generic rule: add support for cwd

2 files changed, 35 insertions, 6 deletions

diff --git a/doc/concepts/built-in-rules.md b/doc/concepts/built-in-rules.md
index 69df06dc..6c80ffb4 100644
--- a/doc/concepts/built-in-rules.md
+++ b/doc/concepts/built-in-rules.md
@@ -91,23 +91,26 @@ precedence to the artifacts over the runfiles; conflicts within
 artifacts or runfiles are resolved in a latest-wins fashion using the
 order of the targets in the evaluated `"deps"` argument.
 
-The fields `"cmds"`, `"sh -c"`, `"out_dirs"`, `"outs"`, and `"env"`
+The fields `"cmds"`, `"cwd"`, `"sh -c"`, `"out_dirs"`, `"outs"`, and `"env"`
 are evaluated fields where `"cmds"`, `"out_dirs"`, and `"outs"`
 have to evaluate to a list of strings, `"sh -c"` has to evalute to
-a list of strings or `null`, and `"env"` has to evaluate to a map
-of strings. During their evaluation, the functions `"outs"` and
+a list of strings or `null`, `"env"` has to evaluate to a map
+of strings, and `"cwd"` has to evaluate to a single string naming a non-upwards
+relative path. During their evaluation, the functions `"outs"` and
 `"runfiles"` can be used to access the logical paths of the artifacts
 and runfiles, respectively, of a target specified in `"deps"`. Here,
 `"env"` specifies the environment in which the action is carried
 out. `"out_dirs"` and `"outs"` define the output directories and
-files, respectively, the action has to produce. Since some artifacts
+files, respectively, the action has to produce; these path are read
+relative to the action root. Since some artifacts
 are to be produced, at least one of `"out_dirs"` or `"outs"` must
 be a non-empty list of strings. It is an error if one or more paths
 are present in both the `"out_dirs"` and `"outs"`. Finally, the
 strings in `"cmds"` are extended by a newline character and joined,
 and command of the action is the result of evaluating the field
 `"sh -c"` (or `["sh", "-c"]` if `"sh -c"` evaluates to `null` or
-`[]`) extended by this string.
+`[]`) extended by this string; the command is executed in the
+subdirectory of the execution root specified by `"cwd"`.
 
 The artifacts of this target are the outputs (as declared by
 `"out_dirs"` and `"outs"`) of this action. Runfiles and provider map are
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 e1cf9c7f..e3cfc028 100644
--- a/src/buildtool/build_engine/target_map/built_in_rules.cpp
+++ b/src/buildtool/build_engine/target_map/built_in_rules.cpp
@@ -42,6 +42,7 @@ namespace {
 auto const kGenericRuleFields =
     std::unordered_set<std::string>{"arguments_config",
                                     "cmds",
+                                    "cwd",
                                     "deps",
                                     "env",
                                     "execution properties",
@@ -1196,6 +1197,31 @@ void GenericRuleWithDeps(
         cmd_ss << x->String();
         cmd_ss << "\n";
     }
+    auto cwd_exp =
+        desc->ReadOptionalExpression("cwd", Expression::kEmptyString);
+    if (not cwd_exp) {
+        return;
+    }
+    auto cwd_value = cwd_exp.Evaluate(
+        param_config, string_fields_fcts, [&logger](auto const& msg) {
+            (*logger)(fmt::format("While evaluating cwd:\n{}", msg), true);
+        });
+    if (not cwd_value) {
+        return;
+    }
+    if (not cwd_value->IsString()) {
+        (*logger)(fmt::format("cwd has to evaluate to a string, but found {}",
+                              cwd_value->ToString()),
+                  true);
+        return;
+    }
+    if (not PathIsNonUpwards(cwd_value->String())) {
+        (*logger)(fmt::format("cwd has to evalute to a non-upwards relative "
+                              "path, but found {}",
+                              cwd_value->ToString()),
+                  true);
+        return;
+    }
     auto const& empty_map_exp = Expression::kEmptyMapExpr;
     auto env_exp = desc->ReadOptionalExpression("env", empty_map_exp);
     if (not env_exp) {
@@ -1341,7 +1367,7 @@ void GenericRuleWithDeps(
         outs,
         out_dirs,
         argv,
-        "",
+        cwd_value->String(),
         env_val,
         std::nullopt,
         false,