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/block.rs

@@ -777,22 +777,30 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
             self.codegen_argument(&mut bx, op, &mut llargs, &fn_abi.args[i]);
         }
-        if let Some(tup) = untuple {
+        let num_untupled = untuple.map(|tup| {
             self.codegen_arguments_untupled(
                 &mut bx,
                 tup,
                 &mut llargs,
                 &fn_abi.args[first_args.len()..],
             )
-        }
+        });
 
         let needs_location =
             instance.map_or(false, |i| i.def.requires_caller_location(self.cx.tcx()));
         if needs_location {
+            let mir_args = if let Some(num_untupled) = num_untupled {
+                first_args.len() + num_untupled
+            } else {
+                args.len()
+            };
             assert_eq!(
                 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] fn's must have 1 more argument in their ABI than in their MIR: {:?} {:?} {:?}",
+                instance,
+                fn_span,
+                fn_abi,
             );
             let location =
                 self.get_caller_location(&mut bx, mir::SourceInfo { span: fn_span, ..source_info });
@@ -1122,7 +1130,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         operand: &mir::Operand<'tcx>,
         llargs: &mut Vec<Bx::Value>,
         args: &[ArgAbi<'tcx, Ty<'tcx>>],
-    ) {
+    ) -> usize {
         let tuple = self.codegen_operand(bx, operand);
 
         // Handle both by-ref and immediate tuples.
@@ -1142,6 +1150,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 self.codegen_argument(bx, op, llargs, &args[i]);
             }
         }
+        tuple.layout.fields.count()
     }
 
     fn get_caller_location(

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();