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feat: fallback registry + install-first resolve + mcpp self fallbacks
1. Fallback registry (src/fallback/registry.cppm): - Compile-time registry of all 17 fallback mechanisms in mcpp - Each entry: id, domain, lifecycle (permanent/compat/workaround), removeBy - Single source of truth for auditing and cleanup planning 2. resolve_xpkg_path priority fix (src/pm/package_fetcher.cppm): - Refactored from: sandbox → copy → install (copy short-circuits install) - Refactored to: sandbox → install → copy (install-first, copy is fallback) - Split into resolve_quick() + copy_from_global() for clarity - Fixes: install path was never exercised when ~/.xlings/ had the package 3. mcpp self fallbacks command (src/cli.cppm): - Lists all registered fallbacks grouped by lifecycle - Shows workarounds (need upstream fix), compat (remove by 1.0), permanent
1 parent 2dfcf48 commit 0651cc3

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Lines changed: 296 additions & 79 deletions

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‎src/cli.cppm‎

Lines changed: 43 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -42,6 +42,7 @@ import mcpp.pm.compat; // 0.0.6: namespace field + dotted-name compat shims
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import mcpp.pm.dep_spec;
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import mcpp.ui;
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import mcpp.log;
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import mcpp.fallback.registry;
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import mcpp.bmi_cache;
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import mcpp.dyndep;
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import mcpp.version_req; // SemVer constraint resolution
@@ -4268,6 +4269,40 @@ int cmd_self_version(const mcpplibs::cmdline::ParsedArgs& /*parsed*/) {
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return 0;
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}
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int cmd_self_fallbacks(const mcpplibs::cmdline::ParsedArgs& /*parsed*/) {
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namespace fb = mcpp::fallback;
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// Group by lifecycle
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std::vector<const fb::Entry*> workarounds, compats, permanents;
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for (std::size_t i = 0; i < fb::kEntryCount; ++i) {
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auto* e = &fb::kEntries[i];
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switch (e->lifecycle) {
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case fb::Lifecycle::workaround: workarounds.push_back(e); break;
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case fb::Lifecycle::compat: compats.push_back(e); break;
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case fb::Lifecycle::permanent: permanents.push_back(e); break;
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}
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}
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std::println("Fallback Registry ({} entries)\n", fb::kEntryCount);
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auto print_group = [](std::string_view title, const std::vector<const fb::Entry*>& entries) {
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if (entries.empty()) return;
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std::println("{}:", title);
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for (auto* e : entries) {
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std::print(" {:<40s} {}", e->id, e->description);
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if (!e->removeBy.empty())
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std::print(" [remove by {}]", e->removeBy);
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std::println("");
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}
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std::println("");
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};
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print_group("WORKAROUNDS (need upstream fix)", workarounds);
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print_group("COMPAT (backward compatibility)", compats);
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print_group("PERMANENT (architecturally required)", permanents);
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return 0;
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}
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std::string upper_ascii(std::string s) {
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for (char& ch : s) {
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if (ch >= 'a' && ch <= 'z') ch = static_cast<char>(ch - 'a' + 'A');
@@ -4648,13 +4683,16 @@ int run(int argc, char** argv) {
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.subcommand(cl::App("explain")
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.description("Show extended description for an error code")
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.arg(cl::Arg("code").help("Error code such as E0001").required()))
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.subcommand(cl::App("fallbacks")
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.description("List all registered fallback mechanisms"))
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.action(wrap_rc([&dispatch_sub](const cl::ParsedArgs& p) {
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return dispatch_sub("self", p, {
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{"doctor", cmd_doctor},
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{"env", cmd_env},
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{"config", cmd_self_config},
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{"version", cmd_self_version},
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{"explain", cmd_explain_action},
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{"doctor", cmd_doctor},
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{"env", cmd_env},
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{"config", cmd_self_config},
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{"version", cmd_self_version},
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{"explain", cmd_explain_action},
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{"fallbacks", cmd_self_fallbacks},
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});
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})))
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‎src/fallback/registry.cppm‎

