Refactor lint buffering to avoid requiring a giant enum

Lint buffering currently relies on a giant enum `BuiltinLintDiag`
containing all the lints that might potentially get buffered. In
addition to being an unwieldy enum in a central crate, this also makes
`rustc_lint_defs` a build bottleneck: it depends on various types from
various crates (with a steady pressure to add more), and many crates
depend on it.

Having all of these variants in a separate crate also prevents detecting
when a variant becomes unused, which we can do with a dedicated type
defined and used in the same crate.

Refactor this to use a dyn trait, to allow using `LintDiagnostic` types
directly.

This requires boxing, but all of this is already on the slow path
(emitting an error).

Because the existing `BuiltinLintDiag` requires some additional types in
order to decorate some variants, which are only available later in
`rustc_lint`, use an enum `DecorateDiagCompat` to handle both the `dyn
LintDiagnostic` case and the `BuiltinLintDiag` case.

2 files changed, 8 insertions, 8 deletions

diff --git a/compiler/rustc_session/src/config/cfg.rs b/compiler/rustc_session/src/config/cfg.rs
index 62891eb4f26..852ae414157 100644
--- a/compiler/rustc_session/src/config/cfg.rs
+++ b/compiler/rustc_session/src/config/cfg.rs
@@ -99,7 +99,7 @@ pub(crate) fn disallow_cfgs(sess: &Session, user_cfgs: &Cfg) {
             EXPLICIT_BUILTIN_CFGS_IN_FLAGS,
             None,
             ast::CRATE_NODE_ID,
-            BuiltinLintDiag::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by },
+            BuiltinLintDiag::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by }.into(),
         )
     };
 
diff --git a/compiler/rustc_session/src/parse.rs b/compiler/rustc_session/src/parse.rs
index 426480f0dba..9048c51d5b4 100644
--- a/compiler/rustc_session/src/parse.rs
+++ b/compiler/rustc_session/src/parse.rs
@@ -11,8 +11,8 @@ use rustc_data_structures::sync::{AppendOnlyVec, Lock};
 use rustc_errors::emitter::{FatalOnlyEmitter, HumanEmitter, stderr_destination};
 use rustc_errors::translation::Translator;
 use rustc_errors::{
-    ColorConfig, Diag, DiagCtxt, DiagCtxtHandle, DiagMessage, EmissionGuarantee, MultiSpan,
-    StashKey,
+    BufferedEarlyLint, ColorConfig, DecorateDiagCompat, Diag, DiagCtxt, DiagCtxtHandle,
+    DiagMessage, EmissionGuarantee, MultiSpan, StashKey,
 };
 use rustc_feature::{GateIssue, UnstableFeatures, find_feature_issue};
 use rustc_span::edition::Edition;
@@ -27,7 +27,7 @@ use crate::errors::{
     FeatureDiagnosticSuggestion, FeatureGateError, SuggestUpgradeCompiler,
 };
 use crate::lint::builtin::UNSTABLE_SYNTAX_PRE_EXPANSION;
-use crate::lint::{BufferedEarlyLint, BuiltinLintDiag, Lint, LintId};
+use crate::lint::{Lint, LintId};
 
 /// Collected spans during parsing for places where a certain feature was
 /// used and should be feature gated accordingly in `check_crate`.
@@ -342,17 +342,17 @@ impl ParseSess {
         lint: &'static Lint,
         span: impl Into<MultiSpan>,
         node_id: NodeId,
-        diagnostic: BuiltinLintDiag,
+        diagnostic: impl Into<DecorateDiagCompat>,
     ) {
-        self.opt_span_buffer_lint(lint, Some(span.into()), node_id, diagnostic)
+        self.opt_span_buffer_lint(lint, Some(span.into()), node_id, diagnostic.into())
     }
 
-    pub fn opt_span_buffer_lint(
+    pub(crate) fn opt_span_buffer_lint(
         &self,
         lint: &'static Lint,
         span: Option<MultiSpan>,
         node_id: NodeId,
-        diagnostic: BuiltinLintDiag,
+        diagnostic: DecorateDiagCompat,
     ) {
         self.buffered_lints.with_lock(|buffered_lints| {
             buffered_lints.push(BufferedEarlyLint {