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.

Author: nnethercote

Cargo.lock

@@ -3666,7 +3666,6 @@ dependencies = [
  "rustc_monomorphize",
  "rustc_parse",
  "rustc_passes",
- "rustc_plugin_impl",
  "rustc_privacy",
  "rustc_query_system",
  "rustc_resolve",
@@ -3956,7 +3955,6 @@ dependencies = [
  "rustc_monomorphize",
  "rustc_parse",
  "rustc_passes",
- "rustc_plugin_impl",
  "rustc_privacy",
  "rustc_query_impl",
  "rustc_query_system",
@@ -4266,21 +4264,6 @@ dependencies = [
  "tracing",
 ]
 
-[[package]]
-name = "rustc_plugin_impl"
-version = "0.0.0"
-dependencies = [
- "libloading 0.7.4",
- "rustc_ast",
- "rustc_errors",
- "rustc_fluent_macro",
- "rustc_lint",
- "rustc_macros",
- "rustc_metadata",
- "rustc_session",
- "rustc_span",
-]
-
 [[package]]
 name = "rustc_privacy"
 version = "0.0.0"

compiler/rustc_codegen_cranelift/scripts/test_rustc_tests.sh

@@ -76,7 +76,6 @@ rm -r tests/run-make/split-debuginfo # same
 rm -r tests/run-make/symbols-include-type-name # --emit=asm not supported
 rm -r tests/run-make/target-specs # i686 not supported by Cranelift
 rm -r tests/run-make/mismatching-target-triples # same
-rm -r tests/run-make/use-extern-for-plugins # same
 
 # requires LTO
 rm -r tests/run-make/cdylib

compiler/rustc_driver_impl/Cargo.toml

@@ -44,7 +44,6 @@ rustc_hir_pretty = { path = "../rustc_hir_pretty" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_metadata = { path = "../rustc_metadata" }
 rustc_parse = { path = "../rustc_parse" }
-rustc_plugin_impl = { path = "../rustc_plugin_impl" }
 rustc_codegen_ssa = { path = "../rustc_codegen_ssa" }
 rustc_session = { path = "../rustc_session" }
 rustc_error_codes = { path = "../rustc_error_codes" }

compiler/rustc_driver_impl/src/lib.rs

@@ -17,8 +17,6 @@
 #[macro_use]
 extern crate tracing;
 
-pub extern crate rustc_plugin_impl as plugin;
-
 use rustc_ast as ast;
 use rustc_codegen_ssa::{traits::CodegenBackend, CodegenErrors, CodegenResults};
 use rustc_data_structures::profiling::{
@@ -129,7 +127,6 @@ pub static DEFAULT_LOCALE_RESOURCES: &[&str] = &[
     rustc_monomorphize::DEFAULT_LOCALE_RESOURCE,
     rustc_parse::DEFAULT_LOCALE_RESOURCE,
     rustc_passes::DEFAULT_LOCALE_RESOURCE,
-    rustc_plugin_impl::DEFAULT_LOCALE_RESOURCE,
     rustc_privacy::DEFAULT_LOCALE_RESOURCE,
     rustc_query_system::DEFAULT_LOCALE_RESOURCE,
     rustc_resolve::DEFAULT_LOCALE_RESOURCE,
@@ -970,16 +967,14 @@ the command line flag directly.
 }
 
 /// Write to stdout lint command options, together with a list of all available lints
-pub fn describe_lints(sess: &Session, lint_store: &LintStore, loaded_plugins: bool) {
+pub fn describe_lints(sess: &Session, lint_store: &LintStore, loaded_lints: bool) {
     safe_println!(
         "
 Available lint options:
     -W <foo>           Warn about <foo>
-    -A <foo>           \
-              Allow <foo>
+    -A <foo>           Allow <foo>
     -D <foo>           Deny <foo>
-    -F <foo>           Forbid <foo> \
-              (deny <foo> and all attempts to override)
+    -F <foo>           Forbid <foo> (deny <foo> and all attempts to override)
 
 "
     );
@@ -998,18 +993,18 @@ Available lint options:
         lints
     }
 
-    let (plugin, builtin): (Vec<_>, _) =
-        lint_store.get_lints().iter().cloned().partition(|&lint| lint.is_plugin);
-    let plugin = sort_lints(sess, plugin);
+    let (loaded, builtin): (Vec<_>, _) =
+        lint_store.get_lints().iter().cloned().partition(|&lint| lint.is_loaded);
+    let loaded = sort_lints(sess, loaded);
     let builtin = sort_lints(sess, builtin);
 
-    let (plugin_groups, builtin_groups): (Vec<_>, _) =
+    let (loaded_groups, builtin_groups): (Vec<_>, _) =
         lint_store.get_lint_groups().partition(|&(.., p)| p);
-    let plugin_groups = sort_lint_groups(plugin_groups);
+    let loaded_groups = sort_lint_groups(loaded_groups);
     let builtin_groups = sort_lint_groups(builtin_groups);
 
     let max_name_len =
-        plugin.iter().chain(&builtin).map(|&s| s.name.chars().count()).max().unwrap_or(0);
+        loaded.iter().chain(&builtin).map(|&s| s.name.chars().count()).max().unwrap_or(0);
     let padded = |x: &str| {
         let mut s = " ".repeat(max_name_len - x.chars().count());
         s.push_str(x);
@@ -1037,7 +1032,7 @@ Available lint options:
 
     let max_name_len = max(
         "warnings".len(),
-        plugin_groups
+        loaded_groups
             .iter()
             .chain(&builtin_groups)
             .map(|&(s, _)| s.chars().count())
@@ -1075,20 +1070,22 @@ Available lint options:
 
     print_lint_groups(builtin_groups, true);
 
