Allow using fn pointers in const fn with unleash miri

src/librustc_mir/transform/qualify_consts.rs

@@ -1407,11 +1407,23 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
                     }
                 }
                 ty::FnPtr(_) => {
+                    let unleash_miri = self
+                        .tcx
+                        .sess
+                        .opts
+                        .debugging_opts
+                        .unleash_the_miri_inside_of_you;
+                    let const_fn_ptr = self.tcx.features().const_fn_ptr;
                     if self.mode.requires_const_checking() {
-                        let mut err = self.tcx.sess.struct_span_err(
-                            self.span,
-                            &format!("function pointers are not allowed in const fn"));
-                        err.emit();
+                        if !(unleash_miri || const_fn_ptr) {
+                            emit_feature_err(
+                                &self.tcx.sess.parse_sess,
+                                sym::const_fn_ptr,
+                                self.span,
+                                GateIssue::Language,
+                                "function pointers in const fn are unstable",
+                            );
+                        }
                     }
                 }
                 _ => {

src/libsyntax/feature_gate/active.rs

@@ -405,6 +405,9 @@ declare_features! (
     /// Allows macro invocations in `extern {}` blocks.
     (active, macros_in_extern, "1.27.0", Some(49476), None),
 
+    /// Allows calling function pointers inside `const` functions.
+    (active, const_fn_ptr, "1.27.0", Some(51909), None),
+
     /// Allows accessing fields of unions inside `const` functions.
     (active, const_fn_union, "1.27.0", Some(51909), None),
 

src/libsyntax_pos/symbol.rs

@@ -198,6 +198,7 @@ symbols! {
         const_compare_raw_pointers,
         const_constructor,
         const_fn,
+        const_fn_ptr,
         const_fn_union,
         const_generics,
         const_indexing,

src/test/ui/consts/const-eval/const_fn_ptr.rs

@@ -0,0 +1,21 @@
+// run-pass
+#![feature(const_fn)]
+#![feature(const_fn_ptr)]
+
+const fn double(x: usize) -> usize { x * 2 }
+const X: fn(usize) -> usize = double;
+
+const fn bar(x: usize) -> usize {
+    X(x)
+}
+
+const fn foo(x: fn(usize) -> usize, y: usize)  -> usize {
+    x(y)
+}
+
+fn main() {
+    const Y: usize = bar(2);
+    assert_eq!(Y, 4);
+    const Z: usize = foo(double, 2);
+    assert_eq!(Z, 4);
+}

src/test/ui/consts/const-eval/const_fn_ptr_fail.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+// FIXME: this should not pass
+
+#![feature(const_fn)]
+#![feature(const_fn_ptr)]
+
+fn double(x: usize) -> usize { x * 2 }
+const X: fn(usize) -> usize = double;
+
+const fn bar(x: usize) -> usize {
+    X(x)
+}
+
+fn main() {}

src/test/ui/consts/const-eval/const_fn_ptr_fail.stderr

@@ -0,0 +1,20 @@
+warning: function is never used: `double`
+  --> $DIR/const_fn_ptr_fail.rs:8:1
+   |
+LL | fn double(x: usize) -> usize { x * 2 }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(dead_code)]` on by default
+
+warning: constant item is never used: `X`
+  --> $DIR/const_fn_ptr_fail.rs:9:1
+   |
+LL | const X: fn(usize) -> usize = double;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: function is never used: `bar`
+  --> $DIR/const_fn_ptr_fail.rs:11:1
+   |
+LL | const fn bar(x: usize) -> usize {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+

src/test/ui/consts/const-eval/const_fn_ptr_fail2.rs

@@ -0,0 +1,21 @@
+#![feature(const_fn)]
+#![feature(const_fn_ptr)]
+
+fn double(x: usize) -> usize { x * 2 }
+const X: fn(usize) -> usize = double;
+
+const fn bar(x: fn(usize) -> usize, y: usize) -> usize {
+    x(y)
+}
+
+const Y: usize = bar(X, 2);
+//~^ ERROR any use of this value will cause an error
+
+const Z: usize = bar(double, 2);
+//~^ ERROR any use of this value will cause an error
+
+
+fn main() {
+    assert_eq!(Y, 4);
+    assert_eq!(Z, 4);
+}

src/test/ui/consts/const-eval/const_fn_ptr_fail2.stderr

@@ -0,0 +1,28 @@
+error: any use of this value will cause an error
+  --> $DIR/const_fn_ptr_fail2.rs:8:5
+   |
+LL |     x(y)
+   |     ^^^^
+   |     |
+   |     calling non-const function `double`
+   |     inside call to `bar` at $DIR/const_fn_ptr_fail2.rs:11:18
+...
+LL | const Y: usize = bar(X, 2);
+   | ---------------------------
+   |
+   = note: `#[deny(const_err)]` on by default
+
+error: any use of this value will cause an error
+  --> $DIR/const_fn_ptr_fail2.rs:8:5
+   |
+LL |     x(y)
+   |     ^^^^
+   |     |
+   |     calling non-const function `double`
+   |     inside call to `bar` at $DIR/const_fn_ptr_fail2.rs:14:18
+...
+LL | const Z: usize = bar(double, 2);
+   | --------------------------------
+
+error: aborting due to 2 previous errors
+

src/test/ui/consts/const-eval/feature-gate-const_fn_ptr.rs

@@ -0,0 +1,11 @@
+#![feature(const_fn)]
+
+fn main() {}
+
+const fn foo() {}
+const X: fn() = foo;
+
+const fn bar() {
+    X()
+    //~^ ERROR function pointers in const fn are unstable
+}

src/test/ui/consts/const-eval/feature-gate-const_fn_ptr.stderr

@@ -0,0 +1,12 @@
+error[E0658]: function pointers in const fn are unstable
+  --> $DIR/feature-gate-const_fn_ptr.rs:9:5
+   |
+LL |     X()
+   |     ^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/51909
+   = help: add `#![feature(const_fn_ptr)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.