Add a flag for pretending to be a stable compiler when bisecting.

src/main.rs

@@ -149,6 +149,13 @@ struct Opts {
     )]
     command_args: Vec<OsString>,
 
+    #[arg(
+        long,
+        help = "Pretend to be a stable compiler (disable features, \
+report a version that looks like a stable version)"
+    )]
+    pretend_to_be_stable: bool,
+
     #[arg(
         long,
         help = "Left bound for search (*without* regression). You can use \

src/toolchains.rs

@@ -313,12 +313,29 @@ impl Toolchain {
     }
 
     fn set_cargo_args_and_envs(&self, cmd: &mut Command, cfg: &Config) {
-        cmd.arg(&format!("+{}", self.rustup_name()));
+        let rustup_name = format!("+{}", self.rustup_name());
+        cmd.arg(&rustup_name);
         if cfg.args.command_args.is_empty() {
             cmd.arg("build");
         } else {
             cmd.args(&cfg.args.command_args);
         }
+        if cfg.args.pretend_to_be_stable && self.is_current_nightly() {
+            // Forbid using features
+            cmd.env(
+                "RUSTFLAGS",
+                format!(
+                    "{} -Zenable-features=",
+                    std::env::var("RUSTFLAGS").unwrap_or_default()
+                ),
+            );
+            // Make rustc report a stable version string derived from the current nightly's version string.
+            let version = rustc_version::version_meta().unwrap().semver;
+            cmd.env(
+                "RUSTC_FORCE_RUSTC_VERSION",
+                format!("{}.{}.{}", version.major, version.minor, version.patch),
+            );
+        }
     }
 
     pub(crate) fn test(&self, cfg: &Config) -> TestOutcome {

tests/cmd/h.stdout

@@ -19,6 +19,8 @@ Options:
       --install <INSTALL>       Install the given artifact
       --preserve                Preserve the downloaded artifacts
       --preserve-target         Preserve the target directory used for builds
+      --pretend-to-be-stable    Pretend to be a stable compiler (disable features, report a version
+                                that looks like a stable version)
       --prompt                  Manually evaluate for regression with prompts
       --regress <REGRESS>       Custom regression definition [default: error] [possible values:
                                 error, success, ice, non-ice, non-error]

tests/cmd/help.stdout

@@ -46,6 +46,10 @@ Options:
       --preserve-target
           Preserve the target directory used for builds
 
+      --pretend-to-be-stable
+          Pretend to be a stable compiler (disable features, report a version that looks like a
+          stable version)
+
       --prompt
           Manually evaluate for regression with prompts