compiler: rustc_abi::Abi => BackendRepr

The initial naming of "Abi" was an awful mistake, conveying wrong ideas
about how psABIs worked and even more about what the enum meant.
It was only meant to represent the way the value would be described to
a codegen backend as it was lowered to that intermediate representation.
It was never meant to mean anything about the actual psABI handling!
The conflation is because LLVM typically will associate a certain form
with a certain ABI, but even that does not hold when the special cases
that actually exist arise, plus the IR annotations that modify the ABI.

Reframe `rustc_abi::Abi` as the `BackendRepr` of the type, and rename
`BackendRepr::Aggregate` as `BackendRepr::Memory`. Unfortunately, due to
the persistent misunderstandings, this too is now incorrect:
- Scattered ABI-relevant code is entangled with BackendRepr
- We do not always pre-compute a correct BackendRepr that reflects how
  we "actually" want this value to be handled, so we leave the backend
  interface to also inject various special-cases here
- In some cases `BackendRepr::Memory` is a "real" aggregate, but in
  others it is in fact using memory, and in some cases it is a scalar!

Our rustc-to-backend lowering code handles this sort of thing right now.
That will eventually be addressed by lifting duplicated lowering code
to either rustc_codegen_ssa or rustc_target as appropriate.

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -11,6 +11,10 @@ use std::num::NonZero;
 
 use either::{Left, Right};
 use hir::def::DefKind;
+use rustc_abi::{
+    BackendRepr, FieldIdx, FieldsShape, Scalar as ScalarAbi, Size, VariantIdx, Variants,
+    WrappingRange,
+};
 use rustc_ast::Mutability;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
@@ -23,9 +27,6 @@ use rustc_middle::mir::interpret::{
 use rustc_middle::ty::layout::{LayoutCx, LayoutOf, TyAndLayout};
 use rustc_middle::ty::{self, Ty};
 use rustc_span::symbol::{Symbol, sym};
-use rustc_target::abi::{
-    Abi, FieldIdx, FieldsShape, Scalar as ScalarAbi, Size, VariantIdx, Variants, WrappingRange,
-};
 use tracing::trace;
 
 use super::machine::AllocMap;
@@ -422,7 +423,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
         // Reset provenance: ensure slice tail metadata does not preserve provenance,
         // and ensure all pointers do not preserve partial provenance.
         if self.reset_provenance_and_padding {
-            if matches!(imm.layout.abi, Abi::Scalar(..)) {
+            if matches!(imm.layout.backend_repr, BackendRepr::Scalar(..)) {
                 // A thin pointer. If it has provenance, we don't have to do anything.
                 // If it does not, ensure we clear the provenance in memory.
                 if matches!(imm.to_scalar(), Scalar::Int(..)) {
@@ -981,7 +982,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                     let elem = layout.field(cx, 0);
 
                     // Fast-path for large arrays of simple types that do not contain any padding.
-                    if elem.abi.is_scalar() {
+                    if elem.backend_repr.is_scalar() {
                         out.add_range(base_offset, elem.size * count);
                     } else {
                         for idx in 0..count {
@@ -1299,19 +1300,19 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValueVisitor<'tcx, M> for ValidityVisitor<'rt,
         // FIXME: We could avoid some redundant checks here. For newtypes wrapping
         // scalars, we do the same check on every "level" (e.g., first we check
         // MyNewtype and then the scalar in there).
-        match val.layout.abi {
-            Abi::Uninhabited => {
+        match val.layout.backend_repr {
+            BackendRepr::Uninhabited => {
                 let ty = val.layout.ty;
                 throw_validation_failure!(self.path, UninhabitedVal { ty });
             }
-            Abi::Scalar(scalar_layout) => {
+            BackendRepr::Scalar(scalar_layout) => {
                 if !scalar_layout.is_uninit_valid() {
                     // There is something to check here.
                     let scalar = self.read_scalar(val, ExpectedKind::InitScalar)?;
                     self.visit_scalar(scalar, scalar_layout)?;
                 }
             }
-            Abi::ScalarPair(a_layout, b_layout) => {
+            BackendRepr::ScalarPair(a_layout, b_layout) => {
                 // We can only proceed if *both* scalars need to be initialized.
                 // FIXME: find a way to also check ScalarPair when one side can be uninit but
                 // the other must be init.
@@ -1322,12 +1323,12 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValueVisitor<'tcx, M> for ValidityVisitor<'rt,
                     self.visit_scalar(b, b_layout)?;
                 }
             }
-            Abi::Vector { .. } => {
+            BackendRepr::Vector { .. } => {
                 // No checks here, we assume layout computation gets this right.
                 // (This is harder to check since Miri does not represent these as `Immediate`. We
                 // also cannot use field projections since this might be a newtype around a vector.)
             }
-            Abi::Aggregate { .. } => {
+            BackendRepr::Memory { .. } => {
                 // Nothing to do.
             }
         }