fix(uv): fail clearly when no conflict variant is selected

CHANGELOG.md

@@ -25,6 +25,8 @@ All notable changes to this project will be documented in this file.
   preserved, per-edge resolution-marker forks use their variant-specific marker
   expressions, and locked versions use exact (`===`) matching so local version
   tags (e.g. `2.12.0` vs `2.12.0+cpu`) do not collide.
+- **Missing `uv` conflict variant.** Fails with a message naming the `_variants`
+  flags, at execution time under `unavailable_package_mode=fail_at_execution`.
 
 ## [2.0.0]
 

README.md

@@ -805,11 +805,13 @@ The transition applies to the repo's whole closure, including sdist builds from
 
 ## Handling Unavailable Packages
 
-A package is unavailable in the selected target environment when it has no compatible wheel (and no sdist to fall back to), when no resolution-marker fork matches, or when the selected conflict variant doesn't include it. (Selecting no variant from a conflict set that has no default is still an analysis error; see [Default Groups](#default-groups).) The `--@rules_pycross//pycross/settings:unavailable_package_mode` flag controls how such packages behave:
+A package is unavailable in the selected target environment when it has no compatible wheel (and no sdist to fall back to), when no resolution-marker fork matches, or when the selected conflict variant doesn't include it. The `--@rules_pycross//pycross/settings:unavailable_package_mode` flag controls how such packages behave:
 
 * **`incompatible` (default)** — unavailable packages are marked [incompatible](https://bazel.build/extending/platforms#skipping-incompatible-targets): targets that depend on them are skipped by `bazel build //...` and `bazel test //...`, and fail analysis when requested explicitly. (`all_requirements` and other platform-conditional `:maybe` aggregates exclude such packages instead.)
 * **`fail_at_execution`** — unavailable packages analyze successfully and provide the usual providers (`PyInfo`, etc.), registering an action that fails only when executed. Building anything that actually needs the package fails with a `No compatible wheel is available ...` error.
 
+Leaving a `uv` conflict set without a default unselected (see [Default Groups](#default-groups)) always fails with a message naming the `_variants` flags to set—at analysis time by default (so `bazel build //...` does not silently skip conflicted targets), or at execution time under `fail_at_execution`.
+
 `fail_at_execution` is useful when running consumers that only *analyze* dependencies—such as type-checking aspects that attach stub packages via implicit attributes, which otherwise see a target without the usual providers and fail even when the package is never built—or when you want wildcard `bazel build //...` / `bazel test //...` runs to fail loudly instead of silently skipping targets with unavailable dependencies:
 
 ```

pycross/private/BUILD.bazel

@@ -123,7 +123,9 @@ bzl_library(
     name = "variant_resolver",
     srcs = ["variant_resolver.bzl"],
     deps = [
+        ":deferred_failure",
         "@bazel_skylib//rules:common_settings",
+        "@rules_python//python:py_info_bzl",
     ],
 )
 

pycross/private/package_repo.bzl

@@ -208,11 +208,13 @@ def _package_repo_impl(rctx):
         "",
     ]
     if variant_items:
-        lock_build_lines.extend([
-            'load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")',
-            'load("@rules_pycross//pycross/private:variant_resolver.bzl", "variant_resolver")',
-            "",
-        ])
+        has_unselected_fallback = any([not vset["default"] for vset in variant_sets])
+        lock_build_lines.append('load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")')
+        if has_unselected_fallback:
+            lock_build_lines.append('load("@rules_pycross//pycross/private:variant_resolver.bzl", "pycross_no_variant_selected", "variant_resolver")')
+        else:
+            lock_build_lines.append('load("@rules_pycross//pycross/private:variant_resolver.bzl", "variant_resolver")')
+        lock_build_lines.append("")
 
     # Check if we need config_setting_group for items in multiple sets.
     needs_config_setting_group = False
@@ -243,7 +245,8 @@ def _package_repo_impl(rctx):
 
     # Emit variant_resolver targets (one per conflict set).
     for vset in variant_sets:
-        resolver_name = "_resolver_" + "_".join(sorted(vset["qnames"]))
+        set_suffix = "_".join(sorted(vset["qnames"]))
+        resolver_name = "_resolver_" + set_suffix
         lock_build_lines.extend([
             "variant_resolver(",
             '    name = "{}",'.format(resolver_name),
@@ -266,6 +269,22 @@ def _package_repo_impl(rctx):
             ")",
             "",
         ])
+        if not vset["default"]:
+            lock_build_lines.extend([
+                "config_setting(",
+                '    name = "_unselected_{}",'.format(set_suffix),
+                '    flag_values = {{":{resolver}": ""}},'.format(resolver = resolver_name),
+                ")",
+                "",
+                "pycross_no_variant_selected(",
+                '    name = "_no_variant_{}",'.format(set_suffix),
+                '    message = "No uv conflict variant selected. Set one of the _variants flags on your pycross repo: {} (e.g. --@<repo>//_variants:{}=True)",'.format(
+                    ", ".join(vset["qnames"]),
+                    vset["qnames"][0],
+                ),
+                ")",
+                "",
+            ])
 
     # Emit config_settings referencing the resolver(s).
     for qname in sorted(variant_items.keys()):