feat: Respect --color=never Cargo option for mutation diagnostics

We already respected the `--color=never` option passed to rustc-driver directly. However, Cargo always forces colored output for its own internal diagnostic processing. This processing strips colors on the Cargo side if no coloring is requested, however this is not detectible from the driver side, and so this never carried over to the detection diagnostics we embed into the mutated crates. We now indicate the requested lack of colored output using an additional MUTEST_CARGO_EXPLICIT_NO_COLOR environment variable, which disables detection diagnostics coloring for Cargo-based invocations.

Author: zalanlevai

cargo-mutest/src/main.rs

@@ -107,6 +107,7 @@ fn main() {
             .arg(clap::Arg::new("PASSED_ARGS").trailing_var_arg(true).allow_hyphen_values(true))
         )
         // Cargo options.
+        .arg(clap::arg!(--color [WHEN] "Output coloring."))
         .next_help_heading("Package Selection")
         .arg(clap::arg!(--workspace "Test all packages in the workspace."))
         .arg(clap::arg!(-p --package [PACKAGE] "Package with the target to analyze."))
@@ -255,6 +256,18 @@ fn main() {
     let i = mutest_args.iter().position(|arg| matches.subcommand_name().is_some_and(|subcommand| arg == subcommand)).expect("subcommand not found in args");
     mutest_args.splice(i.., [mutest_driver_subcommand.to_owned()]);
 
+    if let Some(color) = matches.get_one::<String>("color") {
+        cmd.args(["--color", color]);
+        strip_arg(&mut mutest_args, true, None, Some("color"));
+        if color == "never" {
+            // HACK: Indicate to mutest-driver that the explicit `--color=never` flag was passed.
+            // NOTE: This is needed because Cargo always captures colored output, and
+            //       the Cargo --color flag only controls whether the coloring is stripped before final output.
+            //       This is also required for users because Cargo's rustc invocations disallow using RUSTFLAGS="--color=never".
+            cmd.env("MUTEST_CARGO_EXPLICIT_NO_COLOR", "1");
+        }
+    }
+
     let mut metadata_cmd = cargo_metadata::MetadataCommand::new();
 
     if let Some(manifest_path) = matches.get_one::<String>("manifest-path") {

mutest-emit/src/analysis/diagnostic.rs

@@ -1,3 +1,4 @@
+use std::env;
 use std::io::{self, Write};
 use std::sync::{Arc, Mutex};
 
@@ -80,6 +81,7 @@ pub fn raw_output_full<G: EmissionGuarantee>(
     //       However, this would require mutest_runtime to depend on anstream.
     let color_choice = match color_config {
         ColorConfig::Never => ColorChoice::Never,
+        _ if let Ok("1") = env::var("MUTEST_CARGO_EXPLICIT_NO_COLOR").as_deref() => ColorChoice::Never,
         _ => ColorChoice::AlwaysAnsi,
     };
     let dst: Destination = AutoStream::new(Box::new(shared_buffer), color_choice);