Rebase resume argument projections during state transform

When remapping a resume argument with projections rebase them on top of
the new base.

The case where resume argument has projections is unusual, but might
arise with box syntax where the assignment is performed directly into
the box without an intermediate temporary.

Author: tmiasko

compiler/rustc_mir_transform/src/generator.rs

@@ -342,15 +342,16 @@ impl MutVisitor<'tcx> for TransformVisitor<'tcx> {
             let source_info = data.terminator().source_info;
             // We must assign the value first in case it gets declared dead below
             data.statements.extend(self.make_state(state_idx, v, source_info));
-            let state = if let Some((resume, resume_arg)) = resume {
+            let state = if let Some((resume, mut resume_arg)) = resume {
                 // Yield
                 let state = 3 + self.suspension_points.len();
 
                 // The resume arg target location might itself be remapped if its base local is
                 // live across a yield.
                 let resume_arg =
                     if let Some(&(ty, variant, idx)) = self.remap.get(&resume_arg.local) {
-                        self.make_field(variant, idx, ty)
+                        replace_base(&mut resume_arg, self.make_field(variant, idx, ty), self.tcx);
+                        resume_arg
                     } else {
                         resume_arg
                     };

src/test/ui/generator/yield-in-box.rs

@@ -1,8 +1,10 @@
 // run-pass
-
 // Test that box-statements with yields in them work.
 
-#![feature(generators, box_syntax)]
+#![feature(generators, box_syntax, generator_trait)]
+use std::pin::Pin;
+use std::ops::Generator;
+use std::ops::GeneratorState;
 
 fn main() {
     let x = 0i32;
@@ -15,4 +17,8 @@ fn main() {
             _t => {}
         }
     };
+
+    let mut g = |_| box yield;
+    assert_eq!(Pin::new(&mut g).resume(1), GeneratorState::Yielded(()));
+    assert_eq!(Pin::new(&mut g).resume(2), GeneratorState::Complete(box 2));
 }

src/test/ui/generator/yield-in-box.stderr

@@ -1,5 +1,5 @@
 warning: unused generator that must be used
-  --> $DIR/yield-in-box.rs:9:5
+  --> $DIR/yield-in-box.rs:11:5
    |
 LL | /     || {
 LL | |         let y = 2u32;