perf: avoid test_inverse import-time compilation

[Beforerr]

Summary

Small load-time cleanup: replace the runtime __init__ error hint with a direct test_inverse fallback method. Users still get a friendly error when Test is missing, but package import no longer pays for the error-hint hook.

Benchmark

Measured with julia --startup-file=no --project=@. -e 'using InteractiveUtils; @time_imports using InverseFunctions' after precompile.

• before: 74.2 ms InverseFunctions, 98.73% compilation time
• after: 0.7 ms InverseFunctions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.49%. Comparing base (bbb49fe) to head (441dcba).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   98.48%   98.49%   +0.01%     
==========================================
  Files           6        6              
  Lines         132      133       +1     
==========================================
+ Hits          130      131       +1     
  Misses          2        2              

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[devmotion]

This changes the behaviour: Currently, the error hint with the question is only shown when the extension is actually not loaded. This message here will instead be displayed unconditionally, and users will see an ArgumentError with an incorrect message instead of eg a MethodError.

[Beforerr]

Good catch. I changed this so the fallback no longer changes unrelated call errors.

The fallback is still a varargs method so the Test extension can add its real test_inverse(f, x; ...) method without overwriting during precompilation. But it now only emits the friendly missing-Test message for the normal two-argument call. Other signatures throw MethodError, matching existing behavior better.

[devmotion]

No, even the behaviour of the new version is different from master. There's no point in asking the user to load the extension if it's already loaded.

[Beforerr]

If test is already loaded, the ArgumentError would never be throwed I think.

Since Julia owns MethodError display so to improve load-time compilation,
options are :

1. Exact old exception behavior, faster load: MethodError, but no “Did you forget…” hint.
2. Or friendly message, changed exception: define fallback method, throw ArgumentError / ErrorException.

I felt like keeping original UX with longer load-time is worse as this function is rarely used downstream and require Test to load first to use it anyway.