Commits on Sep 22, 2021

1. 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.

   

   Aaron1011 committed Sep 22, 2021
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