haskell_{library,binary}: add build_tool_depends attribute

prelude/decls/haskell_common.bzl

@@ -50,6 +50,24 @@ def _compiler_flags_arg():
         ),
     }
 
+def _build_tool_depends_arg():
+    return {
+        "build_tool_depends": attrs.list(
+            attrs.dep(),
+            default = [],
+            doc = """
+    Executables (haskell_binary(), export_exe(), or anything else with a
+    single default output) that this rule's own GHC invocation needs
+    reachable via PATH by bare name. The executables can then be used
+    with -pgmL or -pgmF flags for example, either as a command-line
+    arg or in a {-# OPTIONS_GHC ... #-} pragma.
+
+    The build_tool_depends attribute is used to implement Cabal's
+    build-tool-depends: field.
+""",
+        ),
+    }
+
 def _exported_linker_flags_arg():
     return {
         "exported_linker_flags": attrs.list(
@@ -66,4 +84,5 @@ haskell_common = struct(
     deps_arg = _deps_arg,
     compiler_flags_arg = _compiler_flags_arg,
     exported_linker_flags_arg = _exported_linker_flags_arg,
+    build_tool_depends_arg = _build_tool_depends_arg,
 )

prelude/decls/haskell_rules.bzl

@@ -51,6 +51,7 @@ haskell_binary = prelude_rule(
         | haskell_common.srcs_arg()
         | haskell_common.compiler_flags_arg()
         | haskell_common.deps_arg()
+        | haskell_common.build_tool_depends_arg()
         | {
             "deps_query": attrs.option(attrs.query(), default = None),
             "enable_profiling": attrs.bool(default = False),
@@ -175,6 +176,7 @@ haskell_library = prelude_rule(
         | haskell_common.compiler_flags_arg()
         | haskell_common.deps_arg()
         | haskell_common.exported_linker_flags_arg()
+        | haskell_common.build_tool_depends_arg()
         | native_common.link_whole(link_whole_type = attrs.bool(default = False))
         | native_common.preferred_linkage(preferred_linkage_type = attrs.enum(Linkage.values()))
         | {

prelude/haskell/compile.bzl

@@ -308,11 +308,26 @@ def compile(ctx: AnalysisContext, link_style: LinkStyle, enable_profiling: bool,
     # build-action time rather than baking absolute host paths into .bzl
     # or BUCK files (see toolchains/BUCK's `compiler`/`packager`).
     run_cmd = compile_cmd
-    if haskell_toolchain.compile_env:
-        env_exports = "".join([
-            'export {}="{}"; '.format(name, value)
-            for name, value in haskell_toolchain.compile_env.items()
-        ])
+    env_exports = "".join([
+        'export {}="{}"; '.format(name, value)
+        for name, value in haskell_toolchain.compile_env.items()
+    ]) if haskell_toolchain.compile_env else ""
+
+    build_tool_dirs = {
+        dep[DefaultInfo].default_outputs[0].basename: dep[DefaultInfo].default_outputs[0]
+        for dep in ctx.attrs.build_tool_depends
+    }
+    if build_tool_dirs:
+        build_tool_bin_dir = ctx.actions.symlinked_dir(
+            artifact_suffix + "-build-tool-depends",
+            build_tool_dirs,
+            has_content_based_path = False,
+        )
+        run_cmd = cmd_args(
+            ["sh", "-c", env_exports + 'export PATH="$PATH:$1"; shift; exec "$@"', "sh", build_tool_bin_dir],
+            compile_cmd,
+        )
+    elif haskell_toolchain.compile_env:
         run_cmd = cmd_args(["sh", "-c", env_exports + 'exec "$@"', "sh"], compile_cmd)
 
     ctx.actions.run(