Add example.

Author: cjgillot

library/core/src/intrinsics/mir.rs

@@ -254,6 +254,24 @@
 //!  - A debuginfo name can be given to a local using `debug my_name => contents;`.
 //!    For `contents`, we use the same syntax as operands, to support both places and constants.
 //!
+//! ```rust
+//! #![allow(internal_features)]
+//! #![feature(core_intrinsics, custom_mir)]
+//!
+//! use core::intrinsics::mir::*;
+//!
+//! #[custom_mir(dialect = "built")]
+//! fn debuginfo(option: Option<&i32>) {
+//!     mir!(
+//!         debug option => option;
+//!         debug projection => *Field::<&i32>(Variant(option, 1), 0);
+//!         debug constant => 5_usize;
+//!         {
+//!             Return()
+//!         }
+//!     )
+//! }
+//! ```
 
 #![unstable(
     feature = "custom_mir",

tests/mir-opt/building/custom/debuginfo.rs

@@ -63,10 +63,22 @@ fn variant_deref(opt: Option<&i32>) {
     )
 }
 
+// EMIT_MIR debuginfo.constant.built.after.mir
+#[custom_mir(dialect = "built")]
+fn constant() {
+    mir!(
+        debug scalar => 5_usize;
+        {
+            Return()
+        }
+    )
+}
+
 fn main() {
     numbered((5, 6));
     structured(S { x: 5. });
     variant(Some(5));
     variant_deref(Some(&5));
     pointee(&mut Some(5));
+    constant();
 }