Rollup merge of rust-lang#77122 - ecstatic-morse:const-fn-arithmetic, r=RalfJung,oli-obk

Add `#![feature(const_fn_floating_point_arithmetic)]`

cc rust-lang#76618

This is a template for splitting up `const_fn` into granular feature gates. I think this will make it easier, both for us and for users, to track stabilization of each individual feature. We don't *have* to do this, however. We could also keep stabilizing things out from under `const_fn`.

cc @rust-lang/wg-const-eval
r? @oli-obk

Author: RalfJung

compiler/rustc_feature/src/active.rs

@@ -584,6 +584,9 @@ declare_features! (
     /// Allows non trivial generic constants which have to be manually propageted upwards.
     (active, const_evaluatable_checked, "1.48.0", Some(76560), None),
 
+    /// Allows basic arithmetic on floating point types in a `const fn`.
+    (active, const_fn_floating_point_arithmetic, "1.48.0", Some(57241), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -112,6 +112,30 @@ impl NonConstOp for Abort {
     }
 }
 
+#[derive(Debug)]
+pub struct FloatingPointOp;
+impl NonConstOp for FloatingPointOp {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        if ccx.const_kind() == hir::ConstContext::ConstFn {
+            Status::Unstable(sym::const_fn_floating_point_arithmetic)
+        } else {
+            Status::Allowed
+        }
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        feature_err(
+            &ccx.tcx.sess.parse_sess,
+            sym::const_fn_floating_point_arithmetic,
+            span,
+            &format!("floating point arithmetic is not allowed in {}s", ccx.const_kind()),
+        )
+        .emit();
+    }
+}
+
 #[derive(Debug)]
 pub struct NonPrimitiveOp;
 impl NonConstOp for NonPrimitiveOp {

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -540,8 +540,12 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
 
             Rvalue::UnaryOp(_, ref operand) => {
                 let ty = operand.ty(self.body, self.tcx);
-                if !(ty.is_integral() || ty.is_bool()) {
-                    self.check_op(ops::NonPrimitiveOp)
+                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);
+                } else {
+                    span_bug!(self.span, "non-primitive type in `Rvalue::UnaryOp`: {:?}", ty);
                 }
             }
 
@@ -550,7 +554,9 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                 let lhs_ty = lhs.ty(self.body, self.tcx);
                 let rhs_ty = rhs.ty(self.body, self.tcx);
 
-                if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
+                if is_int_bool_or_char(lhs_ty) && is_int_bool_or_char(rhs_ty) {
+                    // Int, bool, and char operations are fine.
+                } else if lhs_ty.is_fn_ptr() || lhs_ty.is_unsafe_ptr() {
                     assert_eq!(lhs_ty, rhs_ty);
                     assert!(
                         op == BinOp::Eq
@@ -563,12 +569,15 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                     );
 
                     self.check_op(ops::RawPtrComparison);
-                }
-
-                if !(lhs_ty.is_integral() || lhs_ty.is_bool() || lhs_ty.is_char())
-                    || !(rhs_ty.is_integral() || rhs_ty.is_bool() || rhs_ty.is_char())
-                {
-                    self.check_op(ops::NonPrimitiveOp)
+                } else if lhs_ty.is_floating_point() || rhs_ty.is_floating_point() {
+                    self.check_op(ops::FloatingPointOp);
+                } else {
+                    span_bug!(
+                        self.span,
+                        "non-primitive type in `Rvalue::BinaryOp`: {:?} ⚬ {:?}",
+                        lhs_ty,
+                        rhs_ty
+                    );
                 }
             }
         }
@@ -867,3 +876,7 @@ fn place_as_reborrow(
         }
     })
 }
+
+fn is_int_bool_or_char(ty: Ty<'_>) -> bool {
+    ty.is_bool() || ty.is_integral() || ty.is_char()
+}

