Filling in the command line didn't work, but I fixed it

[pillmuncher]

This didn't work:

function()
    return ":IncRename " .. vim.fn.expand("<cword>")
end

It had no observable effect whatsoever.

vim.api.nvim_feedkeys() to the rescue:

function()
    local line = ":IncRename " .. vim.fn.expand("<cword>")
    vim.api.nvim_feedkeys(line, "n", true)
end

I don't know if it was my Neovim version (0.9.2) or some other plugin interfering, but now it's working.

[smjonas]

Glad it's working for you now! Does it also not work if you pass expr = true to vim.keymap.set?

[pillmuncher]

Yes, you're right. It was the expr = true. I'm using lazy.nvim as my plugin manager and accidentally had it written like this:

-- plugins/inc_rename.lua
return {
    "smjonas/inc-rename.nvim",
    opts = {},
    keys = {
        {
            "<leader>rn",
            function()
                return ":IncRename " .. vim.fn.expand("<cword>")
            end,
            desc = "[r]e[n]ame",
            mode = { "n" },
           { expr = true },  --  <------------------ 
        },
    },
}

Changing it to

           expr = true,

fixed it. My bad.