Rollup merge of #94113 - Mizobrook-kan:issue-94025, r=estebank

document rustc_middle::mir::Field

cc #94025

compiler/rustc_middle/src/mir/mod.rs

@@ -1841,6 +1841,15 @@ static_assert_size!(PlaceElem<'_>, 24);
 pub type ProjectionKind = ProjectionElem<(), ()>;
 
 rustc_index::newtype_index! {
+    /// A [newtype'd][wrapper] index type in the MIR [control-flow graph][CFG]
+    ///
+    /// 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]
+    /// [wrapper]: https://rustc-dev-guide.rust-lang.org/appendix/glossary.html#newtype
+    /// [CFG]: https://rustc-dev-guide.rust-lang.org/appendix/background.html#cfg
+    /// [mir-datatypes]: https://rustc-dev-guide.rust-lang.org/mir/index.html#mir-data-types
     pub struct Field {
         derive [HashStable]
         DEBUG_FORMAT = "field[{}]"