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.

1 files changed, 0 insertions, 12 deletions

diff --git a/tests/ui-fulldeps/plugin/lint-tool-cmdline-allow.rs b/tests/ui-fulldeps/plugin/lint-tool-cmdline-allow.rs
deleted file mode 100644
index 83a8b3e1a7e..00000000000
--- a/tests/ui-fulldeps/plugin/lint-tool-cmdline-allow.rs
+++ /dev/null
@@ -1,12 +0,0 @@
-// check-pass
-// aux-build:lint-tool-test.rs
-// ignore-stage1
-// compile-flags: -A test-lint
-
-#![feature(plugin)]
-#![plugin(lint_tool_test)] //~ WARNING compiler plugins are deprecated
-
-fn lintme() {}
-//~^ WARNING item is named 'lintme' [clippy::test_lint]
-
-pub fn main() {}