Move mir::Field → abi::FieldIdx

The first PR for https://github.com/rust-lang/compiler-team/issues/606

This is just the move-and-rename, because it's plenty big-and-bitrotty already.  Future PRs will start using `FieldIdx` more broadly, and concomitantly removing `FieldIdx::new`s.

compiler/rustc_middle/src/mir/syntax.rs

@@ -3,7 +3,7 @@
 //! This is in a dedicated file so that changes to this file can be reviewed more carefully.
 //! The intention is that this file only contains datatype declarations, no code.
 
-use super::{BasicBlock, Constant, Field, Local, SwitchTargets, UserTypeProjection};
+use super::{BasicBlock, Constant, Local, SwitchTargets, UserTypeProjection};
 
 use crate::mir::coverage::{CodeRegion, CoverageKind};
 use crate::traits::Reveal;
@@ -16,7 +16,7 @@ use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{self as hir};
 use rustc_hir::{self, GeneratorKind};
-use rustc_target::abi::VariantIdx;
+use rustc_target::abi::{FieldIdx, VariantIdx};
 
 use rustc_ast::Mutability;
 use rustc_span::def_id::LocalDefId;
@@ -888,7 +888,15 @@ pub struct Place<'tcx> {
 #[derive(TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
 pub enum ProjectionElem<V, T> {
     Deref,
-    Field(Field, T),
+
+    /// A field (e.g., `f` in `_1.f`) is one variant of [`ProjectionElem`]. Conceptually,
+    /// rustc can identify that a field projection refers to either two different regions of memory
+    /// or the same one between the base and the 'projection element'.
+    /// Read more about projections in the [rustc-dev-guide][mir-datatypes]
+    ///
+    /// [mir-datatypes]: https://rustc-dev-guide.rust-lang.org/mir/index.html#mir-data-types
+    Field(FieldIdx, T),
+
     /// Index into a slice/array.
     ///
     /// Note that this does not also dereference, and so it does not exactly correspond to slice