cmd/cgo: use the export'ed file/line 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 file/line 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 file/line of the export'ed function is expected in the error message.

Use it 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

So doing, fix typos in comments.

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

Change-Id: I0bf36d5c8c5c0c7df13b00818bc4641009058979
GitHub-Last-Rev: e65839c
GitHub-Pull-Request: #76118
Reviewed-on: https://go-review.googlesource.com/c/go/+/716441
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: ariel-anieli

src/cmd/cgo/main.go

@@ -72,8 +72,8 @@ type File struct {
 	ExpFunc     []*ExpFunc          // exported functions for this file
 	Name        map[string]*Name    // map from Go name to Name
 	NamePos     map[*Name]token.Pos // map from Name to position of the first reference
-	NoCallbacks map[string]bool     // C function names that with #cgo nocallback directive
-	NoEscapes   map[string]bool     // C function names that with #cgo noescape directive
+	NoCallbacks map[string]bool     // C function names with #cgo nocallback directive
+	NoEscapes   map[string]bool     // C function names with #cgo noescape directive
 	Edit        *edit.Buffer
 
 	debugs []*debug // debug data from iterations of gccDebug. Initialized by File.loadDebug.

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 file/line 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)
 				})
 		}