hook max-threads up for roc check

cli/src/build.rs

@@ -350,6 +350,7 @@ pub fn check_file(
     src_dir: PathBuf,
     roc_file_path: PathBuf,
     emit_timings: bool,
+    threading: Threading,
 ) -> Result<(program::Problems, Duration), LoadingProblem> {
     let compilation_start = SystemTime::now();
 
@@ -368,7 +369,7 @@ pub fn check_file(
         target_info,
         // TODO: expose this from CLI?
         RenderTarget::ColorTerminal,
-        Threading::AllAvailable,
+        threading,
     )?;
 
     let buf = &mut String::with_capacity(1024);

cli/src/lib.rs

@@ -62,8 +62,8 @@ pub fn build_app<'a>() -> Command<'a> {
     let flag_max_threads = Arg::new(FLAG_MAX_THREADS)
         .long(FLAG_MAX_THREADS)
         .help("Limit the number of threads (and hence cores) used during compilation.")
-        .takes_value(true)
         .requires(ROC_FILE)
+        .takes_value(true)
         .validator(|s| s.parse::<usize>())
         .required(false);
 
@@ -185,6 +185,7 @@ pub fn build_app<'a>() -> Command<'a> {
         .subcommand(Command::new(CMD_CHECK)
             .about("Check the code for problems, but doesn’t build or run it")
             .arg(flag_time.clone())
+            .arg(flag_max_threads.clone())
             .arg(
                 Arg::new(ROC_FILE)
                     .help("The .roc file of an app to check")

cli/src/main.rs

@@ -6,7 +6,7 @@ use roc_cli::{
     FLAG_NO_LINK, FLAG_TARGET, FLAG_TIME, ROC_FILE,
 };
 use roc_error_macros::user_error;
-use roc_load::LoadingProblem;
+use roc_load::{LoadingProblem, Threading};
 use std::fs::{self, FileType};
 use std::io;
 use std::path::{Path, PathBuf};
@@ -94,7 +94,17 @@ fn main() -> io::Result<()> {
             let roc_file_path = PathBuf::from(filename);
             let src_dir = roc_file_path.parent().unwrap().to_owned();
 
-            match check_file(&arena, src_dir, roc_file_path, emit_timings) {
+            let threading = match matches
+                .value_of(roc_cli::FLAG_MAX_THREADS)
+                .and_then(|s| s.parse::<usize>().ok())
+            {
+                None => Threading::AllAvailable,
+                Some(0) => user_error!("cannot build with at most 0 threads"),
+                Some(1) => Threading::Single,
+                Some(n) => Threading::AtMost(n),
+            };
+
+            match check_file(&arena, src_dir, roc_file_path, emit_timings, threading) {
                 Ok((problems, total_time)) => {
                     println!(
                         "\x1B[{}m{}\x1B[39m {} and \x1B[{}m{}\x1B[39m {} found in {} ms.",