Auto merge of #91342 - RalfJung:fn-abi, r=eddyb,oli-obk

CTFE eval_fn_call: use FnAbi to determine argument skipping and compatibility

This makes use of the `FnAbi` type in CTFE/Miri, which `@eddyb` has been saying for years is what we should do.^^ `FnAbi` is used to
- determine which arguments to skip (rather than the previous heuristic of skipping ZST arguments with the Rust ABI)
- impose further restrictions on whether caller and callee are consistent in how a given argument is passed

I was hoping it would also simplify the code, but that is not the case -- the previous type compatibility checks are still required (AFAIK), only the ZST skipping is gone and that took barely any code. We also need some hacks because `FnAbi` assumes a certain way of implementing `caller_location` (by passing extra arguments), but Miri can just read the caller location from the call stack so it doesn't need those arguments. (The fact that every backend has to separately implement support for these arguments seems suboptimal -- looks like this might have been better implemented on the MIR level.) To avoid having to implement those unnecessary arguments in Miri, we just compute *whether* the argument is present on the caller/callee side, but don't actually pass that argument around.

I have no idea if this looks the way `@eddyb` thinks it should look... but it makes Miri's test suite pass. ;)
One of rustc's tests fails unfortunately (`ui/const-generics/issues/issue-67739.rs`), some const generic code that is evaluated too early -- I think that should raise `TooGeneric` but instead it ICEs. My assumption is this is some FnAbi code that has not been properly tested on polymorphic code, but it might also be me calling that FnAbi code the wrong way.

r? `@oli-obk` `@eddyb`
Fixes #56166
Miri PR at rust-lang/miri#1928

Author: bors

Diff for: compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -260,7 +260,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         args: &[OpTy<'tcx>],
         _ret: Option<(&PlaceTy<'tcx>, mir::BasicBlock)>,
         _unwind: StackPopUnwind, // unwinding is not supported in consts
-    ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
+    ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
         debug!("find_mir_or_eval_fn: {:?}", instance);
 
         // Only check non-glue functions
@@ -279,11 +279,21 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
             if let Some(new_instance) = ecx.hook_special_const_fn(instance, args)? {
                 // We call another const fn instead.
-                return Self::find_mir_or_eval_fn(ecx, new_instance, _abi, args, _ret, _unwind);
+                // However, we return the *original* instance to make backtraces work out
+                // (and we hope this does not confuse the FnAbi checks too much).
+                return Ok(Self::find_mir_or_eval_fn(
+                    ecx,
+                    new_instance,
+                    _abi,
+                    args,
+                    _ret,
+                    _unwind,
+                )?
+                .map(|(body, _instance)| (body, instance)));
             }
         }
         // This is a const fn. Call it.
-        Ok(Some(ecx.load_mir(instance.def, None)?))
+        Ok(Some((ecx.load_mir(instance.def, None)?, instance)))
     }
 
     fn call_intrinsic(

Diff for: compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -8,15 +8,18 @@ use rustc_index::vec::IndexVec;
 use rustc_macros::HashStable;
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::{InterpError, InvalidProgramInfo};
-use rustc_middle::ty::layout::{self, LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
+use rustc_middle::ty::layout::{
+    self, FnAbiError, FnAbiOfHelpers, FnAbiRequest, LayoutError, LayoutOf, LayoutOfHelpers,
+    TyAndLayout,
+};
 use rustc_middle::ty::{
     self, query::TyCtxtAt, subst::SubstsRef, ParamEnv, Ty, TyCtxt, TypeFoldable,
 };
 use rustc_mir_dataflow::storage::AlwaysLiveLocals;
 use rustc_query_system::ich::StableHashingContext;
 use rustc_session::Limit;
 use rustc_span::{Pos, Span};
-use rustc_target::abi::{Align, HasDataLayout, Size, TargetDataLayout};
+use rustc_target::abi::{call::FnAbi, Align, HasDataLayout, Size, TargetDataLayout};
 
 use super::{
     AllocId, GlobalId, Immediate, InterpErrorInfo, InterpResult, MPlaceTy, Machine, MemPlace,
@@ -333,6 +336,24 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> LayoutOfHelpers<'tcx> for InterpC
     }
 }
 
+impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> FnAbiOfHelpers<'tcx> for InterpCx<'mir, 'tcx, M> {
+    type FnAbiOfResult = InterpResult<'tcx, &'tcx FnAbi<'tcx, Ty<'tcx>>>;
+
+    fn handle_fn_abi_err(
+        &self,
+        err: FnAbiError<'tcx>,
+        _span: Span,
+        _fn_abi_request: FnAbiRequest<'tcx>,
+    ) -> InterpErrorInfo<'tcx> {
+        match err {
+            FnAbiError::Layout(err) => err_inval!(Layout(err)).into(),
+            FnAbiError::AdjustForForeignAbi(err) => {
+                err_inval!(FnAbiAdjustForForeignAbi(err)).into()
+            }
+        }
+    }
+}
+
 /// Test if it is valid for a MIR assignment to assign `src`-typed place to `dest`-typed value.
 /// This test should be symmetric, as it is primarily about layout compatibility.
 pub(super) fn mir_assign_valid_types<'tcx>(

Diff for: compiler/rustc_const_eval/src/interpret/machine.rs

@@ -167,7 +167,7 @@ pub trait Machine<'mir, 'tcx>: Sized {
         args: &[OpTy<'tcx, Self::PointerTag>],
         ret: Option<(&PlaceTy<'tcx, Self::PointerTag>, mir::BasicBlock)>,
         unwind: StackPopUnwind,
-    ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>>;
+    ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>>;
 
     /// Execute `fn_val`.  It is the hook's responsibility to advance the instruction
     /// pointer as appropriate.