Introduce a fast path that avoids the debug_tuple abstraction when deriving Debug for unit-like enum variants.

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -65,15 +65,29 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
     // We want to make sure we have the ctxt set so that we can use unstable methods
     let span = cx.with_def_site_ctxt(span);
     let name = cx.expr_lit(span, ast::LitKind::Str(ident.name, ast::StrStyle::Cooked));
+    let fmt = substr.nonself_args[0].clone();
+
+    // Special fast path for unit variants. In the common case of an enum that is entirely unit
+    // variants (i.e. a C-like enum), this fast path allows LLVM to eliminate the entire switch in
+    // favor of a lookup table.
+    if let ast::VariantData::Unit(..) = vdata {
+        let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
+        let expr = cx.expr_call_global(span, fn_path_write_str, vec![fmt, name]);
+        let stmts = vec![cx.stmt_expr(expr)];
+        let block = cx.block(span, stmts);
+        return cx.expr_block(block);
+    }
+
     let builder = Ident::new(sym::debug_trait_builder, span);
     let builder_expr = cx.expr_ident(span, builder);
 
-    let fmt = substr.nonself_args[0].clone();
-
     let mut stmts = Vec::with_capacity(fields.len() + 2);
     let fn_path_finish;
     match vdata {
-        ast::VariantData::Tuple(..) | ast::VariantData::Unit(..) => {
+        ast::VariantData::Unit(..) => {
+            cx.span_bug(span, "unit variants should have been handled above");
+        }
+        ast::VariantData::Tuple(..) => {
             // tuple struct/"normal" variant
             let fn_path_debug_tuple = cx.std_path(&[sym::fmt, sym::Formatter, sym::debug_tuple]);
             let expr = cx.expr_call_global(span, fn_path_debug_tuple, vec![fmt, name]);

compiler/rustc_span/src/symbol.rs

@@ -1418,6 +1418,7 @@ symbols! {
         wrapping_sub,
         wreg,
         write_bytes,
+        write_str,
         x87_reg,
         xer,
         xmm_reg,