Rollup merge of #125596 - nnethercote:rental-hard-error, r=estebank

Convert `proc_macro_back_compat` lint to an unconditional error.

We still check for the `rental`/`allsorts-rental` crates. But now if they are detected we just emit a fatal error, instead of emitting a warning and providing alternative behaviour.

The original "hack" implementing alternative behaviour was added in #73345.

The lint was added in #83127.

The tracking issue is #83125.

The direct motivation for the change is that providing the alternative behaviour is interfering with #125174 and follow-on work.

r? ``@estebank``

compiler/rustc_lint/src/lints.rs

@@ -2649,14 +2649,6 @@ pub struct LegacyDeriveHelpers {
     pub span: Span,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(lint_proc_macro_back_compat)]
-#[note]
-pub struct ProcMacroBackCompat {
-    pub crate_name: String,
-    pub fixed_version: String,
-}
-
 #[derive(LintDiagnostic)]
 #[diag(lint_or_patterns_back_compat)]
 pub struct OrPatternsBackCompat {