feat: support ts_project for only type-checking

Possible scenarios include: noEmit, transpiler+noEmitDeclarations.

examples/no_emit/BUILD.bazel

@@ -9,12 +9,33 @@ 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,
 )
+
+# Shows how to run `tsc` producing no outputs at all via declarations=False.
+ts_project(
+    name = "typecheck_only_noemitdeclarations",
+    srcs = ["b.js"],
+    tsconfig = {
+        "compilerOptions": {
+            "allowJs": True,
+            "noEmit": True,
+            "declaration": False,
+        },
+    },
+)

examples/transpiler/BUILD.bazel

@@ -19,6 +19,14 @@ 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(
@@ -31,12 +39,12 @@ 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(
     name = "no-emit",
-    srcs = ["big.ts"],
+    srcs = ["a.ts"],
     no_emit = True,
     transpiler = babel,
     tsconfig = {
@@ -45,3 +53,17 @@ 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,
+        },
+    },
+)

ts/private/ts_config.bzl

@@ -136,16 +136,16 @@ def _write_tsconfig_rule(ctx):
         content = content.replace("__extends__", extends_path)
 
     filtered_files = _filter_input_files(ctx.files.files, ctx.attr.allow_js, ctx.attr.resolve_json_module)
-    if filtered_files:
-        # Update file paths to be relative to the tsconfig file, including a ./ prefix
-        # to ensure paths are all relative to the config file.
-        package_prefix = ctx.label.package + "/"
-        filtered_files = [
-            "./" + f.short_path.removeprefix(package_prefix)
-            for f in filtered_files
-        ]
-
-        content = content.replace("\"__files__\"", str(filtered_files))
+
+    # Update file paths to be relative to the tsconfig file, including a ./ prefix
+    # to ensure paths are all relative to the config file.
+    package_prefix = ctx.label.package + "/"
+    filtered_files = [
+        "./" + f.short_path.removeprefix(package_prefix)
+        for f in filtered_files
+    ]
+
+    content = content.replace("\"__files__\"", str(filtered_files))
     ctx.actions.write(
         output = ctx.outputs.out,
         content = content,

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
+    no_emit = 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 no_emit:
             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 no_emit:
         # 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")