I propose adding the function os.Copy(dst, src string) that does the exact same thing as the cp command.
This function would help simplify these codes in Go itself:

• go/src/cmd/go/internal/modcmd/vendor.go

  Line 293 in be9e244

  if _, err := io.Copy(w, r); err != nil {

• go/src/cmd/cgo/out.go

  Line 261 in 8d3631f

  _, err = io.Copy(fexp, fgcch)

In the wild, there are 91k results for the search https://github.com/search?p=4&q=os.create+%22io.copy%22&type=Code , and one-tenth of them are copying files.
This implies that this function would help 10k codes out there.

Tentative implementation:

// Copy copies src to dst like the cp command.
func Copy(dst, src string) error {
        if dst == src {
                return ErrInvalid
        }

        srcF, err := Open(src)
        if err != nil {
                return err
        }
        defer srcF.Close()

        info, err := srcF.Stat()
        if err != nil {
                return err
        }

        dstF, err := OpenFile(dst, O_RDWR|O_CREATE|O_TRUNC, info.Mode())
        if err != nil {
                return err
        }
        defer dstF.Close()

        if _, err := io.Copy(dstF, srcF); err != nil {
                return err
        }
        return nil
}

Unsettled questions:

• Should the name be Copy or CopyFile?
• How can the implementation handle errors from File.Close() in a clean manner without defer?
• Should it return the number of bytes that were copied like io.Copy does?