func (s *IssuesService) ListMergeRequestsRelatedToIssue(pid interface{}, issue int, opt *ListMergeRequestsRelatedToIssueOptions, options ...OptionFunc) ([]*MergeRequest, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("/projects/%s/issues/%d/related_merge_requests",
		pathEscape(project),
		issue,
	)
	req, err := s.client.NewRequest("GET", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var m []*MergeRequest
	resp, err := s.client.Do(req, &m)
	if err != nil {
		return nil, resp, err
	}
	return m, resp, err
}
ListMergeRequestsRelatedToIssue gets all the merge requests that are
related to the issue

GitLab API docs:
https://docs.gitlab.com/ce/api/issues.html#list-merge-requests-related-to-issue
package gitlab
import (
	"encoding/json"
	"fmt"
	"strings"
	"time"
)
type IssuesService struct {
	client    *Client
	timeStats *timeStatsService
}
type IssueAuthor struct {
	ID        int    `json:"id"`
	State     string `json:"state"`
	WebURL    string `json:"web_url"`
	Name      string `json:"name"`
	AvatarURL string `json:"avatar_url"`
	Username  string `json:"username"`
}
type IssueAssignee struct {
	ID        int    `json:"id"`
	State     string `json:"state"`
	WebURL    string `json:"web_url"`
	Name      string `json:"name"`
	AvatarURL string `json:"avatar_url"`
	Username  string `json:"username"`
}
type IssueLinks struct {
	Self       string `json:"self"`
	Notes      string `json:"notes"`
	AwardEmoji string `json:"award_emoji"`
	Project    string `json:"project"`
}
type Issue struct {
	ID                int              `json:"id"`
	IID               int              `json:"iid"`
	ProjectID         int              `json:"project_id"`
	Milestone         *Milestone       `json:"milestone"`
	Author            *IssueAuthor     `json:"author"`
	Description       string           `json:"description"`
	State             string           `json:"state"`
	Assignees         []*IssueAssignee `json:"assignees"`
	Assignee          *IssueAssignee   `json:"assignee"`
	Upvotes           int              `json:"upvotes"`
	Downvotes         int              `json:"downvotes"`
	Labels            []string         `json:"labels"`
	Title             string           `json:"title"`
	UpdatedAt         *time.Time       `json:"updated_at"`
	CreatedAt         *time.Time       `json:"created_at"`
	ClosedAt          *time.Time       `json:"closed_at"`
	Subscribed        bool             `json:"subscribed"`
	UserNotesCount    int              `json:"user_notes_count"`
	DueDate           *ISOTime         `json:"due_date"`
	WebURL            string           `json:"web_url"`
	TimeStats         *TimeStats       `json:"time_stats"`
	Confidential      bool             `json:"confidential"`
	Weight            int              `json:"weight"`
	DiscussionLocked  bool             `json:"discussion_locked"`
	Links             *IssueLinks      `json:"_links"`
	IssueLinkID       int              `json:"issue_link_id"`
	MergeRequestCount int              `json:"merge_requests_count"`
}
func (i Issue) String() string {
	return Stringify(i)
}
type Labels []string
func (l *Labels) MarshalJSON() ([]byte, error) {
	return json.Marshal(strings.Join(*l, ","))
}
type ListIssuesOptions struct {
	ListOptions
	State           *string    `url:"state,omitempty" json:"state,omitempty"`
	Labels          Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	Milestone       *string    `url:"milestone,omitempty" json:"milestone,omitempty"`
	Scope           *string    `url:"scope,omitempty" json:"scope,omitempty"`
	AuthorID        *int       `url:"author_id,omitempty" json:"author_id,omitempty"`
	AssigneeID      *int       `url:"assignee_id,omitempty" json:"assignee_id,omitempty"`
	MyReactionEmoji *string    `url:"my_reaction_emoji,omitempty" json:"my_reaction_emoji,omitempty"`
	IIDs            []int      `url:"iids[],omitempty" json:"iids,omitempty"`
	OrderBy         *string    `url:"order_by,omitempty" json:"order_by,omitempty"`
	Sort            *string    `url:"sort,omitempty" json:"sort,omitempty"`
	Search          *string    `url:"search,omitempty" json:"search,omitempty"`
	CreatedAfter    *time.Time `url:"created_after,omitempty" json:"created_after,omitempty"`
	CreatedBefore   *time.Time `url:"created_before,omitempty" json:"created_before,omitempty"`
	UpdatedAfter    *time.Time `url:"updated_after,omitempty" json:"updated_after,omitempty"`
	UpdatedBefore   *time.Time `url:"updated_before,omitempty" json:"updated_before,omitempty"`
}
func (s *IssuesService) ListIssues(opt *ListIssuesOptions, options ...OptionFunc) ([]*Issue, *Response, error) {
	req, err := s.client.NewRequest("GET", "issues", opt, options)
	if err != nil {
		return nil, nil, err
	}
	var i []*Issue
	resp, err := s.client.Do(req, &i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type ListGroupIssuesOptions struct {
	ListOptions
	State           *string    `url:"state,omitempty" json:"state,omitempty"`
	Labels          Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	IIDs            []int      `url:"iids[],omitempty" json:"iids,omitempty"`
	Milestone       *string    `url:"milestone,omitempty" json:"milestone,omitempty"`
	Scope           *string    `url:"scope,omitempty" json:"scope,omitempty"`
	AuthorID        *int       `url:"author_id,omitempty" json:"author_id,omitempty"`
	AssigneeID      *int       `url:"assignee_id,omitempty" json:"assignee_id,omitempty"`
	MyReactionEmoji *string    `url:"my_reaction_emoji,omitempty" json:"my_reaction_emoji,omitempty"`
