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_lint/src/builtin.rs

@@ -16,6 +16,7 @@
 use std::fmt::Write;
 
 use ast::token::TokenKind;
+use rustc_abi::BackendRepr;
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_ast::visit::{FnCtxt, FnKind};
 use rustc_ast::{self as ast, *};
@@ -40,7 +41,6 @@ use rustc_span::edition::Edition;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{Ident, Symbol, kw, sym};
 use rustc_span::{BytePos, InnerSpan, Span};
-use rustc_target::abi::Abi;
 use rustc_target::asm::InlineAsmArch;
 use rustc_trait_selection::infer::{InferCtxtExt, TyCtxtInferExt};
 use rustc_trait_selection::traits::misc::type_allowed_to_implement_copy;
@@ -2466,7 +2466,9 @@ impl<'tcx> LateLintPass<'tcx> for InvalidValue {
 
             // Check if this ADT has a constrained layout (like `NonNull` and friends).
             if let Ok(layout) = cx.tcx.layout_of(cx.param_env.and(ty)) {
-                if let Abi::Scalar(scalar) | Abi::ScalarPair(scalar, _) = &layout.abi {
+                if let BackendRepr::Scalar(scalar) | BackendRepr::ScalarPair(scalar, _) =
+                    &layout.backend_repr
+                {
                     let range = scalar.valid_range(cx);
                     let msg = if !range.contains(0) {
                         "must be non-null"

compiler/rustc_lint/src/foreign_modules.rs

@@ -217,7 +217,7 @@ fn structurally_same_type<'tcx>(
         // `extern` blocks cannot be generic, so we'll always get a layout here.
         let a_layout = tcx.layout_of(param_env.and(a)).unwrap();
         let b_layout = tcx.layout_of(param_env.and(b)).unwrap();
-        assert_eq!(a_layout.abi, b_layout.abi);
+        assert_eq!(a_layout.backend_repr, b_layout.backend_repr);
         assert_eq!(a_layout.size, b_layout.size);
         assert_eq!(a_layout.align, b_layout.align);
     }

compiler/rustc_lint/src/types.rs

@@ -1,6 +1,7 @@
 use std::iter;
 use std::ops::ControlFlow;
 
+use rustc_abi::{BackendRepr, TagEncoding, Variants, WrappingRange};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::DiagMessage;
 use rustc_hir::{Expr, ExprKind};
@@ -13,7 +14,6 @@ use rustc_session::{declare_lint, declare_lint_pass, impl_lint_pass};
 use rustc_span::def_id::LocalDefId;
 use rustc_span::symbol::sym;
 use rustc_span::{Span, Symbol, source_map};
-use rustc_target::abi::{Abi, TagEncoding, Variants, WrappingRange};
 use rustc_target::spec::abi::Abi as SpecAbi;
 use tracing::debug;
 use {rustc_ast as ast, rustc_hir as hir};
@@ -776,8 +776,8 @@ pub(crate) fn repr_nullable_ptr<'tcx>(
             bug!("should be able to compute the layout of non-polymorphic type");
         }
 
-        let field_ty_abi = &field_ty_layout.ok()?.abi;
-        if let Abi::Scalar(field_ty_scalar) = field_ty_abi {
+        let field_ty_abi = &field_ty_layout.ok()?.backend_repr;
+        if let BackendRepr::Scalar(field_ty_scalar) = field_ty_abi {
             match field_ty_scalar.valid_range(&tcx) {
                 WrappingRange { start: 0, end }
                     if end == field_ty_scalar.size(&tcx).unsigned_int_max() - 1 =>