Auto merge of #65182 - anp:reify-shim, r=eddyb

Add `Instance::resolve_for_fn_ptr` (RFC 2091 #2/N)

Supercedes: #65082
Depends on: #65037
Tracking issue: #47809
[RFC text](https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md)

steps taken:

* [x] add a `ReifyShim` that is similar to `VirtualShim` in behavior (see #54183)
* [x] add `ty::Instance::resolve_for_fn_ptr` (leave `ty::Instance::resolve_vtable` alone), migrate appropriate callers
* [x] `resolve_for_fn_ptr` returns the shim if calling a `#[track_caller]` function

Author: bors

src/librustc/mir/mono.rs

@@ -386,6 +386,7 @@ impl<'tcx> CodegenUnit<'tcx> {
                             tcx.hir().as_local_hir_id(def_id)
                         }
                         InstanceDef::VtableShim(..) |
+                        InstanceDef::ReifyShim(..) |
                         InstanceDef::Intrinsic(..) |
                         InstanceDef::FnPtrShim(..) |
                         InstanceDef::Virtual(..) |

src/librustc/ty/instance.rs

@@ -1,3 +1,4 @@
+use crate::hir::CodegenFnAttrFlags;
 use crate::hir::Unsafety;
 use crate::hir::def::Namespace;
 use crate::hir::def_id::DefId;
@@ -25,6 +26,14 @@ pub enum InstanceDef<'tcx> {
     /// `<T as Trait>::method` where `method` receives unsizeable `self: Self`.
     VtableShim(DefId),
 
+    /// `fn()` pointer where the function itself cannot be turned into a pointer.
+    ///
+    /// One example in the compiler today is functions annotated with `#[track_caller]`, which
+    /// must have their implicit caller location argument populated for a call. Because this is a
+    /// required part of the function's ABI but can't be tracked as a property of the function
+    /// pointer, we create a single "caller location" at the site where the function is reified.
+    ReifyShim(DefId),
+
     /// `<fn() as FnTrait>::call_*`
     /// `DefId` is `FnTrait::call_*`
     FnPtrShim(DefId, Ty<'tcx>),
@@ -123,6 +132,7 @@ impl<'tcx> InstanceDef<'tcx> {
         match *self {
             InstanceDef::Item(def_id) |
             InstanceDef::VtableShim(def_id) |
+            InstanceDef::ReifyShim(def_id) |
             InstanceDef::FnPtrShim(def_id, _) |
             InstanceDef::Virtual(def_id, _) |
             InstanceDef::Intrinsic(def_id, ) |
@@ -178,6 +188,9 @@ impl<'tcx> fmt::Display for Instance<'tcx> {
             InstanceDef::VtableShim(_) => {
                 write!(f, " - shim(vtable)")
             }
+            InstanceDef::ReifyShim(_) => {
+                write!(f, " - shim(reify)")
+            }
             InstanceDef::Intrinsic(_) => {
                 write!(f, " - intrinsic")
             }
@@ -290,6 +303,30 @@ impl<'tcx> Instance<'tcx> {
         result
     }
 
+    pub fn resolve_for_fn_ptr(
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> Option<Instance<'tcx>> {
+        debug!("resolve(def_id={:?}, substs={:?})", def_id, substs);
+        Instance::resolve(tcx, param_env, def_id, substs).map(|resolved| {
+            let has_track_caller = |def| tcx.codegen_fn_attrs(def).flags
+                .contains(CodegenFnAttrFlags::TRACK_CALLER);
+
+            match resolved.def {
+                InstanceDef::Item(def_id) if has_track_caller(def_id) => {
+                    debug!(" => fn pointer created for function with #[track_caller]");
+                    Instance {
+                        def: InstanceDef::ReifyShim(def_id),
+                        substs,
+                    }
+                },
+                _ => resolved,
+            }
+        })
+    }
+
     pub fn resolve_for_vtable(
         tcx: TyCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,

