feat(third_party/bazel): Check in rules_haskell from Tweag

third_party/bazel/rules_haskell/tests/ghc/BUILD.bazel

@@ -0,0 +1,3 @@
+load("//tests/ghc:ghc.bzl", "ghc_help")
+
+ghc_help(name = "ghc_help")

third_party/bazel/rules_haskell/tests/ghc/ghc.bzl

@@ -0,0 +1,19 @@
+"""Runs ghc --help"""
+
+hs_toolchain = "@io_tweag_rules_haskell//haskell:toolchain"
+
+def _impl(ctx):
+    output = ctx.outputs.out
+    ghc = ctx.toolchains[hs_toolchain].tools.ghc
+    ctx.actions.run_shell(
+        inputs = [ghc],
+        outputs = [output],
+        progress_message = "Printing ghc help message",
+        command = "%s --help > %s" % (ghc.path, output.path),
+    )
+
+ghc_help = rule(
+    implementation = _impl,
+    outputs = {"out": "out_file"},
+    toolchains = [hs_toolchain],
+)