
+    /// Information about the given target platform, learned by querying rustc.
+    ///
+    /// Returns `None` if the target platform described by `kind` can't be found.
+    pub fn get_info(&self, kind: CompileKind) -> Option<&TargetInfo> {
         match kind {
-            CompileKind::Host => &self.host_info,
-            CompileKind::Target(s) => &self.target_info[&s],
+            CompileKind::Host => Some(&self.host_info),
+            CompileKind::Target(s) => self.target_info.get(&s),
         }
     }
 

src/cargo/core/compiler/compilation.rs

@@ -129,16 +129,7 @@ impl<'cfg> Compilation<'cfg> {
                 .info(CompileKind::Host)
                 .sysroot_host_libdir
                 .clone(),
-            sysroot_target_libdir: bcx
-                .all_kinds
-                .iter()
-                .map(|&kind| {
-                    (
-                        kind,
-                        bcx.target_data.info(kind).sysroot_target_libdir.clone(),
-                    )
-                })
-                .collect(),
+            sysroot_target_libdir: get_sysroot_target_libdir(bcx)?,
             tests: Vec::new(),
             binaries: Vec::new(),
             cdylibs: Vec::new(),
@@ -384,6 +375,35 @@ fn fill_rustc_tool_env(mut cmd: ProcessBuilder, unit: &Unit) -> ProcessBuilder {
     cmd
 }
 
+fn get_sysroot_target_libdir(
+    bcx: &BuildContext<'_, '_>,
+) -> CargoResult<HashMap<CompileKind, PathBuf>> {
+    bcx.all_kinds
+        .iter()
+        .map(|&kind| {
+            let Some(info) = bcx.target_data.get_info(kind) else {
+                let target = match kind {
+                    CompileKind::Host => "host".to_owned(),
+                    CompileKind::Target(s) => s.short_name().to_owned(),
+                };
+
+                let dependency = bcx
+                    .unit_graph
+                    .iter()
+                    .find_map(|(u, _)| (u.kind == kind).then_some(u.pkg.summary().package_id()))
+                    .unwrap();
+
+                anyhow::bail!(
+                    "could not find specification for target `{target}`.\n  \
+                    Dependency `{dependency}` requires to build for target `{target}`."
+                )
+            };
+
+            Ok((kind, info.sysroot_target_libdir.clone()))
+        })
+        .collect()
+}
+
 fn target_runner(
     bcx: &BuildContext<'_, '_>,
     kind: CompileKind,