compiler/rustc_span/src/symbol.rs

@@ -352,6 +352,7 @@ symbols! {
         const_evaluatable_checked,
         const_extern_fn,
         const_fn,
+        const_fn_floating_point_arithmetic,
         const_fn_transmute,
         const_fn_union,
         const_generics,

library/core/src/lib.rs

@@ -82,6 +82,7 @@
 #![feature(const_pin)]
 #![feature(const_fn_union)]
 #![feature(const_fn)]
+#![cfg_attr(not(bootstrap), feature(const_fn_floating_point_arithmetic))]
 #![feature(const_generics)]
 #![feature(const_option)]
 #![feature(const_precise_live_drops)]

library/std/src/lib.rs

@@ -236,6 +236,7 @@
 #![feature(clamp)]
 #![feature(concat_idents)]
 #![feature(const_cstr_unchecked)]
+#![cfg_attr(not(bootstrap), feature(const_fn_floating_point_arithmetic))]
 #![feature(const_fn_transmute)]
 #![feature(const_fn)]
 #![feature(const_ip)]

src/test/ui/consts/const-extern-fn/const-extern-fn-min-const-fn.rs

@@ -4,7 +4,7 @@ const extern fn unsize(x: &[u8; 3]) -> &[u8] { x }
 const unsafe extern "C" fn closure() -> fn() { || {} }
 //~^ ERROR function pointers in const fn are unstable
 const unsafe extern fn use_float() { 1.0 + 1.0; }
-//~^ ERROR only int, `bool` and `char` operations are stable in const fn
+//~^ ERROR floating point arithmetic
 const extern "C" fn ptr_cast(val: *const u8) { val as usize; }
 //~^ ERROR casting pointers to integers
 

src/test/ui/consts/const-extern-fn/const-extern-fn-min-const-fn.stderr

@@ -7,14 +7,14 @@ LL | const unsafe extern "C" fn closure() -> fn() { || {} }
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
-error[E0723]: only int, `bool` and `char` operations are stable in const fn
+error[E0658]: floating point arithmetic is not allowed in constant functions
   --> $DIR/const-extern-fn-min-const-fn.rs:6:38
    |
 LL | const unsafe extern fn use_float() { 1.0 + 1.0; }
    |                                      ^^^^^^^^^
    |
-   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
-   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+   = note: see issue #57241 <https://github.com/rust-lang/rust/issues/57241> for more information
+   = help: add `#![feature(const_fn_floating_point_arithmetic)]` to the crate attributes to enable
 
 error[E0658]: casting pointers to integers in constant functions is unstable
   --> $DIR/const-extern-fn-min-const-fn.rs:8:48

src/test/ui/consts/const_fn_floating_point_arithmetic.gated.stderr

@@ -0,0 +1,8 @@
+error: fatal error triggered by #[rustc_error]
+  --> $DIR/const_fn_floating_point_arithmetic.rs:20:1
+   |
+LL | fn main() {}
+   | ^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/consts/const_fn_floating_point_arithmetic.rs

@@ -0,0 +1,20 @@
+// gate-test-const_fn_floating_point_arithmetic
+
+// revisions: stock gated
+
+#![feature(rustc_attrs)]
+#![cfg_attr(gated, feature(const_fn_floating_point_arithmetic))]
+
+const fn add(f: f32) -> f32 { f + 2.0 }
+//[stock]~^ floating point arithmetic
+const fn sub(f: f32) -> f32 { 2.0 - f }
+//[stock]~^ floating point arithmetic
+const fn mul(f: f32, g: f32) -> f32 { f * g }
+//[stock]~^ floating point arithmetic
+const fn div(f: f32, g: f32) -> f32 { f / g }
+//[stock]~^ floating point arithmetic
+const fn neg(f: f32) -> f32 { -f }
+//[stock]~^ floating point arithmetic
+
+#[rustc_error]
+fn main() {} //[gated]~ fatal error triggered by #[rustc_error]