src/librustc/ty/mod.rs

@@ -3026,6 +3026,7 @@ impl<'tcx> TyCtxt<'tcx> {
                 self.optimized_mir(did)
             }
             ty::InstanceDef::VtableShim(..) |
+            ty::InstanceDef::ReifyShim(..) |
             ty::InstanceDef::Intrinsic(..) |
             ty::InstanceDef::FnPtrShim(..) |
             ty::InstanceDef::Virtual(..) |

src/librustc/ty/structural_impls.rs

@@ -761,6 +761,8 @@ impl<'a, 'tcx> Lift<'tcx> for ty::InstanceDef<'a> {
                 Some(ty::InstanceDef::Item(def_id)),
             ty::InstanceDef::VtableShim(def_id) =>
                 Some(ty::InstanceDef::VtableShim(def_id)),
+            ty::InstanceDef::ReifyShim(def_id) =>
+                Some(ty::InstanceDef::ReifyShim(def_id)),
             ty::InstanceDef::Intrinsic(def_id) =>
                 Some(ty::InstanceDef::Intrinsic(def_id)),
             ty::InstanceDef::FnPtrShim(def_id, ref ty) =>
@@ -966,6 +968,7 @@ impl<'tcx> TypeFoldable<'tcx> for ty::instance::Instance<'tcx> {
             def: match self.def {
                 Item(did) => Item(did.fold_with(folder)),
                 VtableShim(did) => VtableShim(did.fold_with(folder)),
+                ReifyShim(did) => ReifyShim(did.fold_with(folder)),
                 Intrinsic(did) => Intrinsic(did.fold_with(folder)),
                 FnPtrShim(did, ty) => FnPtrShim(
                     did.fold_with(folder),
@@ -994,7 +997,7 @@ impl<'tcx> TypeFoldable<'tcx> for ty::instance::Instance<'tcx> {
         use crate::ty::InstanceDef::*;
         self.substs.visit_with(visitor) ||
         match self.def {
-            Item(did) | VtableShim(did) | Intrinsic(did) | Virtual(did, _) => {
+            Item(did) | VtableShim(did) | ReifyShim(did) | Intrinsic(did) | Virtual(did, _) => {
                 did.visit_with(visitor)
             },
             FnPtrShim(did, ty) | CloneShim(did, ty) => {

src/librustc_codegen_ssa/callee.rs

@@ -18,6 +18,23 @@ pub fn resolve_and_get_fn<'tcx, Cx: CodegenMethods<'tcx>>(
     )
 }
 
+pub fn resolve_and_get_fn_for_ptr<'tcx,
+    Cx: Backend<'tcx> + MiscMethods<'tcx> + TypeMethods<'tcx>
+>(
+    cx: &Cx,
+    def_id: DefId,
+    substs: SubstsRef<'tcx>,
+) -> Cx::Value {
+    cx.get_fn(
+        ty::Instance::resolve_for_fn_ptr(
+            cx.tcx(),
+            ty::ParamEnv::reveal_all(),
+            def_id,
+            substs
+        ).unwrap()
+    )
+}
+
 pub fn resolve_and_get_fn_for_vtable<'tcx,
     Cx: Backend<'tcx> + MiscMethods<'tcx> + TypeMethods<'tcx>
 >(

src/librustc_codegen_ssa/mir/rvalue.rs

@@ -190,7 +190,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                     bug!("reifying a fn ptr that requires const arguments");
                                 }
                                 OperandValue::Immediate(
-                                    callee::resolve_and_get_fn(bx.cx(), def_id, substs))
+                                    callee::resolve_and_get_fn_for_ptr(bx.cx(), def_id, substs))
                             }
                             _ => {
                                 bug!("{} cannot be reified to a fn ptr", operand.layout.ty)

src/librustc_mir/interpret/cast.rs

@@ -44,7 +44,14 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         if self.tcx.has_attr(def_id, sym::rustc_args_required_const) {
                             bug!("reifying a fn ptr that requires const arguments");
                         }
-                        let instance = self.resolve(def_id, substs)?;
+
+                        let instance = ty::Instance::resolve_for_fn_ptr(
+                            *self.tcx,
+                            self.param_env,
+                            def_id,
+                            substs,
+                        ).ok_or_else(|| err_inval!(TooGeneric))?;
+
                         let fn_ptr = self.memory.create_fn_alloc(FnVal::Instance(instance));
                         self.write_scalar(Scalar::Ptr(fn_ptr.into()), dest)?;
                     }

