Auto merge of #116412 - nnethercote:rm-plugin-support, r=bjorn3

Remove support for compiler plugins.

They've been deprecated for four years.

This commit includes the following changes.
- It eliminates the `rustc_plugin_impl` crate.
- It changes the language used for lints in `compiler/rustc_driver_impl/src/lib.rs` and `compiler/rustc_lint/src/context.rs`. External lints are now called "loaded" lints, rather than "plugins" to avoid confusion with the old plugins. This only has a tiny effect on the output of `-W help`.
- E0457 and E0498 are no longer used.
- E0463 is narrowed, now only relating to unfound crates, not plugins.
- The `plugin` feature was moved from "active" to "removed".
- It removes the entire plugins chapter from the unstable book.
- It removes quite a few tests, mostly all of those in `tests/ui-fulldeps/plugin/`.

Closes #29597.

r? `@ghost`

3 files changed, 1 insertions, 116 deletions

diff --git a/src/doc/book b/src/doc/book
-Subproject 3dca2fc50b922a8efb94903b9fee8bb42ab48f3
+Subproject 5b6c1ceaa62ecbd6caef08df39b33b3938e99de
diff --git a/src/doc/guide-plugins.md b/src/doc/guide-plugins.md
index 6c511548789..ccb9bb2ee83 100644
--- a/src/doc/guide-plugins.md
+++ b/src/doc/guide-plugins.md
@@ -1,4 +1,3 @@
 % The (old) Rust Compiler Plugins Guide
 
-This content has moved into
-[the Unstable Book](unstable-book/language-features/plugin.html).
+Support for plugins has been removed.
diff --git a/src/doc/unstable-book/src/language-features/plugin.md b/src/doc/unstable-book/src/language-features/plugin.md
deleted file mode 100644
index d9421a428f1..00000000000
--- a/src/doc/unstable-book/src/language-features/plugin.md
+++ /dev/null
@@ -1,114 +0,0 @@
-# `plugin`
-
-The tracking issue for this feature is: [#29597]
-
-[#29597]: https://github.com/rust-lang/rust/issues/29597
-
-
-This feature is part of "compiler plugins." It will often be used with the
-`rustc_private` feature.
-
-------------------------
-
-`rustc` can load compiler plugins, which are user-provided libraries that
-extend the compiler's behavior with new lint checks, etc.
-
-A plugin is a dynamic library crate with a designated *registrar* function that
-registers extensions with `rustc`. Other crates can load these extensions using
-the crate attribute `#![plugin(...)]`.  See the
-`rustc_driver::plugin` documentation for more about the
-mechanics of defining and loading a plugin.
-
-In the vast majority of cases, a plugin should *only* be used through
-`#![plugin]` and not through an `extern crate` item.  Linking a plugin would
-pull in all of librustc_ast and librustc as dependencies of your crate.  This is
-generally unwanted unless you are building another plugin.
-
-The usual practice is to put compiler plugins in their own crate, separate from
-any `macro_rules!` macros or ordinary Rust code meant to be used by consumers
-of a library.
-
-# Lint plugins
-
-Plugins can extend [Rust's lint
-infrastructure](../../reference/attributes/diagnostics.md#lint-check-attributes) with
-additional checks for code style, safety, etc. Now let's write a plugin
-[`lint-plugin-test.rs`](https://github.com/rust-lang/rust/blob/master/tests/ui-fulldeps/plugin/auxiliary/lint-plugin-test.rs)
-that warns about any item named `lintme`.
-
-```rust,ignore (requires-stage-2)
-#![feature(rustc_private)]
-
-extern crate rustc_ast;
-
-// Load rustc as a plugin to get macros
-extern crate rustc_driver;
-extern crate rustc_lint;
-#[macro_use]
-extern crate rustc_session;
-
-use rustc_ast::ast;
-use rustc_driver::plugin::Registry;
-use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
-
-declare_lint!(TEST_LINT, Warn, "Warn about items named 'lintme'");
-
-declare_lint_pass!(Pass => [TEST_LINT]);
-
-impl EarlyLintPass for Pass {
-    fn check_item(&mut self, cx: &EarlyContext, it: &ast::Item) {
-        if it.ident.name.as_str() == "lintme" {
-            cx.lint(TEST_LINT, "item is named 'lintme'", |lint| lint.set_span(it.span));
-        }
-    }
-}
-
-#[no_mangle]
-fn __rustc_plugin_registrar(reg: &mut Registry) {
-    reg.lint_store.register_lints(&[&TEST_LINT]);
-    reg.lint_store.register_early_pass(|| Box::new(Pass));
-}
-```
-
-Then code like
-
-```rust,ignore (requires-plugin)
-#![feature(plugin)]
-#![plugin(lint_plugin_test)]
-
-fn lintme() { }
-```
-
-will produce a compiler warning:
-
-```txt
-foo.rs:4:1: 4:16 warning: item is named 'lintme', #[warn(test_lint)] on by default
-foo.rs:4 fn lintme() { }
-         ^~~~~~~~~~~~~~~
-```
-
-The components of a lint plugin are:
-
-* one or more `declare_lint!` invocations, which define static `Lint` structs;
-
-* a struct holding any state needed by the lint pass (here, none);
-
-* a `LintPass`
-  implementation defining how to check each syntax element. A single
-  `LintPass` may call `span_lint` for several different `Lint`s, but should
-  register them all through the `get_lints` method.
-
-Lint passes are syntax traversals, but they run at a late stage of compilation
-where type information is available. `rustc`'s [built-in
-lints](https://github.com/rust-lang/rust/blob/master/compiler/rustc_lint_defs/src/builtin.rs)
-mostly use the same infrastructure as lint plugins, and provide examples of how
-to access type information.
-
-Lints defined by plugins are controlled by the usual [attributes and compiler
-flags](../../reference/attributes/diagnostics.md#lint-check-attributes), e.g.
-`#[allow(test_lint)]` or `-A test-lint`. These identifiers are derived from the
-first argument to `declare_lint!`, with appropriate case and punctuation
-conversion.
-
-You can run `rustc -W help foo.rs` to see a list of lints known to `rustc`,
-including those provided by plugins loaded by `foo.rs`.