use UnitFor::is_for_host to detect build deps

Although `CompileKind::is_host` is currently used for build dependencies prior
to unit graph sharing, it's not a guarantee. So we use `UnitFor::is_for_host`
to make detection more future-proof.

src/cargo/ops/cargo_compile/mod.rs

@@ -1225,7 +1225,15 @@ fn rebuild_unit_graph_shared(
     let new_roots = roots
         .iter()
         .map(|root| {
-            traverse_and_share(interner, &mut memo, &mut result, &unit_graph, root, to_host)
+            traverse_and_share(
+                interner,
+                &mut memo,
+                &mut result,
+                &unit_graph,
+                root,
+                false,
+                to_host,
+            )
         })
         .collect();
     let new_scrape_units = scrape_units
@@ -1246,6 +1254,7 @@ fn traverse_and_share(
     new_graph: &mut UnitGraph,
     unit_graph: &UnitGraph,
     unit: &Unit,
+    unit_is_for_host: bool,
     to_host: CompileKind,
 ) -> Unit {
     if let Some(new_unit) = memo.get(unit) {
@@ -1256,8 +1265,15 @@ fn traverse_and_share(
     let new_deps: Vec<_> = unit_graph[unit]
         .iter()
         .map(|dep| {
-            let new_dep_unit =
-                traverse_and_share(interner, memo, new_graph, unit_graph, &dep.unit, to_host);
+            let new_dep_unit = traverse_and_share(
+                interner,
+                memo,
+                new_graph,
+                unit_graph,
+                &dep.unit,
+                dep.unit_for.is_for_host(),
+                to_host,
+            );
             new_dep_unit.hash(&mut dep_hash);
             UnitDep {
                 unit: new_dep_unit,
@@ -1286,7 +1302,7 @@ fn traverse_and_share(
     // If this is a build dependency, and it's not shared with runtime dependencies, we can weaken
     // its debuginfo level to optimize build times. We do nothing if it's an artifact dependency,
     // as it and its debuginfo may end up embedded in the main program.
-    if unit.kind.is_host() && profile.debuginfo.is_deferred() && !unit.artifact.is_true() {
+    if unit_is_for_host && profile.debuginfo.is_deferred() && !unit.artifact.is_true() {
         // We create a "probe" test to see if a unit with the same explicit debuginfo level exists
         // in the graph. This is the level we'd expect if it was set manually or the default value
         // set by a profile for a runtime dependency: its canonical value.