Lines changed: 175 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,175 @@
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// mcpp.fallback.registry — compile-time fallback metadata registry.
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//
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// Every fallback/workaround in mcpp is registered here with its lifecycle,
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// domain, and removal plan. This provides a single source of truth for
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// auditing, logging, and the `mcpp self fallbacks` command.
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//
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// Usage:
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// auto* fb = mcpp::fallback::find("xpkg.copy_from_global");
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// mcpp::log::verbose("fetcher", std::format("[fallback:{}] {}", fb->id, fb->description));
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export module mcpp.fallback.registry;
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import std;
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export namespace mcpp::fallback {
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enum class Lifecycle {
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permanent, // architecturally required (multi-platform, retry logic)
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compat, // backward compatibility, remove by specified version
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workaround, // works around external bug, remove when upstream fixed
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};
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struct Entry {
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std::string_view id; // unique key, e.g. "xpkg.copy_from_global"
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std::string_view domain; // "package" | "config" | "toolchain" | "build" | "dependency" | "manifest"
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std::string_view description; // one-line human-readable description
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Lifecycle lifecycle;
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std::string_view removeBy; // version string "1.0" or "" (permanent/workaround)
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std::string_view upstreamIssue; // "xlings#123" or "" or description of upstream issue
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};
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// ─── Registry entries ────────────────────────────────────────────────
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inline constexpr Entry kEntries[] = {
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// ─── Package fetch & install ─────────────────────────────────────
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{ .id = "xpkg.copy_from_global",
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.domain = "package",
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.description = "copy xpkg from ~/.xlings/ when sandbox install fails",
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.lifecycle = Lifecycle::workaround,
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.removeBy = {},
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.upstreamIssue = "xlings XLINGS_HOME propagation / NDJSON large-package install",
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},
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{ .id = "xpkg.install_direct_before_ndjson",
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.domain = "package",
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.description = "try direct xlings install before NDJSON interface for large packages",
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.lifecycle = Lifecycle::workaround,
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.removeBy = {},
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.upstreamIssue = "xlings NDJSON interface unreliable for large packages",
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},
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{ .id = "xpkg.install_dir_scan",
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.domain = "package",
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.description = "last-resort scan xpkgs/ directory for matching package dir",
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.lifecycle = Lifecycle::compat,
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.removeBy = "1.0",
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},
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{ .id = "xpkg.lua_candidates",
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.domain = "package",
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.description = "multi-candidate xpkg .lua file lookup for legacy naming",
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.lifecycle = Lifecycle::compat,
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.removeBy = "1.0",
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},
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// ─── xlings binary acquisition ───────────────────────────────────
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{ .id = "xlings_binary.vendored_env",
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.domain = "config",
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.description = "MCPP_VENDORED_XLINGS env override (Windows runtime workaround)",
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.lifecycle = Lifecycle::workaround,
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.removeBy = {},
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.upstreamIssue = "Windows xlings binary may lack runtime after copy",
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},
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{ .id = "xlings_binary.system_which",
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.domain = "config",
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.description = "find xlings in PATH when bundled binary unavailable",
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.lifecycle = Lifecycle::permanent,
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},
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// ─── Toolchain probing ───────────────────────────────────────────
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{ .id = "probe.sysroot_compiler",
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.domain = "toolchain",
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.description = "gcc -print-sysroot direct probe",
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.lifecycle = Lifecycle::permanent,
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},
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{ .id = "probe.sysroot_cfg",
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.domain = "toolchain",
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.description = "parse clang++.cfg for --sysroot line",
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.lifecycle = Lifecycle::permanent,
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},
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{ .id = "probe.sysroot_xcrun",
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.domain = "toolchain",
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.description = "macOS xcrun --show-sdk-path fallback",
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.lifecycle = Lifecycle::permanent,
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},
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{ .id = "probe.sysroot_xlings_remap",
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.domain = "toolchain",
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.description = "remap xlings build-time sysroot path to mcpp registry",
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.lifecycle = Lifecycle::workaround,
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.removeBy = {},
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.upstreamIssue = "xlings bakes build-host absolute path into gcc specs",
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},
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{ .id = "sysroot.symlink_kernel_headers",
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.domain = "toolchain",
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.description = "symlink linux-headers xpkg into sysroot when missing",
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.lifecycle = Lifecycle::workaround,
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.removeBy = {},
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.upstreamIssue = "xlings sysroot may lack kernel headers after init",
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},
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{ .id = "sysroot.symlink_glibc_headers",
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.domain = "toolchain",
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.description = "symlink glibc xpkg headers into sysroot when missing",
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.lifecycle = Lifecycle::workaround,
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.removeBy = {},
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.upstreamIssue = "xlings sysroot may lack glibc headers after init",
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},
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// ─── Build system ────────────────────────────────────────────────
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{ .id = "build.ninja_incremental_retry",
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.domain = "build",
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.description = "full rebuild when incremental ninja build fails",
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.lifecycle = Lifecycle::permanent,
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},
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{ .id = "build.dyndep_opt_out",
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.domain = "build",
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.description = "MCPP_NINJA_DYNDEP=0 disables P1689 dynamic deps",
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.lifecycle = Lifecycle::permanent,
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},
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// ─── Dependency resolution ───────────────────────────────────────
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{ .id = "deps.multi_version_mangle",
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.domain = "dependency",
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.description = "cross-major version coexistence via module name mangling",
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.lifecycle = Lifecycle::permanent,
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},
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// ─── Backward compatibility / migration ──────────────────────────
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{ .id = "compat.dotted_package_name",
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.domain = "manifest",
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.description = "split legacy 'ns.name' dotted package names",
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.lifecycle = Lifecycle::compat,
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.removeBy = "1.0",
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},
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{ .id = "compat.config_index_migration",
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.domain = "config",
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.description = "rename mcpp-index to mcpplibs in config files",
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.lifecycle = Lifecycle::compat,
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.removeBy = "1.0",
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},
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};
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inline constexpr std::size_t kEntryCount = sizeof(kEntries) / sizeof(kEntries[0]);
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// ─── Query API ───────────────────────────────────────────────────────
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constexpr const Entry* find(std::string_view id) {
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for (auto& e : kEntries)
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if (e.id == id) return &e;
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return nullptr;
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}
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inline constexpr const char* lifecycle_str(Lifecycle l) {
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switch (l) {
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case Lifecycle::permanent: return "permanent";
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case Lifecycle::compat: return "compat";
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case Lifecycle::workaround: return "workaround";
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}
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return "unknown";
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}
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} // namespace mcpp::fallback

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