Fix G118 false positive when cancel is stored in returned struct field

[ccojocar]

Summary

Fixes #1591

The G118 analyzer (detectLostCancel) reported a false positive when a cancel function was stored in a struct field and the struct was returned to the caller:

type Foo struct{ Cancel func() }

func NewFoo() Foo {
    _, cancel := context.WithCancel(context.Background())
    foo := Foo{Cancel: cancel}
    return foo
}

func main() {
    foo := NewFoo()
    foo.Cancel()
}

Root cause

In isCancelCalled, when the cancel value is stored into a struct field via a Store to a FieldAddr, the trace adds the FieldAddr address to the queue. But that FieldAddr's only referrer is the Store itself — dead end. The struct is subsequently loaded and returned, transferring responsibility to the caller, but this link was never followed.

The existing isCancelCalledViaStructField only checks methods on the same receiver type, which doesn't apply when the struct has no methods and the cancel is consumed by a plain caller.

Fix

Added isStructFieldReturnedFromFunc that checks whether the struct base pointer from a FieldAddr is loaded and returned from the enclosing function. Called from the Store+FieldAddr branch in isCancelCalled — if the struct is returned, treat the cancel as transferred (same as a direct cancel return).

Testing

• Added a test sample matching the exact pattern from the issue (expected 0 issues)
• All existing G118 tests continue to pass
• Verified manually that genuinely lost cancels (struct not returned) are still detected

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.87%. Comparing base (59a9da0) to head (89a15c0).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
analyzers/context_propagation.go	85.71%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1593   +/-   ##
=======================================
  Coverage   80.87%   80.87%           
=======================================
  Files         104      104           
  Lines        9906     9920   +14     
=======================================
+ Hits         8011     8023   +12     
- Misses       1409     1410    +1     
- Partials      486      487    +1     

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