Rollup merge of #142608 - workingjubilee:redescribe-rustc_target-more-accurately, r=wesleywiser

Refresh module-level docs for `rustc_target::spec`

We have long since gone on a curveball from the flexible-target-specification RFC by introducing stability and soundness promises to the language and compiler which we often struggle with extending to target-specific implementation details. Indeed, we often *literally cannot*. We also have modified the search algorithm details. Update the comments for `rustc_target::spec` considerably.

compiler/rustc_target/src/spec/mod.rs

@@ -6,33 +6,36 @@
 //! to a new platform, and allows for an unprecedented level of control over how
 //! the compiler works.
 //!
-//! # Using custom targets
+//! # Using targets and target.json
 //!
-//! A target tuple, as passed via `rustc --target=TUPLE`, will first be
-//! compared against the list of built-in targets. This is to ease distributing
-//! rustc (no need for configuration files) and also to hold these built-in
-//! targets as immutable and sacred. If `TUPLE` is not one of the built-in
-//! targets, rustc will check if a file named `TUPLE` exists. If it does, it
-//! will be loaded as the target configuration. If the file does not exist,
-//! rustc will search each directory in the environment variable
-//! `RUST_TARGET_PATH` for a file named `TUPLE.json`. The first one found will
-//! be loaded. If no file is found in any of those directories, a fatal error
-//! will be given.
+//! Invoking "rustc --target=${TUPLE}" will result in rustc initiating the [`Target::search`] by
+//! - checking if "$TUPLE" is a complete path to a json (ending with ".json") and loading if so
+//! - checking builtin targets for "${TUPLE}"
+//! - checking directories in "${RUST_TARGET_PATH}" for "${TUPLE}.json"
+//! - checking for "${RUSTC_SYSROOT}/lib/rustlib/${TUPLE}/target.json"
 //!
-//! Projects defining their own targets should use
-//! `--target=path/to/my-awesome-platform.json` instead of adding to
-//! `RUST_TARGET_PATH`.
+//! Code will then be compiled using the first discovered target spec.
 //!
 //! # Defining a new target
 //!
-//! Targets are defined using [JSON](https://json.org/). The `Target` struct in
-//! this module defines the format the JSON file should take, though each
-//! underscore in the field names should be replaced with a hyphen (`-`) in the
-//! JSON file. Some fields are required in every target specification, such as
-//! `llvm-target`, `target-endian`, `target-pointer-width`, `data-layout`,
-//! `arch`, and `os`. In general, options passed to rustc with `-C` override
-//! the target's settings, though `target-feature` and `link-args` will *add*
-//! to the list specified by the target, rather than replace.
+//! Targets are defined using a struct which additionally has serialization to and from [JSON].
+//! The `Target` struct in this module loosely corresponds with the format the JSON takes.
+//! We usually try to make the fields equivalent but we have given up on a 1:1 correspondence
+//! between the JSON and the actual structure itself.
+//!
+//! Some fields are required in every target spec, and they should be embedded in Target directly.
+//! Optional keys are in TargetOptions, but Target derefs to it, for no practical difference.
+//! Most notable is the "data-layout" field which specifies Rust's notion of sizes and alignments
+//! for several key types, such as f64, pointers, and so on.
+//!
+//! At one point we felt `-C` options should override the target's settings, like in C compilers,
+//! but that was an essentially-unmarked route for making code incorrect and Rust unsound.
+//! Confronted with programmers who prefer a compiler with a good UX instead of a lethal weapon,
+//! we have almost-entirely recanted that notion, though we hope "target modifiers" will offer
+//! a way to have a decent UX yet still extend the necessary compiler controls, without
+//! requiring a new target spec for each and every single possible target micro-variant.
+//!
+//! [JSON]: https://json.org
 
 use std::borrow::Cow;
 use std::collections::BTreeMap;