[nextest-runner] use 1 thread in the miri environment by default

Define a new profile `default-miri`, and use it when `miri` is detected.

See rust-lang/miri#2398 (comment)
for discussion.

Author: sunshowers

cargo-nextest/src/dispatch.rs

@@ -937,8 +937,17 @@ impl App {
         profile_name: Option<&str>,
         config: &'cfg NextestConfig,
     ) -> Result<NextestProfile<'cfg>> {
+        let profile_name = profile_name.unwrap_or_else(|| {
+            // The "official" way to detect a miri environment is with MIRI_SYSROOT.
+            // https://github.com/rust-lang/miri/pull/2398#issuecomment-1190747685
+            if std::env::var_os("MIRI_SYSROOT").is_some() {
+                NextestConfig::DEFAULT_MIRI_PROFILE
+            } else {
+                NextestConfig::DEFAULT_PROFILE
+            }
+        });
         let profile = config
-            .profile(profile_name.unwrap_or(NextestConfig::DEFAULT_PROFILE))
+            .profile(profile_name)
             .map_err(ExpectedError::profile_not_found)?;
         let store_dir = profile.store_dir();
         std::fs::create_dir_all(&store_dir).map_err(|err| ExpectedError::StoreDirCreateError {

nextest-runner/default-config.toml

@@ -84,3 +84,8 @@ leak-timeout = "100ms"
 # reports across different test runs, it may be useful to provide separate names
 # for each report.
 report-name = "nextest-run"
+
+# This profile is activated if MIRI_SYSROOT is set.
+[profile.default-miri]
+# Miri tests take up a lot of memory, so only run 1 test at a time by default.
+test-threads = 1

nextest-runner/src/config.rs

@@ -48,6 +48,9 @@ impl NextestConfig {
     /// The name of the default profile.
     pub const DEFAULT_PROFILE: &'static str = "default";
 
+    /// The name of the default profile used for miri.
+    pub const DEFAULT_MIRI_PROFILE: &'static str = "default-miri";
+
     /// Reads the nextest config from the given file, or if not specified from `.config/nextest.toml`
     /// in the workspace root.
     ///