cmd/cgo: use the export'ed details in error messages

When an unpinned Go pointer (or a pointer to an unpinned Go pointer) is
returned from Go to C,

>     1  package main
>     2
>     3  import (
>     4          "C"
>     5  )
>     6
>     7  //export foo
>     8  func foo(CLine *C.char) string {
>     9          return C.GoString(CLine)
>    10  }
>    11
>    12
>    13  func main() {
>    14  }

The error message mentions the details of the cgo wrapper,

> panic: runtime error: cgo result is unpinned Go pointer or points to unpinned Go pointer
>
> goroutine 17 [running, locked to thread]:
> panic({0x798f2341a4c0?, 0xc000112000?})
>         /usr/lib/go/src/runtime/panic.go:802 +0x168
> runtime.cgoCheckArg(0x798f23417e20, 0xc000066e50, 0x0?, 0x0, {0x798f233f5a62, 0x42})
>         /usr/lib/go/src/runtime/cgocall.go:679 +0x35b
> runtime.cgoCheckResult({0x798f23417e20, 0xc000066e50})
>         /usr/lib/go/src/runtime/cgocall.go:795 +0x4b
> _cgoexp_3c910ddb72c4_foo(0x7ffc9fa9bfa0)
>         _cgo_gotypes.go:65 +0x5d
> runtime.cgocallbackg1(0x798f233ec780, 0x7ffc9fa9bfa0, 0x0)
>         /usr/lib/go/src/runtime/cgocall.go:446 +0x289
> runtime.cgocallbackg(0x798f233ec780, 0x7ffc9fa9bfa0, 0x0)
>         /usr/lib/go/src/runtime/cgocall.go:350 +0x132
> runtime.cgocallbackg(0x798f233ec780, 0x7ffc9fa9bfa0, 0x0)
>         <autogenerated>:1 +0x2b
> runtime.cgocallback(0x0, 0x0, 0x0)
>         /usr/lib/go/src/runtime/asm_amd64.s:1082 +0xcd
> runtime.goexit({})
>         /usr/lib/go/src/runtime/asm_amd64.s:1693 +0x1

The cgo wrapper (_cgoexp_3c910ddb72c4_foo) is located in a temporary
build artifact (_cgo_gotypes.go)

> $ go tool cgo -objdir objdir parse.go
> $ cat -n objdir/_cgo_gotypes.go | sed -n '55,70p'
>     55  //go:cgo_export_dynamic foo
>     56  //go:linkname _cgoexp_d48770e267d1_foo _cgoexp_d48770e267d1_foo
>     57  //go:cgo_export_static _cgoexp_d48770e267d1_foo
>     58  func _cgoexp_d48770e267d1_foo(a *struct {
>     59                  p0 *_Ctype_char
>     60                  r0 string
>     61          }) {
>     62          a.r0 = foo(a.p0)
>     63          _cgoCheckResult(a.r0)
>     64  }

The details of the export'ed function are expected in the error message.

Use them in error messages.

> panic: runtime error: cgo result is unpinned Go pointer or points to unpinned Go pointer
>
> goroutine 17 [running, locked to thread]:
> panic({0x7df72b1d8ae0?, 0x3ec8a1790030?})
>         /mnt/go/src/runtime/panic.go:877 +0x16f
> runtime.cgoCheckArg(0x7df72b1d62c0, 0x3ec8a16eee50, 0x68?, 0x0, {0x7df72b1ad44c, 0x42})
>         /mnt/go/src/runtime/cgocall.go:679 +0x35b
> runtime.cgoCheckResult({0x7df72b1d62c0, 0x3ec8a16eee50})
>         /mnt/go/src/runtime/cgocall.go:795 +0x4b
> _cgoexp_3c910ddb72c4_foo(0x7ffca1b21020)
>         /mnt/tmp/parse.go:8 +0x5d
> runtime.cgocallbackg1(0x7df72b1a4360, 0x7ffca1b21020, 0x0)
>         /mnt/go/src/runtime/cgocall.go:446 +0x289
> runtime.cgocallbackg(0x7df72b1a4360, 0x7ffca1b21020, 0x0)
>         /mnt/go/src/runtime/cgocall.go:350 +0x132
> runtime.cgocallbackg(0x7df72b1a4360, 0x7ffca1b21020, 0x0)
>         <autogenerated>:1 +0x2b
> runtime.cgocallback(0x0, 0x0, 0x0)
>         /mnt/go/src/runtime/asm_amd64.s:1101 +0xcd
> runtime.goexit({})
>         /mnt/go/src/runtime/asm_amd64.s:1712 +0x1

Link: https://web.archive.org/web/20251008114504/https://dave.cheney.net/2018/01/08/gos-hidden-pragmas
Suggested-by: Keith Randall <[email protected]>
For #75856

Author: ariel-anieli

src/cmd/cgo/out.go

@@ -1144,6 +1144,10 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 					if !p.hasPointer(nil, atype, false) {
 						return
 					}
+
+					// Use the export'ed details in error messages.
+					pos := fset.Position(exp.Func.Pos())
+					fmt.Fprintf(fgo2, "//line %s:%d\n", pos.Filename, pos.Line)
 					fmt.Fprintf(fgo2, "\t_cgoCheckResult(a.r%d)\n", i)
 				})
 		}