add -C overflow-checks option

In addition to defining and handling the new option, we also add a
method on librustc::Session for determining the necessity of overflow
checks.  This method provides a single point to sort out the three (!)
different ways for turning on overflow checks: -C debug-assertions, -C
overflow-checks, and -Z force-overflow-checks.

Fixes #33134.

src/librustc/session/config.rs

@@ -804,6 +804,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "save all temporary output files during compilation"),
     rpath: bool = (false, parse_bool, [UNTRACKED],
         "set rpath values in libs/exes"),
+    overflow_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "use overflow checks for integer arithmetic"),
     no_prepopulate_passes: bool = (false, parse_bool, [TRACKED],
         "don't pre-populate the pass manager with a list of passes"),
     no_vectorize_loops: bool = (false, parse_bool, [TRACKED],
@@ -2345,6 +2347,10 @@ mod tests {
         opts.cg.llvm_args = vec![String::from("1"), String::from("2")];
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
 
+        opts = reference.clone();
+        opts.cg.overflow_checks = Some(true);
+        assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
+
         opts = reference.clone();
         opts.cg.no_prepopulate_passes = true;
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());

src/librustc/session/mod.rs

@@ -372,6 +372,11 @@ impl Session {
     pub fn nonzeroing_move_hints(&self) -> bool {
         self.opts.debugging_opts.enable_nonzeroing_move_hints
     }
+    pub fn overflow_checks(&self) -> bool {
+        self.opts.cg.overflow_checks
+            .or(self.opts.debugging_opts.force_overflow_checks)
+            .unwrap_or(self.opts.debug_assertions)
+    }
 
     pub fn must_not_eliminate_frame_pointers(&self) -> bool {
         self.opts.debuginfo != DebugInfoLevel::NoDebugInfo ||