Auto merge of #54568 - levex:issue-54130, r=nagisa

codegen_llvm: check inline assembly constraints with LLVM

---%<---
Hey all,

As issue #54130 highlights, constraints are not checked and passing bad constraints to LLVM can crash it since a `Verify()` call is placed inside an assertion (see: `src/llvm/lib/IR/InlineAsm.cpp:39`).

As this is my first PR to the Rust compiler (woot! 🎉), there might be better ways of achieving this result. In particular, I am not too happy about generating an error in codegen; it would be much nicer if we did it earlier. However, @rkruppe [noted on IRC](https://botbot.me/mozilla/rustc/2018-09-25/?msg=104791581&page=1) that this should be fine for an unstable feature and a much better solution than the _status quo_, which is an ICE.

Thanks!
--->%---

LLVM provides a way of checking whether the constraints and the actual
inline assembly make sense. This commit introduces a check before
emitting code for the inline assembly. If LLVM rejects the inline
assembly (or its constraints), then the compiler emits an error E0668
("malformed inline assembly").

Fixes: #54130
Signed-off-by: Levente Kurusa \<lkurusa@acm.org\>

src/test/ui/inline-asm-bad-constraint.rs

@@ -0,0 +1,47 @@
+// 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 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that the compiler will catch invalid inline assembly constraints.
+
+#![feature(asm)]
+
+extern "C" {
+    fn foo(a: usize);
+}
+
+fn main() {
+    bad_register_constraint();
+    bad_input();
+    wrong_size_output();
+}
+
+// Issue #54130
+fn bad_register_constraint() {
+    let rax: u64;
+    unsafe {
+        asm!("" :"={rax"(rax)) //~ ERROR E0668
+    };
+    println!("Accumulator is: {}", rax);
+}
+
+// Issue #54376
+fn bad_input() {
+    unsafe {
+        asm!("callq $0" : : "0"(foo)) //~ ERROR E0668
+    };
+}
+
+fn wrong_size_output() {
+    let rax: u64 = 0;
+    unsafe {
+        asm!("addb $1, $0" : "={rax}"((0i32, rax))); //~ ERROR E0668
+    }
+    println!("rax: {}", rax);
+}

src/test/ui/inline-asm-bad-constraint.stderr

@@ -0,0 +1,21 @@
+error[E0668]: malformed inline assembly
+  --> $DIR/inline-asm-bad-constraint.rs:29:9
+   |
+LL |         asm!("" :"={rax"(rax)) //~ ERROR E0668
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0668]: malformed inline assembly
+  --> $DIR/inline-asm-bad-constraint.rs:37:9
+   |
+LL |         asm!("callq $0" : : "0"(foo)) //~ ERROR E0668
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0668]: malformed inline assembly
+  --> $DIR/inline-asm-bad-constraint.rs:44:9
+   |
+LL |         asm!("addb $1, $0" : "={rax}"((0i32, rax))); //~ ERROR E0668
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0668`.