"generic" rule: verify staging conflicts on inputs

The "generic" rules deliberately resolves conflicts on identical
paths in a latest-wins fashion (seeing all artifacts as later than
all runfiles) to allow an easy way to define actions. However, the
inputs stage obtained by this resolution can still contain conflicts
and those are an error. Properly detect those. Also clarify in the
documentation, that only conflicts on identical paths are resolved
in the described priority, not semantic overlap.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/buildtool/build_engine/target_map/built_in_rules.cpp b/src/buildtool/build_engine/target_map/built_in_rules.cpp
index 220502c4..534d2fea 100644
--- a/src/buildtool/build_engine/target_map/built_in_rules.cpp
+++ b/src/buildtool/build_engine/target_map/built_in_rules.cpp
@@ -1369,9 +1369,18 @@ void GenericRuleWithDeps(
     for (auto const& dep : dependency_values) {
         inputs = ExpressionPtr{Expression::map_t{inputs, (*dep)->RunFiles()}};
     }
+    auto inputs_conflict = BuildMaps::Target::Utils::tree_conflict(inputs);
     for (auto const& dep : dependency_values) {
         inputs = ExpressionPtr{Expression::map_t{inputs, (*dep)->Artifacts()}};
     }
+    // While syntactical conflicts are resolved in a latest wins (with artifacts
+    // after runfiles), semantic path conclicts are an error.
+    if (inputs_conflict) {
+        (*logger)(fmt::format("Input artifacts have staging conflict on {}",
+                              nlohmann::json(*inputs_conflict).dump()),
+                  /*fatal=*/true);
+        return;
+    }
     std::vector<Tree::Ptr> trees{};
     inputs = BuildMaps::Target::Utils::add_dir_for(
         cwd_value->String(), inputs, &trees);