Support safe intrinsics with fallback bodies

Turn `is_val_statically_known` into such an intrinsic to demonstrate. It is perfectly safe to call after all.

Author: oli-obk

Diff for: compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -71,9 +71,13 @@ fn equate_intrinsic_type<'tcx>(
 
 /// Returns the unsafety of the given intrinsic.
 pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -> hir::Unsafety {
-    let has_safe_attr = match tcx.has_attr(intrinsic_id, sym::rustc_safe_intrinsic) {
-        true => hir::Unsafety::Normal,
-        false => hir::Unsafety::Unsafe,
+    let has_safe_attr = if tcx.has_attr(intrinsic_id, sym::rustc_intrinsic) {
+        tcx.fn_sig(intrinsic_id).skip_binder().unsafety()
+    } else {
+        match tcx.has_attr(intrinsic_id, sym::rustc_safe_intrinsic) {
+            true => hir::Unsafety::Normal,
+            false => hir::Unsafety::Unsafe,
+        }
     };
     let is_in_list = match tcx.item_name(intrinsic_id.into()) {
         // When adding a new intrinsic to this list,
@@ -117,6 +121,7 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -
         | sym::forget
         | sym::black_box
         | sym::variant_count
+        | sym::is_val_statically_known
         | sym::ptr_mask
         | sym::debug_assertions => hir::Unsafety::Normal,
         _ => hir::Unsafety::Unsafe,

Diff for: library/core/src/intrinsics.rs

@@ -2513,9 +2513,7 @@ extern "rust-intrinsic" {
 /// use std::hint::unreachable_unchecked;
 /// use std::intrinsics::is_val_statically_known;
 ///
-/// unsafe {
-///    if !is_val_statically_known(0) { unreachable_unchecked(); }
-/// }
+/// if !is_val_statically_known(0) { unsafe { unreachable_unchecked(); } }
 /// ```
 ///
 /// This also means that the following code's behavior is unspecified; it
@@ -2527,9 +2525,7 @@ extern "rust-intrinsic" {
 /// # #![allow(internal_features)]
 /// use std::intrinsics::is_val_statically_known;
 ///
-/// unsafe {
-///     assert_eq!(is_val_statically_known(0), is_val_statically_known(0));
-/// }
+/// assert_eq!(is_val_statically_known(0), is_val_statically_known(0));
 /// ```
 ///
 /// Unsafe code may not rely on `is_val_statically_known` returning any
@@ -2544,7 +2540,7 @@ extern "rust-intrinsic" {
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
-pub const unsafe fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
+pub const fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
     false
 }
 
@@ -2564,7 +2560,7 @@ pub const unsafe fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
 #[rustc_const_unstable(feature = "delayed_debug_assertions", issue = "none")]
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
-pub(crate) const unsafe fn debug_assertions() -> bool {
+pub(crate) const fn debug_assertions() -> bool {
     cfg!(debug_assertions)
 }
 

Diff for: src/tools/miri/tests/pass/intrinsics.rs

@@ -37,7 +37,7 @@ fn main() {
     let mut saw_false = false;
 
     for _ in 0..50 {
-        if unsafe { intrinsics::is_val_statically_known(0) } {
+        if intrinsics::is_val_statically_known(0) {
             saw_true = true;
         } else {
             saw_false = true;

Diff for: tests/codegen/is_val_statically_known.rs

@@ -11,7 +11,7 @@ pub enum B {
 
 #[inline]
 pub fn _u32(a: u32) -> i32 {
-    if unsafe { is_val_statically_known(a) } { 1 } else { 0 }
+    if is_val_statically_known(a) { 1 } else { 0 }
 }
 
 // CHECK-LABEL: @_u32_true(
@@ -30,7 +30,7 @@ pub fn _u32_false(a: u32) -> i32 {
 
 #[inline]
 pub fn _bool(b: bool) -> i32 {
-    if unsafe { is_val_statically_known(b) } { 3 } else { 2 }
+    if is_val_statically_known(b) { 3 } else { 2 }
 }
 
 // CHECK-LABEL: @_bool_true(

Diff for: tests/ui/consts/is_val_statically_known.rs

@@ -4,7 +4,7 @@
 
 use std::intrinsics::is_val_statically_known;
 
-const CONST_TEST: bool = unsafe { is_val_statically_known(0) };
+const CONST_TEST: bool = is_val_statically_known(0);
 
 fn main() {
     if CONST_TEST {