Temporaries not extended in format_args!() with only one placeholder

[m-ou-se]

format_args!() lifetime-extends its arguments, except when it has only one placeholder:

let a = format_args!("{}", String::new());

dbg!(a); // ERROR


let b = format_args!("{}{}", String::new(), String::new());

dbg!(b); // OK

It should lifetime-extend the temporaries in both cases.

(Originally reported in #145422, see #145422 (comment).)

[m-ou-se]

This should be fixable by simply removing these 20 lines:

rust/compiler/rustc_ast_lowering/src/format.rs

Lines 490 to 509 in 4356e83

	} else if argmap.len() == 1 && arguments.len() == 1 {
	// Only one argument, so we don't need to make the `args` tuple.
	//
	// Generate:
	// super let args = [<core::fmt::Argument>::new_display(&arg)];
	let args = ctx.arena.alloc_from_iter(argmap.iter().map(
	|(&(arg_index, ty), &placeholder_span)| {
	let arg = &arguments[arg_index];
	let placeholder_span =
	placeholder_span.unwrap_or(arg.expr.span).with_ctxt(macsp.ctxt());
	let arg = ctx.lower_expr(&arg.expr);
	let ref_arg = ctx.arena.alloc(ctx.expr_ref(arg.span.with_ctxt(macsp.ctxt()), arg));
	make_argument(ctx, placeholder_span, ref_arg, ty)
	},
	));
	let args = ctx.arena.alloc(ctx.expr(macsp, hir::ExprKind::Array(args)));
	let args_ident = Ident::new(sym::args, macsp);
	let (args_pat, args_hir_id) = ctx.pat_ident(macsp, args_ident);
	let let_statement = ctx.stmt_super_let_pat(macsp, args_pat, Some(args));
	(vec![let_statement], ctx.arena.alloc(ctx.expr_ident_mut(macsp, args_ident, args_hir_id)))

My only worry is that removing that case degrades compilation performance slightly.