Auto merge of #2980 - RalfJung:span, r=RalfJung

work around custom_mir span

Cc #113549

Author: bors

src/tools/miri/tests/fail/function_calls/arg_inplace_observe_after.rs

@@ -5,17 +5,17 @@ pub struct S(i32);
 
 #[custom_mir(dialect = "runtime", phase = "optimized")]
 fn main() {
-    // FIXME: the span is not great (probably caused by custom MIR)
-    mir! { //~ERROR: uninitialized
+    mir! {
         let unit: ();
+        let _observe: i32;
         {
             let non_copy = S(42);
             // This could change `non_copy` in-place
             Call(unit, after_call, change_arg(Move(non_copy)))
         }
         after_call = {
             // So now we must not be allowed to observe non-copy again.
-            let _observe = non_copy.0;
+            _observe = non_copy.0; //~ERROR: uninitialized
             Return()
         }
 

src/tools/miri/tests/fail/function_calls/arg_inplace_observe_after.stderr

@@ -1,20 +1,13 @@
 error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> $DIR/arg_inplace_observe_after.rs:LL:CC
    |
-LL | /     mir! {
-LL | |         let unit: ();
-LL | |         {
-LL | |             let non_copy = S(42);
-...  |
-LL | |
-LL | |     }
-   | |_____^ using uninitialized data, but this operation requires initialized memory
+LL |             _observe = non_copy.0;
+   |             ^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE:
-   = note: inside `main` at RUSTLIB/core/src/intrinsics/mir.rs:LL:CC
-   = note: this error originates in the macro `::core::intrinsics::mir::__internal_remove_let` which comes from the expansion of the macro `mir` (in Nightly builds, run with -Z macro-backtrace for more info)
+   = note: inside `main` at $DIR/arg_inplace_observe_after.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace