Add a stable flag to control codegen UB checks

Author: saethlin

compiler/rustc_interface/src/tests.rs

@@ -609,6 +609,7 @@ fn test_codegen_options_tracking_hash() {
     tracked!(debug_assertions, Some(true));
     tracked!(debuginfo, DebugInfo::Limited);
     tracked!(embed_bitcode, false);
+    tracked!(extra_ub_checks, Some(true));
     tracked!(force_frame_pointers, Some(false));
     tracked!(force_unwind_tables, Some(true));
     tracked!(inline_threshold, Some(0xf007ba11));

compiler/rustc_mir_transform/src/check_alignment.rs

@@ -18,7 +18,7 @@ impl<'tcx> MirPass<'tcx> for CheckAlignment {
         if sess.target.llvm_target == "i686-pc-windows-msvc" {
             return false;
         }
-        sess.opts.debug_assertions
+        sess.extra_ub_checks()
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

compiler/rustc_session/src/options.rs

@@ -1316,6 +1316,8 @@ options! {
         "emit bitcode in rlibs (default: yes)"),
     extra_filename: String = (String::new(), parse_string, [UNTRACKED],
         "extra data to put in each output filename"),
+    extra_ub_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "insert extra runtime checks in codegen that catch Undefined Behavior"),
     force_frame_pointers: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "force use of the frame pointers"),
     #[rustc_lint_opt_deny_field_access("use `Session::must_emit_unwind_tables` instead of this field")]

compiler/rustc_session/src/session.rs

@@ -1104,6 +1104,10 @@ impl Session {
         self.opts.cg.overflow_checks.unwrap_or(self.opts.debug_assertions)
     }
 
+    pub fn extra_ub_checks(&self) -> bool {
+        self.opts.cg.extra_ub_checks.unwrap_or(self.opts.debug_assertions)
+    }
+
     pub fn relocation_model(&self) -> RelocModel {
         self.opts.cg.relocation_model.unwrap_or(self.target.relocation_model)
     }

src/doc/rustc/src/codegen-options/index.md

@@ -140,6 +140,17 @@ This option allows you to put extra data in each output filename. It takes a
 string to add as a suffix to the filename. See the [`--emit`
 flag][option-emit] for more information.
 
+## extra-ub-checks
+
+This flag controls whether the compiler inserts runtime checks during code generation
+to catch Undefined Behavior.
+
+* `y`, `yes`, `on`, `true`, or no value: insert such checks regardless of debug-assertions.
+* `n`, `no`, `off`, `false`: do not emit such checks regardless of debug-assertions.
+
+If not specified, extra UB checks are enabled if
+[debug-assertions](#debug-assertions) are enabled, disabled otherwise.
+
 ## force-frame-pointers
 
 This flag forces the use of frame pointers. It takes one of the following

src/tools/miri/src/lib.rs

@@ -138,5 +138,5 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zmir-emit-retag",
     "-Zmir-keep-place-mention",
     "-Zmir-opt-level=0",
-    "-Zmir-enable-passes=-CheckAlignment",
+    "-Cextra-ub-checks=false",
 ];

src/tools/miri/tests/fail/unaligned_pointers/alignment.rs

@@ -1,5 +1,4 @@
 //@normalize-stderr-test: "\| +\^+" -> "| ^"
-//@compile-flags: -Cdebug-assertions=no
 
 fn main() {
     // No retry needed, this fails reliably.

src/tools/miri/tests/fail/unaligned_pointers/atomic_unaligned.rs

@@ -1,4 +1,4 @@
-//@compile-flags: -Zmiri-symbolic-alignment-check -Cdebug-assertions=no
+//@compile-flags: -Zmiri-symbolic-alignment-check
 #![feature(core_intrinsics)]
 
 fn main() {

src/tools/miri/tests/fail/unaligned_pointers/drop_in_place.rs

@@ -1,5 +1,3 @@
-//@compile-flags: -Cdebug-assertions=no
-
 #[repr(transparent)]
 struct HasDrop(u8);
 

src/tools/miri/tests/fail/unaligned_pointers/dyn_alignment.rs

@@ -1,5 +1,5 @@
 // should find the bug even without, but gets masked by optimizations
-//@compile-flags: -Zmiri-disable-stacked-borrows -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-stacked-borrows
 //@normalize-stderr-test: "but found [0-9]+" -> "but found $$ALIGN"
 
 #[repr(align(256))]