src/librustc_mir/interpret/terminator.rs

@@ -263,6 +263,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 Ok(())
             }
             ty::InstanceDef::VtableShim(..) |
+            ty::InstanceDef::ReifyShim(..) |
             ty::InstanceDef::ClosureOnceShim { .. } |
             ty::InstanceDef::FnPtrShim(..) |
             ty::InstanceDef::DropGlue(..) |

src/librustc_mir/monomorphize/collector.rs

@@ -721,10 +721,12 @@ fn visit_fn_use<'tcx>(
     output: &mut Vec<MonoItem<'tcx>>,
 ) {
     if let ty::FnDef(def_id, substs) = ty.kind {
-        let instance = ty::Instance::resolve(tcx,
-                                             ty::ParamEnv::reveal_all(),
-                                             def_id,
-                                             substs).unwrap();
+        let resolver = if is_direct_call {
+            ty::Instance::resolve
+        } else {
+            ty::Instance::resolve_for_fn_ptr
+        };
+        let instance = resolver(tcx, ty::ParamEnv::reveal_all(), def_id, substs).unwrap();
         visit_instance_use(tcx, instance, is_direct_call, output);
     }
 }
@@ -747,6 +749,7 @@ fn visit_instance_use<'tcx>(
             }
         }
         ty::InstanceDef::VtableShim(..) |
+        ty::InstanceDef::ReifyShim(..) |
         ty::InstanceDef::Virtual(..) |
         ty::InstanceDef::DropGlue(_, None) => {
             // don't need to emit shim if we are calling directly.
@@ -773,6 +776,7 @@ fn should_monomorphize_locally<'tcx>(tcx: TyCtxt<'tcx>, instance: &Instance<'tcx
     let def_id = match instance.def {
         ty::InstanceDef::Item(def_id) => def_id,
         ty::InstanceDef::VtableShim(..) |
+        ty::InstanceDef::ReifyShim(..) |
         ty::InstanceDef::ClosureOnceShim { .. } |
         ty::InstanceDef::Virtual(..) |
         ty::InstanceDef::FnPtrShim(..) |

src/librustc_mir/monomorphize/partitioning.rs

@@ -339,6 +339,7 @@ fn mono_item_visibility(
 
         // These are all compiler glue and such, never exported, always hidden.
         InstanceDef::VtableShim(..) |
+        InstanceDef::ReifyShim(..) |
         InstanceDef::FnPtrShim(..) |
         InstanceDef::Virtual(..) |
         InstanceDef::Intrinsic(..) |
@@ -677,6 +678,7 @@ fn characteristic_def_id_of_mono_item<'tcx>(
             let def_id = match instance.def {
                 ty::InstanceDef::Item(def_id) => def_id,
                 ty::InstanceDef::VtableShim(..) |
+                ty::InstanceDef::ReifyShim(..) |
                 ty::InstanceDef::FnPtrShim(..) |
                 ty::InstanceDef::ClosureOnceShim { .. } |
                 ty::InstanceDef::Intrinsic(..) |

src/librustc_mir/shim.rs

@@ -66,9 +66,12 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceDef<'tcx>) -> &'tcx
                 Some(arg_tys)
             )
         }
-        ty::InstanceDef::Virtual(def_id, _) => {
-            // We are generating a call back to our def-id, which the
-            // codegen backend knows to turn to an actual virtual call.
+        // We are generating a call back to our def-id, which the
+        // codegen backend knows to turn to an actual virtual call.
+        ty::InstanceDef::Virtual(def_id, _) |
+        // ...or we are generating a direct call to a function for which indirect calls must be
+        // codegen'd differently than direct ones (example: #[track_caller])
+        ty::InstanceDef::ReifyShim(def_id) => {
             build_call_shim(
                 tcx,
                 def_id,