Auto merge of #34128 - eddyb:mir-trans-fixes, r=luqmana

[MIR] Fix MIR trans edge cases that showed up on crater.

These fixes cover all of the [regressions found by crater](https://gist.github.com/nikomatsakis/88ce89ed06ef7f7f19bfd1e221d7f7ec) (for #34096).

Two of them were `Pair` edge cases (ZSTs and constants) causing LLVM assertions, the other one was  causing stack overflows in debug scripts compiled in debug mode, due to the `fn_ret_cast` `alloca` ending up in a loop.

src/librustc_trans/mir/block.rs

@@ -894,8 +894,11 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
         //   bitcasting to the struct type yields invalid cast errors.
 
         // We instead thus allocate some scratch space...
-        let llscratch = bcx.alloca(llcast_ty, "fn_ret_cast");
-        bcx.with_block(|bcx| base::call_lifetime_start(bcx, llscratch));
+        let llscratch = bcx.with_block(|bcx| {
+            let alloca = base::alloca(bcx, llcast_ty, "fn_ret_cast");
+            base::call_lifetime_start(bcx, alloca);
+            alloca
+        });
 
         // ...where we first store the value...
         bcx.store(op.immediate(), llscratch);

src/librustc_trans/mir/constant.rs

@@ -125,10 +125,10 @@ impl<'tcx> Const<'tcx> {
         let llty = type_of::immediate_type_of(ccx, self.ty);
         let llvalty = val_ty(self.llval);
 
-        let val = if common::type_is_imm_pair(ccx, self.ty) {
+        let val = if llty == llvalty && common::type_is_imm_pair(ccx, self.ty) {
             let (a, b) = self.get_pair();
             OperandValue::Pair(a, b)
-        } else if common::type_is_immediate(ccx, self.ty) && llty == llvalty {
+        } else if llty == llvalty && common::type_is_immediate(ccx, self.ty) {
             // If the types match, we can use the value directly.
             OperandValue::Immediate(self.llval)
         } else {

src/librustc_trans/mir/mod.rs

@@ -124,7 +124,12 @@ impl<'tcx> TempRef<'tcx> {
             // Zero-size temporaries aren't always initialized, which
             // doesn't matter because they don't contain data, but
             // we need something in the operand.
-            let val = OperandValue::Immediate(common::C_nil(ccx));
+            let nil = common::C_nil(ccx);
+            let val = if common::type_is_imm_pair(ccx, ty) {
+                OperandValue::Pair(nil, nil)
+            } else {
+                OperandValue::Immediate(nil)
+            };
             let op = OperandRef {
                 val: val,
                 ty: ty

src/test/run-pass/mir_constval_adts.rs

@@ -15,21 +15,23 @@ struct Point {
     _y: i32,
 }
 
+#[derive(PartialEq, Eq, Debug)]
+struct Newtype<T>(T);
+
 const STRUCT: Point = Point { _x: 42, _y: 42 };
 const TUPLE1: (i32, i32) = (42, 42);
 const TUPLE2: (&'static str, &'static str) = ("hello","world");
+const PAIR_NEWTYPE: (Newtype<i32>, Newtype<i32>) = (Newtype(42), Newtype(42));
 
 #[rustc_mir]
-fn mir() -> (Point, (i32, i32), (&'static str, &'static str)){
+fn mir() -> (Point, (i32, i32), (&'static str, &'static str), (Newtype<i32>, Newtype<i32>)) {
     let struct1 = STRUCT;
     let tuple1 = TUPLE1;
     let tuple2 = TUPLE2;
-    (struct1, tuple1, tuple2)
+    let pair_newtype = PAIR_NEWTYPE;
+    (struct1, tuple1, tuple2, pair_newtype)
 }
 
-#[derive(PartialEq, Eq, Debug)]
-struct Newtype<T>(T);
-
 const NEWTYPE: Newtype<&'static str> = Newtype("foobar");
 
 #[rustc_mir]
@@ -39,7 +41,7 @@ fn test_promoted_newtype_str_ref() {
 }
 
 fn main(){
-    assert_eq!(mir(), (STRUCT, TUPLE1, TUPLE2));
+    assert_eq!(mir(), (STRUCT, TUPLE1, TUPLE2, PAIR_NEWTYPE));
     test_promoted_newtype_str_ref();
 }
 

src/test/run-pass/mir_trans_calls.rs

@@ -147,6 +147,16 @@ fn test_fn_transmute_zst(x: ()) -> [(); 1] {
     })
 }
 
+#[rustc_mir]
+fn test_fn_ignored_pair() -> ((), ()) {
+    ((), ())
+}
+
+#[rustc_mir]
+fn test_fn_ignored_pair_0() {
+    test_fn_ignored_pair().0
+}
+
 fn main() {
     assert_eq!(test1(1, (2, 3), &[4, 5, 6]), (1, (2, 3), &[4, 5, 6][..]));
     assert_eq!(test2(98), 98);
@@ -169,4 +179,6 @@ fn main() {
 
     assert_eq!(test_fn_nil_call(&(|| 42)), 42);
     assert_eq!(test_fn_transmute_zst(()), [()]);
+
+    assert_eq!(test_fn_ignored_pair_0(), ());
 }