Add support for `cfg(overflow_checks)`

This PR adds support for detecting if overflow checks are enabled in similar fashion as debug_assertions are detected.
Possible use-case of this, for example, if we want to use checked integer casts in builds with overflow checks, e.g.

```rust
pub fn cast(val: usize)->u16 {
    if cfg!(overflow_checks) {
        val.try_into().unwrap()
    }
    else{
        vas as _
    }
}
```

Resolves #91130.
Tracking issue: #111466.

4 files changed, 9 insertions, 0 deletions

diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index f4615445f8e..fe8c630666b 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -321,6 +321,8 @@ declare_features! (
     (active, c_unwind, "1.52.0", Some(74990), None),
     /// Allows using C-variadics.
     (active, c_variadic, "1.34.0", Some(44930), None),
+    /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.
+    (active, cfg_overflow_checks, "CURRENT_RUSTC_VERSION", Some(111466), None),
     /// Allows the use of `#[cfg(sanitize = "option")]`; set when -Zsanitizer is used.
     (active, cfg_sanitize, "1.41.0", Some(39699), None),
     /// Allows `cfg(target_abi = "...")`.
diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index fe05d4590e7..61cfbf5c5e5 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/compiler/rustc_feature/src/builtin_attrs.rs
@@ -24,6 +24,7 @@ pub type GatedCfg = (Symbol, Symbol, GateFn);
 /// `cfg(...)`'s that are feature gated.
 const GATED_CFGS: &[GatedCfg] = &[
     // (name in cfg, feature, function to check if the feature is enabled)
+    (sym::overflow_checks, sym::cfg_overflow_checks, cfg_fn!(cfg_overflow_checks)),
     (sym::target_abi, sym::cfg_target_abi, cfg_fn!(cfg_target_abi)),
     (sym::target_thread_local, sym::cfg_target_thread_local, cfg_fn!(cfg_target_thread_local)),
     (
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index aa3cb03bad8..e2b8d3eea2d 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1060,6 +1060,9 @@ fn default_configuration(sess: &Session) -> CrateConfig {
     if sess.opts.debug_assertions {
         ret.insert((sym::debug_assertions, None));
     }
+    if sess.overflow_checks() {
+        ret.insert((sym::overflow_checks, None));
+    }
     // JUSTIFICATION: before wrapper fn is available
     #[allow(rustc::bad_opt_access)]
     if sess.opts.crate_types.contains(&CrateType::ProcMacro) {
@@ -1209,6 +1212,7 @@ impl CrateCheckConfig {
             sym::windows,
             sym::proc_macro,
             sym::debug_assertions,
+            sym::overflow_checks,
             sym::target_thread_local,
         ] {
             self.expecteds.entry(name).or_insert_with(no_values);
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 60efcb768cb..9951d8f4fc1 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -463,6 +463,7 @@ symbols! {
         cfg_doctest,
         cfg_eval,
         cfg_hide,
+        cfg_overflow_checks,
         cfg_panic,
         cfg_sanitize,
         cfg_target_abi,
@@ -1065,6 +1066,7 @@ symbols! {
         or_patterns,
         other,
         out,
+        overflow_checks,
         overlapping_marker_traits,
         owned_box,
         packed,