feat: support ts_project for only type-checking

Possible scenarios include: noEmit, transpiler+noEmitDeclarations.

examples/no_emit/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@aspect_rules_ts//ts:defs.bzl", "ts_project")
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
 load("@bazel_skylib//rules:write_file.bzl", "write_file")
 
 write_file(
@@ -9,12 +10,46 @@ write_file(
     ],
 )
 
-# Shows how to run `tsc` producing no outputs at all.
+write_file(
+    name = "gen_js",
+    out = "b.js",
+    content = [
+        "export const b = 43",
+    ],
+)
+
+# Shows how to run `tsc` producing no outputs at all via no_emit=True.
 # It will use a validation action to type-check the file:
 # https://bazel.build/extending/rules#validation_actions
 # Run bazel with --norun_validations to skip type-checking.
 ts_project(
-    name = "typecheck_only",
+    name = "typecheck_only_noemit",
     srcs = ["a.ts"],
-    no_emit = True,
+    tsconfig = {
+        "compilerOptions": {
+            "noEmit": True,
+        },
+    },
+)
+
+# Shows how to run `tsc` producing no outputs at all via declarations=False.
+ts_project(
+    name = "typecheck_nodeclarations_js",
+    srcs = ["b.js"],
+    out_dir = "typecheck_nodeclarations_js",
+    tsconfig = {
+        "compilerOptions": {
+            "allowJs": True,
+            "noEmit": True,
+        },
+    },
+)
+
+build_test(
+    name = "targets_test",
+    targets = [
+        # Ensure the _typecheck targets are declared despite no outputs.
+        ":typecheck_only_noemit_typecheck",
+        ":typecheck_nodeclarations_js_typecheck",
+    ],
 )

examples/transpiler/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@aspect_rules_ts//ts:defs.bzl", "ts_project")
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
 
 # Note, Bazel 6 starlark has lambda so maybe we can stop using partial
 load("@bazel_skylib//rules:write_file.bzl", "write_file")
@@ -19,6 +20,14 @@ write_file(
     ],
 )
 
+write_file(
+    name = "gen_a",
+    out = "a.ts",
+    content = [
+        "export const b: number = 43",
+    ],
+)
+
 # Runs babel to transpile ts -> js
 # and tsc to type-check
 ts_project(
@@ -31,7 +40,7 @@ ts_project(
 )
 
 # Runs babel to transpile ts -> js
-# and does not produce any declaration outputs.
+# and does not produce any declaration outputs due to noEmit=True.
 # `tsc` is used for type-check only, as a validation action
 # (run bazel with --norun_validations to skip typechecking)
 ts_project(
@@ -45,3 +54,39 @@ ts_project(
         },
     },
 )
+
+# Runs babel to transpile ts -> js
+# and does not produce any declaration outputs due to declaration=False.
+ts_project(
+    name = "no-declarations",
+    srcs = ["a.ts"],
+    out_dir = "build-nodecls",
+    transpiler = babel,
+    tsconfig = {
+        "compilerOptions": {
+            "declaration": False,
+        },
+    },
+)
+
+build_test(
+    name = "targets_test",
+    targets = [
+        # babel ts_project
+        ":babel",
+        ":babel_typecheck",
+        # babel outputted js
+        "big.js",  # NOTE: does not implement out_dir in this test
+        # tsc outputted dts
+        "build-babel/big.d.ts",
+
+        # no-emit for type-checking
+        ":no-emit",
+        ":no-emit_typecheck",
+
+        # babel outputted .js with no declarations
+        ":no-declarations",
+        ":no-declarations_typecheck",
+        "a.js",  # NOTE: does not implement out_dir in this test
+    ],
+)

ts/private/ts_project.bzl

@@ -177,43 +177,12 @@ See https://github.com/aspect-build/rules_ts/issues/361 for more details.
 
     typings_srcs = [s for s in srcs_inputs if _lib.is_typings_src(s.path)]
 
-    if len(js_outs) + len(typings_outs) < 1 and not ctx.attr.no_emit:
-        label = "//{}:{}".format(ctx.label.package, ctx.label.name)
-        if len(typings_srcs) > 0:
-            no_outs_msg = """ts_project target {target} only has typings in srcs.
-Since there is no `tsc` action to perform, there are no generated outputs.
-
-For "typecheck-only" the TypeScript 'noEmit' feature can be used.
-
-For grouping typings this should be changed to js_library, which can be done by running:
-
-    buildozer 'new_load @aspect_rules_js//js:defs.bzl js_library' //{pkg}:__pkg__
-    buildozer 'set kind js_library' {target}
-    buildozer 'remove declaration' {target}
-
-""".format(
-                target = label,
-                pkg = ctx.label.package,
-            )
-        elif ctx.attr.transpile != 0:
-            no_outs_msg = """ts_project target %s is configured to produce no outputs.
-
-This might be because
-- you configured it with `noEmit`
-- the `srcs` are empty
-- `srcs` has elements producing non-ts outputs
-""" % label
-        else:
-            no_outs_msg = "ts_project target %s with custom transpiler needs 'declaration = True'." % label
-        fail(no_outs_msg + """
-This is an error because Bazel does not run actions unless their outputs are needed for the requested targets to build.
-""")
-
     # Make sure the user has acknowledged that transpiling is slow
     if len(outputs) > 0 and ctx.attr.transpile == -1 and not ctx.attr.emit_declaration_only and not options.default_to_tsc_transpiler:
         fail(transpiler_selection_required)
 
     output_types = typings_outs + typing_maps_outs + typings_srcs
+    use_tsc_noemit = ctx.attr.no_emit or (ctx.attr.transpile == 0 and not ctx.attr.declaration)
 
     # Default outputs (DefaultInfo files) is what you see on the command-line for a built
     # library, and determines what files are used by a simple non-provider-aware downstream
@@ -225,7 +194,7 @@ This is an error because Bazel does not run actions unless their outputs are nee
     else:
         # We must avoid tsc writing any JS files in this case, as tsc was only run for typings, and some other
         # action will try to write the JS files. We must avoid collisions where two actions write the same file.
-        if not ctx.attr.no_emit:
+        if not use_tsc_noemit:
             arguments.add("--emitDeclarationOnly")
 
         # We don't produce any DefaultInfo outputs in this case, because we avoid running the tsc action
@@ -236,7 +205,7 @@ This is an error because Bazel does not run actions unless their outputs are nee
 
     stdout_file = ""
 
-    if ctx.attr.no_emit:
+    if use_tsc_noemit:
         # The type-checking action still need to produce some output, so we output the stdout
         # to a .typecheck file that ends up in the typecheck output group.
         typecheck_output = ctx.actions.declare_file(ctx.attr.name + ".typecheck")