"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.

(cherry-picked from 5e104a526cf76fe75312d2fd288a3c88f506fb0a)

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 9623f816..a227c30b 100644
--- a/src/buildtool/build_engine/target_map/built_in_rules.cpp
+++ b/src/buildtool/build_engine/target_map/built_in_rules.cpp
@@ -1351,9 +1351,18 @@ 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);