-    match (loaded_plugins, plugin.len(), plugin_groups.len()) {
+    match (loaded_lints, loaded.len(), loaded_groups.len()) {
         (false, 0, _) | (false, _, 0) => {
-            safe_println!("Lint tools like Clippy can provide additional lints and lint groups.");
+            safe_println!("Lint tools like Clippy can load additional lints and lint groups.");
+        }
+        (false, ..) => panic!("didn't load additional lints but got them anyway!"),
+        (true, 0, 0) => {
+            safe_println!("This crate does not load any additional lints  or lint groups.")
         }
-        (false, ..) => panic!("didn't load lint plugins but got them anyway!"),
-        (true, 0, 0) => safe_println!("This crate does not load any lint plugins or lint groups."),
         (true, l, g) => {
             if l > 0 {
-                safe_println!("Lint checks provided by plugins loaded by this crate:\n");
-                print_lints(plugin);
+                safe_println!("Lint checks loaded by this crate:\n");
+                print_lints(loaded);
             }
             if g > 0 {
-                safe_println!("Lint groups provided by plugins loaded by this crate:\n");
-                print_lint_groups(plugin_groups, false);
+                safe_println!("Lint groups loaded by this crate:\n");
+                print_lint_groups(loaded_groups, false);
             }
         }
     }
@@ -1105,7 +1102,7 @@ pub fn describe_flag_categories(handler: &EarlyErrorHandler, matches: &Matches)
         rustc_errors::FatalError.raise();
     }
 
-    // Don't handle -W help here, because we might first load plugins.
+    // Don't handle -W help here, because we might first load additional lints.
     let debug_flags = matches.opt_strs("Z");
     if debug_flags.iter().any(|x| *x == "help") {
         describe_debug_flags();

compiler/rustc_error_codes/src/error_codes/E0457.md

@@ -1,3 +1,5 @@
+#### Note: this error code is no longer emitted by the compiler`
+
 Plugin `..` only found in rlib format, but must be available in dylib format.
 
 Erroneous code example:

compiler/rustc_error_codes/src/error_codes/E0463.md

@@ -1,16 +1,13 @@
-A plugin/crate was declared but cannot be found.
+A crate was declared but cannot be found.
 
 Erroneous code example:
 
 ```compile_fail,E0463
-#![feature(plugin)]
-#![plugin(cookie_monster)] // error: can't find crate for `cookie_monster`
-extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`
+extern crate foo; // error: can't find crate
 ```
 
 You need to link your code to the relevant crate in order to be able to use it
-(through Cargo or the `-L` option of rustc example). Plugins are crates as
-well, and you link to them the same way.
+(through Cargo or the `-L` option of rustc, for example).
 
 ## Common causes
 

compiler/rustc_error_codes/src/error_codes/E0498.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 The `plugin` attribute was malformed.
 
 Erroneous code example:
 
-```compile_fail,E0498
+```ignore (E0498 is no longer emitted)
 #![feature(plugin)]
 #![plugin(foo(args))] // error: invalid argument
 #![plugin(bar="test")] // error: invalid argument

compiler/rustc_feature/src/active.rs

@@ -524,8 +524,6 @@ declare_features! (
     (active, object_safe_for_dispatch, "1.40.0", Some(43561), None),
     /// Allows using `#[optimize(X)]`.
     (active, optimize_attribute, "1.34.0", Some(54882), None),
-    /// Allows using `#![plugin(myplugin)]`.
-    (active, plugin, "1.0.0", Some(29597), None),
     /// Allows exhaustive integer pattern matching on `usize` and `isize`.
     (active, precise_pointer_size_matching, "1.32.0", Some(56354), None),
     /// Allows macro attributes on expressions, statements and non-inline modules.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -417,24 +417,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         naked_functions, experimental!(naked)
     ),
 
-    // Plugins:
-    BuiltinAttribute {
-        name: sym::plugin,
-        only_local: false,
-        type_: CrateLevel,
-        template: template!(List: "name"),
-        duplicates: DuplicatesOk,
-        gate: Gated(
-            Stability::Deprecated(
-                "https://github.com/rust-lang/rust/pull/64675",
-                Some("may be removed in a future compiler version"),
-            ),
-            sym::plugin,
-            "compiler plugins are deprecated",
-            cfg_fn!(plugin)
-        ),
-    },
-
     // Testing:
     gated!(
         test_runner, CrateLevel, template!(List: "path"), ErrorFollowing, custom_test_frameworks,

compiler/rustc_feature/src/removed.rs

@@ -155,9 +155,12 @@ declare_features! (
      Some("removed in favor of `#![feature(marker_trait_attr)]`")),
     (removed, panic_implementation, "1.28.0", Some(44489), None,
      Some("subsumed by `#[panic_handler]`")),
+    /// Allows using `#![plugin(myplugin)]`.
+    (removed, plugin, "1.74.0", Some(29597), None, // njn: version?
+     Some("plugins are no longer supported")),
     /// Allows using `#[plugin_registrar]` on functions.
     (removed, plugin_registrar, "1.54.0", Some(29597), None,
-     Some("a __rustc_plugin_registrar symbol must now be defined instead")),
+     Some("plugins are no longer supported")),
     (removed, proc_macro_expr, "1.27.0", Some(54727), None,
      Some("subsumed by `#![feature(proc_macro_hygiene)]`")),
     (removed, proc_macro_gen, "1.27.0", Some(54727), None,