Support #[track_caller] on closures and generators

This PR allows applying a `#[track_caller]` attribute to a
closure/generator expression. The attribute as interpreted as applying
to the compiler-generated implementation of the corresponding trait
method (`FnOnce::call_once`, `FnMut::call_mut`, `Fn::call`, or
`Generator::resume`).

This feature does not have its own feature gate - however, it requires
`#![feature(stmt_expr_attributes)]` in order to actually apply
an attribute to a closure or generator.

This is implemented in the same way as for functions - an extra
location argument is appended to the end of the ABI. For closures,
this argument is *not* part of the 'tupled' argument storing the
parameters - the final closure argument for `#[track_caller]` closures
is no longer a tuple.

For direct (monomorphized) calls, the necessary support was already
implemented - we just needeed to adjust some assertions around checking
the ABI and argument count to take closures into account.

For calls through a trait object, more work was needed.
When creating a `ReifyShim`, we need to create a shim
for the trait method (e.g. `FnOnce::call_mut`) - unlike normal
functions, closures are never invoked directly, and always go through a
trait method.

Additional handling was needed for `InstanceDef::ClosureOnceShim`. In
order to pass location information throgh a direct (monomorphized) call
to `FnOnce::call_once` on an `FnMut` closure, we need to make
`ClosureOnceShim` aware of `#[tracked_caller]`. A new field
`track_caller` is added to `ClosureOnceShim` - this is used by
`InstanceDef::requires_caller` location, allowing codegen to
pass through the extra location argument.

Since `ClosureOnceShim.track_caller` is only used by codegen,
we end up generating two identical MIR shims - one for
`track_caller == true`, and one for `track_caller == false`. However,
these two shims are used by the entire crate (i.e. it's two shims total,
not two shims per unique closure), so this shouldn't a big deal.

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -258,6 +258,8 @@ fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     let mut idx = 0;
     let mut llarg_idx = fx.fn_abi.ret.is_indirect() as usize;
 
+    let mut num_untupled = None;
+
     let args = mir
         .args_iter()
         .enumerate()
@@ -286,6 +288,11 @@ fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
                     let pr_field = place.project_field(bx, i);
                     bx.store_fn_arg(arg, &mut llarg_idx, pr_field);
                 }
+                assert_eq!(
+                    None,
+                    num_untupled.replace(tupled_arg_tys.len()),
+                    "Replaced existing num_tupled"
+                );
 
                 return LocalRef::Place(place);
             }
@@ -362,10 +369,17 @@ fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         .collect::<Vec<_>>();
 
     if fx.instance.def.requires_caller_location(bx.tcx()) {
+        let mir_args = if let Some(num_untupled) = num_untupled {
+            // Subtract off the tupled argument that gets 'expanded'
+            args.len() - 1 + num_untupled
+        } else {
+            args.len()
+        };
         assert_eq!(
             fx.fn_abi.args.len(),
-            args.len() + 1,
-            "#[track_caller] fn's must have 1 more argument in their ABI than in their MIR",
+            mir_args + 1,
+            "#[track_caller] instance {:?} must have 1 more argument in their ABI than in their MIR",
+            fx.instance
         );
 
         let arg = fx.fn_abi.args.last().unwrap();