Allow packages to force depwarns

[timholy]

Fixes #37164 in a gentler way than proposed in the OP, by allowing packages to call depwarn(args...; force=true). No macro equivalent is provided in this PR.

[fredrikekre]

Is there an advantage of using depwarn rather than just @warn if you want to show them unconditionally? I guess the ability to turn them into hard errors with --depwarn=error?

[timholy]

@warn shows the line containing the @warn statement itself; depwarn shows the caller. With JuliaImages/ImageCore.jl#140, julia test/deprecated.jl yields

julia> include("deprecated.jl")
[ Info: Deprecation warnings are expected
┌ Warning: `convert(Array{BGR}, img)` will soon switch to returning an array with non-concrete element type, which adds flexibility but
│ with great cost to performance. To maintain the current behavior, use `BGR.(img)` instead.
└ @ ImageCore ~/.julia/dev/ImageCore/src/deprecations.jl:6
┌ Info: It is recommended that you fix this now to avoid breakage when a new version is released and this warning is removed.
└ Tip: to see all deprecation warnings together with code locations, launch Julia with `--depwarn=yes` and rerun your code.

ImageCore/src/deprecations.jl:6 is this line, which is not very useful because what the user wants to know is where in my code did I call a deprecated method from?

Whereas julia --depwarn=yes test/deprecated.jl yields

julia> include("deprecated.jl")
[ Info: Deprecation warnings are expected
┌ Warning: `convert(Array{BGR}, img)` will soon switch to returning an array with non-concrete element type, which adds flexibility but
│ with great cost to performance. To maintain the current behavior, use `BGR.(img)` instead.
│   caller = macro expansion at deprecated.jl:8 [inlined]
└ @ Core ~/.julia/dev/ImageCore/test/deprecated.jl:8
┌ Warning: `convert(Array{Lab}, img)` will soon switch to returning an array with non-concrete element type, which adds flexibility but
│ with great cost to performance. To maintain the current behavior, use `Lab.(img)` instead.
│   caller = macro expansion at deprecated.jl:12 [inlined]
└ @ Core ~/.julia/dev/ImageCore/test/deprecated.jl:12
...

Ignore the 2-vs-1 distinction and focus on the fact that now the file & line number indicates the caller (i.e., here and here).

[johnnychen94]

Is there an advantage of using depwarn rather than just @warn if you want to show them unconditionally?

Another advantage is to not show thousands of warnings for the same function call 😆 . For example, in a for loop.

[fredrikekre]

Another advantage is to not show thousands of warnings for the same function call

For that you can use maxlog

julia> function f()
           for i in 1:10
               @warn "hello, world" maxlog=1
           end
       end
f (generic function with 1 method)

julia> f()
┌ Warning: hello, world
└ @ Main REPL[2]:3

but yea, the wrong source of the warning is indeed annoying.