Introduce proper build.compiletest-allow-stage0 config option

In favor of the adhoc `COMPILETEST_FORCE_STAGE0` env var.

bootstrap.example.toml

@@ -465,6 +465,11 @@
 # What custom diff tool to use for displaying compiletest tests.
 #build.compiletest-diff-tool = <none>
 
+# Whether to allow `compiletest` self-tests and `compiletest`-managed test
+# suites to be run against the stage 0 rustc. This is only intended to be used
+# when the stage 0 compiler is actually built from in-tree sources.
+#build.compiletest-allow-stage0 = false
+
 # Whether to use the precompiled stage0 libtest with compiletest.
 #build.compiletest-use-stage0-libtest = true
 

src/bootstrap/src/core/build_steps/test.rs

@@ -723,8 +723,8 @@ impl Step for CompiletestTest {
         let mut cargo = tool::prepare_tool_cargo(
             builder,
             compiler,
-            // compiletest uses libtest internals; make it use the in-tree std to make sure it never breaks
-            // when std sources change.
+            // compiletest uses libtest internals; make it use the in-tree std to make sure it never
+            // breaks when std sources change.
             Mode::ToolStd,
             host,
             Kind::Test,
@@ -1612,12 +1612,11 @@ impl Step for Compiletest {
             return;
         }
 
-        if builder.top_stage == 0 && env::var("COMPILETEST_FORCE_STAGE0").is_err() {
+        if builder.top_stage == 0 && !builder.config.compiletest_allow_stage0 {
             eprintln!("\
 ERROR: `--stage 0` runs compiletest on the stage0 (precompiled) compiler, not your local changes, and will almost always cause tests to fail
-HELP: to test the compiler, use `--stage 1` instead
-HELP: to test the standard library, use `--stage 0 library/std` instead
-NOTE: if you're sure you want to do this, please open an issue as to why. In the meantime, you can override this with `COMPILETEST_FORCE_STAGE0=1`."
+HELP: to test the compiler or standard library, omit the stage or explicitly use `--stage 1` instead
+NOTE: if you're sure you want to do this, please open an issue as to why. In the meantime, you can override this with `--set build.compiletest-allow-stage0=true`."
             );
             crate::exit!(1);
         }

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

@@ -298,8 +298,16 @@ pub struct Config {
     /// Command for visual diff display, e.g. `diff-tool --color=always`.
     pub compiletest_diff_tool: Option<String>,
 
+    /// Whether to allow running both `compiletest` self-tests and `compiletest`-managed test suites
+    /// against the stage 0 (rustc, std).
+    ///
+    /// This is only intended to be used when the stage 0 compiler is actually built from in-tree
+    /// sources.
+    pub compiletest_allow_stage0: bool,
+
     /// Whether to use the precompiled stage0 libtest with compiletest.
     pub compiletest_use_stage0_libtest: bool,
+
     /// Default value for `--extra-checks`
     pub tidy_extra_checks: Option<String>,
     pub is_running_on_ci: bool,
@@ -749,6 +757,7 @@ impl Config {
             optimized_compiler_builtins,
             jobs,
             compiletest_diff_tool,
+            compiletest_allow_stage0,
             compiletest_use_stage0_libtest,
             tidy_extra_checks,
             ccache,
@@ -1020,8 +1029,12 @@ impl Config {
 
         config.optimized_compiler_builtins =
             optimized_compiler_builtins.unwrap_or(config.channel != "dev");
+
         config.compiletest_diff_tool = compiletest_diff_tool;
+
+        config.compiletest_allow_stage0 = compiletest_allow_stage0.unwrap_or(false);
         config.compiletest_use_stage0_libtest = compiletest_use_stage0_libtest.unwrap_or(true);
+
         config.tidy_extra_checks = tidy_extra_checks;
 
         let download_rustc = config.download_rustc_commit.is_some();

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

@@ -68,6 +68,7 @@ define_config! {
         optimized_compiler_builtins: Option<bool> = "optimized-compiler-builtins",
         jobs: Option<u32> = "jobs",
         compiletest_diff_tool: Option<String> = "compiletest-diff-tool",
+        compiletest_allow_stage0: Option<bool> = "compiletest-allow-stage0",
         compiletest_use_stage0_libtest: Option<bool> = "compiletest-use-stage0-libtest",
         tidy_extra_checks: Option<String> = "tidy-extra-checks",
         ccache: Option<StringOrBool> = "ccache",