Sema: emit cast to null panics for function pointers

Closes #14676

Author: Vexu

src/Sema.zig

@@ -19626,7 +19626,7 @@ fn zirIntToPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Ai
     }
 
     try sema.requireRuntimeBlock(block, src, operand_src);
-    if (block.wantSafety() and try sema.typeHasRuntimeBits(elem_ty)) {
+    if (block.wantSafety() and (try sema.typeHasRuntimeBits(elem_ty) or elem_ty.zigTypeTag() == .Fn)) {
         if (!ptr_ty.isAllowzeroPtr()) {
             const is_non_zero = try block.addBinOp(.cmp_neq, operand_coerced, .zero_usize);
             try sema.addSafetyCheck(block, is_non_zero, .cast_to_null);
@@ -19852,7 +19852,7 @@ fn zirPtrCast(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air
 
     try sema.requireRuntimeBlock(block, src, null);
     if (block.wantSafety() and operand_ty.ptrAllowsZero() and !dest_ty.ptrAllowsZero() and
-        try sema.typeHasRuntimeBits(dest_ty.elemType2()))
+        (try sema.typeHasRuntimeBits(dest_ty.elemType2()) or dest_ty.elemType2().zigTypeTag() == .Fn))
     {
         const ptr_int = try block.addUnOp(.ptrtoint, ptr);
         const is_non_zero = try block.addBinOp(.cmp_neq, ptr_int, .zero_usize);
@@ -27718,7 +27718,7 @@ fn coerceCompatiblePtrs(
     try sema.requireRuntimeBlock(block, inst_src, null);
     const inst_allows_zero = inst_ty.zigTypeTag() != .Pointer or inst_ty.ptrAllowsZero();
     if (block.wantSafety() and inst_allows_zero and !dest_ty.ptrAllowsZero() and
-        try sema.typeHasRuntimeBits(dest_ty.elemType2()))
+        (try sema.typeHasRuntimeBits(dest_ty.elemType2()) or dest_ty.elemType2().zigTypeTag() == .Fn))
     {
         const actual_ptr = if (inst_ty.isSlice())
             try sema.analyzeSlicePtr(block, inst_src, inst, inst_ty)

test/cases/safety/pointer casting to null function pointer.zig

@@ -0,0 +1,23 @@
+const std = @import("std");
+
+pub fn panic(message: []const u8, stack_trace: ?*std.builtin.StackTrace, _: ?usize) noreturn {
+    _ = stack_trace;
+    if (std.mem.eql(u8, message, "cast causes pointer to be null")) {
+        std.process.exit(0);
+    }
+    std.process.exit(1);
+}
+
+fn getNullPtr() ?*const anyopaque {
+    return null;
+}
+pub fn main() !void {
+    const null_ptr: ?*const anyopaque = getNullPtr();
+    const required_ptr: *align(1) const fn() void = @ptrCast(*align(1) const fn() void, null_ptr);
+    _ = required_ptr;
+    return error.TestFailed;
+}
+
+// run
+// backend=llvm
+// target=native