diff --git a/src/librustc_codegen_llvm/mir/block.rs b/src/librustc_codegen_llvm/mir/block.rs index 556baeba39e0a..1669059a760f4 100644 --- a/src/librustc_codegen_llvm/mir/block.rs +++ b/src/librustc_codegen_llvm/mir/block.rs @@ -428,9 +428,20 @@ impl<'a, 'tcx> FunctionCx<'a, 'tcx> { let intrinsic = intrinsic.as_ref().map(|s| &s[..]); if intrinsic == Some("transmute") { - let &(ref dest, target) = destination.as_ref().unwrap(); - self.codegen_transmute(&bx, &args[0], dest); - funclet_br(self, bx, target); + if let Some(destination_ref) = destination.as_ref() { + let &(ref dest, target) = destination_ref; + self.codegen_transmute(&bx, &args[0], dest); + funclet_br(self, bx, target); + } else { + // If we are trying to transmute to an uninhabited type, + // it is likely there is no allotted destination. In fact, + // transmuting to an uninhabited type is UB, which means + // we can do what we like. Here, we declare that transmuting + // into an uninhabited type is impossible, so anything following + // it must be unreachable. + assert_eq!(bx.cx.layout_of(sig.output()).abi, layout::Abi::Uninhabited); + bx.unreachable(); + } return; } diff --git a/src/test/compile-fail/never_transmute_never.rs b/src/test/compile-fail/never_transmute_never.rs new file mode 100644 index 0000000000000..cbcc47f60c313 --- /dev/null +++ b/src/test/compile-fail/never_transmute_never.rs @@ -0,0 +1,33 @@ +// Copyright 2018 The Rust Project Developers. See the COPYRIGHT +// file at the top-level directory of this distribution and at +// http://rust-lang.org/COPYRIGHT. +// +// Licensed under the Apache License, Version 2.0 or the MIT license +// , at your +// option. This file may not be copied, modified, or distributed +// except according to those terms. + +// compile-pass + +#![crate_type="lib"] + +#![feature(never_type)] +#![allow(dead_code)] +#![allow(unreachable_code)] +#![allow(unused_variables)] + +struct Foo; + +pub fn f(x: !) -> ! { + x +} + +pub fn ub() { + // This is completely undefined behaviour, + // but we still want to make sure it compiles. + let x: ! = unsafe { + std::mem::transmute::(Foo) + }; + f(x) +}