What Does `:=` Do?

What the title says. I can’t figure it out, and the docs just provide the not at all useful description:
x := y is a synonym for x = x : y

Which apparently means something different from what I expected to, given the following returns an error rather than a range:

julia> x = 2
2

julia> y = 4
4

julia> x:=y
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That’s actually an error in the docs that I believe was recently fixed. := doesn’t do anything. It’s basically just reserved syntax for macros.

E.g. some people want domain specific languages embedded in julia might have a syntax like

@model begin
    x := 1
    y := x + 1
    ...
end
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Can you link to the docs? That is not valid syntax in 1.*.

Interesting – what are some examples of macros that use this?

Here’s what I get in the help> mode in 1.6:

help?> :=
search:

  x := y is a synonym for x = x : y

This note gets automatically generated for a bunch of infix operators but was done erroneously for :.

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