postfix macroが導入されれば foo.placeholder!(e => { ... }) と出来るので構文レベルに無理やり組み込む必要はなさそう
ScalaのPlaceholder Syntaxみたいなやつだ(提案されてる構文はかなり微妙だが...) https://internals.rust-lang.org/t/pre-rfc-extended-dot-operator/9304 …
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