Rails text_field_tag id attribute - ruby-on-rails

Question

I'm going through Michael Hartl's Rails tutorial. One of the exercises for chapter 8 is to rewrite the code using form_tag instead of form_for. After viewing the source that form_for was giving me and looking up each tag in the Rails documentation, I was able to successfully put it together.
My question is: How does Rails know what id attribute to give the <input> tag created by text_field_tag, so that it matches the id of the <label> tag created by the label_tag that precedes it?
Here's the code for the form:
<%= form_tag(sessions_path) do %>
<%= label_tag "session_email", "Email" %>
<%= text_field_tag "session[email]" %>
<%= label_tag "session_password", "Password" %>
<%= password_field_tag "session[password]" %>
<%= button_tag("Sign in", name: "commit", type: "submit") %>
<% end %>
Here's the resulting html:
<form accept-charset="UTF-8" action="/sessions" method="post"><div style="margin:0;padding:0;display:inline"><input name="utf8" type="hidden" value="✓" /><input name="authenticity_token" type="hidden" value="c9a5KJveRSOQyDo/ckUtpmQAbjw2f1alnB6Dn1lu3XU=" /></div>
<label for="session_email">Email</label>
<input id="session_email" name="session[email]" type="text" value="{:id=>"session_email"}" />
<label for="session_password">Password</label>
<input id="session_password" name="session[password]" type="password" value="{:id=>"session_password"}" />
<button name="commit" type="submit">Sign in</button>

Answer 1

Note that since Rails 5.1, the recommended form helper to use is form_with.
And form_with no longer automatically generates id's and class's for DOM elements any more like form_for and form_tag used to.
This has implications for adding markup like labels which used to link up to their corresponding input fields automatically. Now, for a label to properly pair up to an input field (with the for attribute) you need to explicitly add the id attribute to the input.
Example
ERB:
<%= form_with model: User.new do |f| %>
<%= f.label :name %>
<%= f.text_field :name, id: :user_name %>
<% end %>
which generates HTML:
<form action="/users" method="post">
<label for="user_name">Name</label>
<input id="user_name" type="text" name="user[name]">
</form>
See also
https://medium.com/#tinchorb/form-with-building-html-forms-in-rails-5-1-f30bd60ef52d, specifically the heading "No more auto-generated ids and classes".