Rollup merge of #128596 - RalfJung:const_fn_floating_point_arithmetic, r=nnethercote

stabilize const_fn_floating_point_arithmetic

Part of https://github.com/rust-lang/rust/issues/128288
Fixes https://github.com/rust-lang/rust/issues/57241

The existing test `tests/ui/consts/const_let_eq_float.rs`  ([link](https://github.com/RalfJung/rust/blob/const_fn_floating_point_arithmetic/tests/ui/consts/const_let_eq_float.rs)) covers the basics, and also Miri has extensive tests covering the interpreter's float machinery. Also, that machinery can already be used on stable inside `const`/`static` initializers, just not inside `const fn`.

This was explicitly called out in https://github.com/rust-lang/rfcs/pull/3514 so in a sense t-lang just recently already FCP'd this, but let's hear from them whether they want another FCP for the stabilization here or whether that was covered by the FCP for the RFC.
Cc ``@rust-lang/lang``

### Open items

- [x] Update the Reference: https://github.com/rust-lang/reference/pull/1566

2 files changed, 12 insertions, 48 deletions

diff --git a/compiler/rustc_const_eval/src/check_consts/check.rs b/compiler/rustc_const_eval/src/check_consts/check.rs
index 86a5afa65ba..c80b3e673b0 100644
--- a/compiler/rustc_const_eval/src/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/check_consts/check.rs
@@ -575,10 +575,8 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
 
             Rvalue::UnaryOp(_, operand) => {
                 let ty = operand.ty(self.body, self.tcx);
-                if is_int_bool_or_char(ty) {
-                    // Int, bool, and char operations are fine.
-                } else if ty.is_floating_point() {
-                    self.check_op(ops::FloatingPointOp);
+                if is_int_bool_float_or_char(ty) {
+                    // Int, bool, float, and char operations are fine.
                 } else {
                     span_bug!(self.span, "non-primitive type in `Rvalue::UnaryOp`: {:?}", ty);
                 }
@@ -588,8 +586,8 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 let lhs_ty = lhs.ty(self.body, self.tcx);
                 let rhs_ty = rhs.ty(self.body, self.tcx);
 
-                if is_int_bool_or_char(lhs_ty) && is_int_bool_or_char(rhs_ty) {
-                    // Int, bool, and char operations are fine.
+                if is_int_bool_float_or_char(lhs_ty) && is_int_bool_float_or_char(rhs_ty) {
+                    // Int, bool, float, and char operations are fine.
                 } else if lhs_ty.is_fn_ptr() || lhs_ty.is_unsafe_ptr() {
                     assert_matches!(
                         op,
@@ -603,8 +601,6 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     );
 
                     self.check_op(ops::RawPtrComparison);
-                } else if lhs_ty.is_floating_point() || rhs_ty.is_floating_point() {
-                    self.check_op(ops::FloatingPointOp);
                 } else {
                     span_bug!(
                         self.span,
@@ -1009,8 +1005,8 @@ fn place_as_reborrow<'tcx>(
     }
 }
 
-fn is_int_bool_or_char(ty: Ty<'_>) -> bool {
-    ty.is_bool() || ty.is_integral() || ty.is_char()
+fn is_int_bool_float_or_char(ty: Ty<'_>) -> bool {
+    ty.is_bool() || ty.is_integral() || ty.is_char() || ty.is_floating_point()
 }
 
 fn emit_unstable_in_stable_error(ccx: &ConstCx<'_, '_>, span: Span, gate: Symbol) {
diff --git a/compiler/rustc_const_eval/src/check_consts/ops.rs b/compiler/rustc_const_eval/src/check_consts/ops.rs
index 259114dbdc2..93fafa60557 100644
--- a/compiler/rustc_const_eval/src/check_consts/ops.rs
+++ b/compiler/rustc_const_eval/src/check_consts/ops.rs
@@ -55,28 +55,6 @@ pub trait NonConstOp<'tcx>: std::fmt::Debug {
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx>;
 }
 
-#[derive(Debug)]
-pub struct FloatingPointOp;
-impl<'tcx> NonConstOp<'tcx> for FloatingPointOp {
-    fn status_in_item(&self, ccx: &ConstCx<'_, 'tcx>) -> Status {
-        if ccx.const_kind() == hir::ConstContext::ConstFn {
-            Status::Unstable(sym::const_fn_floating_point_arithmetic)
-        } else {
-            Status::Allowed
-        }
-    }
-
-    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
-    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        feature_err(
-            &ccx.tcx.sess,
-            sym::const_fn_floating_point_arithmetic,
-            span,
-            format!("floating point arithmetic is not allowed in {}s", ccx.const_kind()),
-        )
-    }
-}
-
 /// A function call where the callee is a pointer.
 #[derive(Debug)]
 pub struct FnCallIndirect;
@@ -440,22 +418,12 @@ impl<'tcx> NonConstOp<'tcx> for CellBorrow {
         DiagImportance::Secondary
     }
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        // FIXME: Maybe a more elegant solution to this if else case
-        if let hir::ConstContext::Static(_) = ccx.const_kind() {
-            ccx.dcx().create_err(errors::InteriorMutableDataRefer {
-                span,
-                opt_help: true,
-                kind: ccx.const_kind(),
-                teach: ccx.tcx.sess.teach(E0492),
-            })
-        } else {
-            ccx.dcx().create_err(errors::InteriorMutableDataRefer {
-                span,
-                opt_help: false,
-                kind: ccx.const_kind(),
-                teach: ccx.tcx.sess.teach(E0492),
-            })
-        }
+        ccx.dcx().create_err(errors::InteriorMutableDataRefer {
+            span,
+            opt_help: matches!(ccx.const_kind(), hir::ConstContext::Static(_)),
+            kind: ccx.const_kind(),
+            teach: ccx.tcx.sess.teach(E0492),
+        })
     }
 }