Rollup merge of #134941 - workingjubilee:rustc-abi-normal, r=Noratrieb

compiler: Add a statement-of-intent to `rustc_abi`

This just documents the most basic idea of what the crate is even for in my view, rather than leaving that strewn about GitHub issues, PR reviews, and Zulip streams. In particular, I hope to make it clearer what code should go in `rustc_abi` and what should not, which is of immediate relevance to contributors.

I considered going even further and explaining ideas like "ABI compatibility", prologues, and so on. However, because of the cross-cutting nature of ABI, I think such explanations should probably live in the place for cross-cutting documents: the rustc dev guide. This is only meant to be a quick "by the way".

compiler/rustc_abi/src/lib.rs

@@ -8,6 +8,38 @@
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
+/*! ABI handling for rustc
+
+## What is an "ABI"?
+
+Literally, "application binary interface", which means it is everything about how code interacts,
+at the machine level, with other code. This means it technically covers all of the following:
+- object binary format for e.g. relocations or offset tables
+- in-memory layout of types
+- procedure calling conventions
+
+When we discuss "ABI" in the context of rustc, we are probably discussing calling conventions.
+To describe those `rustc_abi` also covers type layout, as it must for values passed on the stack.
+Despite `rustc_abi` being about calling conventions, it is good to remember these usages exist.
+You will encounter all of them and more if you study target-specific codegen enough!
+Even in general conversation, when someone says "the Rust ABI is unstable", it may allude to
+either or both of
+- `repr(Rust)` types have a mostly-unspecified layout
+- `extern "Rust" fn(A) -> R` has an unspecified calling convention
+
+## Crate Goal
+
+ABI is a foundational concept, so the `rustc_abi` crate serves as an equally foundational crate.
+It cannot carry all details relevant to an ABI: those permeate code generation and linkage.
+Instead, `rustc_abi` is intended to provide the interface for reasoning about the binary interface.
+It should contain traits and types that other crates then use in their implementation.
+For example, a platform's `extern "C" fn` calling convention will be implemented in `rustc_target`
+but `rustc_abi` contains the types for calculating layout and describing register-passing.
+This makes it easier to describe things in the same way across targets, codegen backends, and
+even other Rust compilers, such as rust-analyzer!
+
+*/
+
 use std::fmt;
 #[cfg(feature = "nightly")]
 use std::iter::Step;