compiletest: Use the new non-libtest executor by default (2)

(Re-landing rust-lang#139998, with a compiler change to inhibit download-rustc.)

Currently the new executor can be explicitly disabled by passing the `-N` flag
to compiletest (e.g. `./x test ui -- -N`), but eventually that flag will be
removed, alongside the removal of the libtest dependency.

Author: Zalathar

src/tools/compiletest/src/common.rs

@@ -414,10 +414,13 @@ pub struct Config {
     /// ABI tests.
     pub minicore_path: Utf8PathBuf,
 
-    /// If true, run tests with the "new" executor that was written to replace
-    /// compiletest's dependency on libtest. Eventually this will become the
-    /// default, and the libtest dependency will be removed.
-    pub new_executor: bool,
+    /// If true, disable the "new" executor, and use the older libtest-based
+    /// executor to run tests instead. This is a temporary fallback, to make
+    /// manual comparative testing easier if bugs are found in the new executor.
+    ///
+    /// FIXME(Zalathar): Eventually remove this flag and remove the libtest
+    /// dependency.
+    pub no_new_executor: bool,
 }
 
 impl Config {

src/tools/compiletest/src/lib.rs

@@ -202,7 +202,7 @@ pub fn parse_config(args: Vec<String>) -> Config {
             "COMMAND",
         )
         .reqopt("", "minicore-path", "path to minicore aux library", "PATH")
-        .optflag("n", "new-executor", "enables the new test executor instead of using libtest")
+        .optflag("N", "no-new-executor", "disables the new test executor, and uses libtest instead")
         .optopt(
             "",
             "debugger",
@@ -448,7 +448,7 @@ pub fn parse_config(args: Vec<String>) -> Config {
 
         minicore_path: opt_path(matches, "minicore-path"),
 
-        new_executor: matches.opt_present("new-executor"),
+        no_new_executor: matches.opt_present("no-new-executor"),
     }
 }
 
@@ -575,9 +575,10 @@ pub fn run_tests(config: Arc<Config>) {
     // Delegate to the executor to filter and run the big list of test structures
     // created during test discovery. When the executor decides to run a test,
     // it will return control to the rest of compiletest by calling `runtest::run`.
-    let res = if config.new_executor {
+    let res = if !config.no_new_executor {
         Ok(executor::run_tests(&config, tests))
     } else {
+        // FIXME(Zalathar): Eventually remove the libtest executor entirely.
         crate::executor::libtest::execute_tests(&config, tests)
     };