Record the fact that rustc_peek has no side effects

This means that calls to `rustc_peek` are no longer indirect definitions
of any locals in the body. Other pure intrinsics (e.g. `transmute`)
could also be added to the list of exceptions.

Author: ecstatic-morse

src/librustc_mir/dataflow/impls/reaching_defs.rs

@@ -85,7 +85,21 @@ fn has_side_effects(
     terminator: &mir::Terminator<'tcx>,
 ) -> bool {
     match &terminator.kind {
-        mir::TerminatorKind::Call { .. } => true,
+        mir::TerminatorKind::Call { func, .. } => {
+            // Special-case some intrinsics that do not have side effects.
+            if let mir::Operand::Constant(func) = func {
+                if let ty::FnDef(def_id, _) = func.ty.sty {
+                    if let Abi::RustIntrinsic = tcx.fn_sig(def_id).abi() {
+                        match tcx.item_name(def_id) {
+                            sym::rustc_peek => return false,
+                            _ => (),
+                        }
+                    }
+                }
+            }
+
+            true
+        }
 
         // Types with special drop glue may mutate their environment.
         | mir::TerminatorKind::Drop { location: place, .. }

src/test/ui/mir-dataflow/reaching-defs-1.rs

@@ -22,7 +22,7 @@ fn foo(test: bool) -> (i32, i32) {
     }
 
     unsafe { rustc_peek(&x); }
-    //~^ ERROR rustc_peek: [16: "x=2", 17: "rustc_peek(&x)", 20: "x=3"]
+    //~^ ERROR rustc_peek: [16: "x=2", 20: "x=3"]
 
     unsafe { rustc_peek(&y); }
     //~^ ERROR rustc_peek: [13: "y=1", 21: "y=4"]

src/test/ui/mir-dataflow/reaching-defs-1.stderr

@@ -4,7 +4,7 @@ error: rustc_peek: [16: "x=2"]
 LL |         unsafe { rustc_peek(&x); }
    |                  ^^^^^^^^^^^^^^
 
-error: rustc_peek: [16: "x=2", 17: "rustc_peek(&x)", 20: "x=3"]
+error: rustc_peek: [16: "x=2", 20: "x=3"]
   --> $DIR/reaching-defs-1.rs:24:14
    |
 LL |     unsafe { rustc_peek(&x); }