slightly clarify unit graph sharing

Author: lqd

src/cargo/ops/cargo_compile/mod.rs

@@ -1265,8 +1265,17 @@ fn traverse_and_share(
             }
         })
         .collect();
+    // Here, we have recursively traversed this unit's dependencies, and hashed them: we can
+    // finalize the dep hash.
     let new_dep_hash = dep_hash.finish();
-    let new_kind = if unit.kind == to_host {
+
+    // This is the key part of the sharing process: if the unit is a runtime dependency, whose
+    // target is the same as the host, we canonicalize the compile kind to `CompileKind::Host`.
+    // A possible host dependency counterpart to this unit would have that kind, and if such a unit
+    // exists in the current `unit_graph`, they will unify in the new unit graph map `new_graph`.
+    // The resulting unit graph will have be optimized with less units, thanks to sharing these host
+    // dependencies.
+    let canonical_kind = if unit.kind == to_host {
         CompileKind::Host
     } else {
         unit.kind
@@ -1311,7 +1320,7 @@ fn traverse_and_share(
         &unit.pkg,
         &unit.target,
         profile,
-        new_kind,
+        canonical_kind,
         unit.mode,
         unit.features.clone(),
         unit.is_std,