Remove obsolete option build.compiletest-use-stage0-libtest

Author: Zalathar

src/bootstrap/src/core/config/config.rs

@@ -533,8 +533,6 @@ impl Config {
             optimized_compiler_builtins: build_optimized_compiler_builtins,
             jobs: build_jobs,
             compiletest_diff_tool: build_compiletest_diff_tool,
-            // No longer has any effect; kept (for now) to avoid breaking people's configs.
-            compiletest_use_stage0_libtest: _,
             tidy_extra_checks: build_tidy_extra_checks,
             ccache: build_ccache,
             exclude: build_exclude,

src/bootstrap/src/core/config/toml/build.rs

@@ -72,9 +72,6 @@ define_config! {
         jobs: Option<u32> = "jobs",
         compiletest_diff_tool: Option<String> = "compiletest-diff-tool",
         compiletest_allow_stage0: Option<bool> = "compiletest-allow-stage0",
-        /// No longer has any effect; kept (for now) to avoid breaking people's configs.
-        /// FIXME(#146929): Remove this in 2026.
-        compiletest_use_stage0_libtest: Option<bool> = "compiletest-use-stage0-libtest",
         tidy_extra_checks: Option<String> = "tidy-extra-checks",
         ccache: Option<StringOrBool> = "ccache",
         exclude: Option<Vec<PathBuf>> = "exclude",

src/bootstrap/src/utils/change_tracker.rs

@@ -656,4 +656,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Warning,
         summary: "GDB and LLDB are no longer automatically discovered from the environment. If you want to use path discovery for them, you can opt in using `build.gdb = \"discover\"` or `build.lldb = \"discover\"`.",
     },
+    ChangeInfo {
+        change_id: 159878,
+        severity: ChangeSeverity::Warning,
+        summary: "Obsolete option `build.compiletest-use-stage0-libtest` has no effect and has been removed.",
+    },
 ];