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At first, the thing from my question (the last one was, obviously, made really bad and miss-called its object) is called inlay-hints and, specially for rust, is provided directly from rust-analyzer. It was implemented in https://github.com/simrat39/rust-tools.nvim for Neovim and needs some configuration work to turn it on, but I use it in my case anyway at the moment.
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More globally, there is no a particular plugin that'd implement it in general. It is sad, but you can get some easier signature help by https://github.com/ray-x/lsp_signature.nvim one (thanks for u/eliizaak for this solution).
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