Emit a warning when Miri is used with optimizations

Author: saethlin

src/bin/miri.rs

@@ -30,6 +30,8 @@ use rustc_middle::{
     },
     ty::{query::ExternProviders, TyCtxt},
 };
+use rustc_session::config::OptLevel;
+
 use rustc_session::{config::CrateType, search_paths::PathKind, CtfeBacktrace};
 
 use miri::{BacktraceStyle, ProvenanceMode, RetagFields};
@@ -82,6 +84,15 @@ impl rustc_driver::Callbacks for MiriCompilerCalls {
                 env::set_current_dir(cwd).unwrap();
             }
 
+            if tcx.sess.opts.optimize != OptLevel::No {
+                tcx.sess.warn("Miri does not support optimizations.");
+            } else if tcx.sess.mir_opt_level() > 0 {
+                tcx.sess.warn("You have explicitly enabled MIR optimizations, overriding Miri's default \
+                    which is to completely disable them. Any optimizations may hide UB that Miri would \
+                    otherwise detect, and it is not necessarily possible to predict what kind of UB will \
+                    be missed.");
+            }
+
             if let Some(return_code) = miri::eval_entry(tcx, entry_def_id, entry_type, config) {
                 std::process::exit(
                     i32::try_from(return_code).expect("Return value was too large!"),

src/lib.rs

@@ -129,6 +129,5 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zmir-emit-retag",
     "-Zmir-opt-level=0",
     "--cfg=miri",
-    "-Cdebug-assertions=on",
     "-Zextra-const-ub-checks",
 ];