	OrderBy         *string    `url:"order_by,omitempty" json:"order_by,omitempty"`
	Sort            *string    `url:"sort,omitempty" json:"sort,omitempty"`
	Search          *string    `url:"search,omitempty" json:"search,omitempty"`
	CreatedAfter    *time.Time `url:"created_after,omitempty" json:"created_after,omitempty"`
	CreatedBefore   *time.Time `url:"created_before,omitempty" json:"created_before,omitempty"`
	UpdatedAfter    *time.Time `url:"updated_after,omitempty" json:"updated_after,omitempty"`
	UpdatedBefore   *time.Time `url:"updated_before,omitempty" json:"updated_before,omitempty"`
}
func (s *IssuesService) ListGroupIssues(pid interface{}, opt *ListGroupIssuesOptions, options ...OptionFunc) ([]*Issue, *Response, error) {
	group, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("groups/%s/issues", pathEscape(group))
	req, err := s.client.NewRequest("GET", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var i []*Issue
	resp, err := s.client.Do(req, &i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type ListProjectIssuesOptions struct {
	ListOptions
	IIDs            []int      `url:"iids[],omitempty" json:"iids,omitempty"`
	State           *string    `url:"state,omitempty" json:"state,omitempty"`
	Labels          Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	Milestone       *string    `url:"milestone,omitempty" json:"milestone,omitempty"`
	Scope           *string    `url:"scope,omitempty" json:"scope,omitempty"`
	AuthorID        *int       `url:"author_id,omitempty" json:"author_id,omitempty"`
	AssigneeID      *int       `url:"assignee_id,omitempty" json:"assignee_id,omitempty"`
	MyReactionEmoji *string    `url:"my_reaction_emoji,omitempty" json:"my_reaction_emoji,omitempty"`
	OrderBy         *string    `url:"order_by,omitempty" json:"order_by,omitempty"`
	Sort            *string    `url:"sort,omitempty" json:"sort,omitempty"`
	Search          *string    `url:"search,omitempty" json:"search,omitempty"`
	CreatedAfter    *time.Time `url:"created_after,omitempty" json:"created_after,omitempty"`
	CreatedBefore   *time.Time `url:"created_before,omitempty" json:"created_before,omitempty"`
	UpdatedAfter    *time.Time `url:"updated_after,omitempty" json:"updated_after,omitempty"`
	UpdatedBefore   *time.Time `url:"updated_before,omitempty" json:"updated_before,omitempty"`
}
func (s *IssuesService) ListProjectIssues(pid interface{}, opt *ListProjectIssuesOptions, options ...OptionFunc) ([]*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues", pathEscape(project))
	req, err := s.client.NewRequest("GET", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var i []*Issue
	resp, err := s.client.Do(req, &i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
func (s *IssuesService) GetIssue(pid interface{}, issue int, options ...OptionFunc) (*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues/%d", pathEscape(project), issue)
	req, err := s.client.NewRequest("GET", u, nil, options)
	if err != nil {
		return nil, nil, err
	}
	i := new(Issue)
	resp, err := s.client.Do(req, i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type CreateIssueOptions struct {
	Title                              *string    `url:"title,omitempty" json:"title,omitempty"`
	Description                        *string    `url:"description,omitempty" json:"description,omitempty"`
	Confidential                       *bool      `url:"confidential,omitempty" json:"confidential,omitempty"`
	AssigneeIDs                        []int      `url:"assignee_ids,omitempty" json:"assignee_ids,omitempty"`
	MilestoneID                        *int       `url:"milestone_id,omitempty" json:"milestone_id,omitempty"`
	Labels                             Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	CreatedAt                          *time.Time `url:"created_at,omitempty" json:"created_at,omitempty"`
	DueDate                            *ISOTime   `url:"due_date,omitempty" json:"due_date,omitempty"`
	MergeRequestToResolveDiscussionsOf *int       `url:"merge_request_to_resolve_discussions_of,omitempty" json:"merge_request_to_resolve_discussions_of,omitempty"`
	DiscussionToResolve                *string    `url:"discussion_to_resolve,omitempty" json:"discussion_to_resolve,omitempty"`
	Weight                             *int       `url:"weight,omitempty" json:"weight,omitempty"`
}
func (s *IssuesService) CreateIssue(pid interface{}, opt *CreateIssueOptions, options ...OptionFunc) (*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues", pathEscape(project))
	req, err := s.client.NewRequest("POST", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	i := new(Issue)
	resp, err := s.client.Do(req, i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type UpdateIssueOptions struct {
	Title            *string    `url:"title,omitempty" json:"title,omitempty"`
	Description      *string    `url:"description,omitempty" json:"description,omitempty"`
	Confidential     *bool      `url:"confidential,omitempty" json:"confidential,omitempty"`
	AssigneeIDs      []int      `url:"assignee_ids,omitempty" json:"assignee_ids,omitempty"`
	MilestoneID      *int       `url:"milestone_id,omitempty" json:"milestone_id,omitempty"`
	Labels           Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	StateEvent       *string    `url:"state_event,omitempty" json:"state_event,omitempty"`
	UpdatedAt        *time.Time `url:"updated_at,omitempty" json:"updated_at,omitempty"`
	DueDate          *ISOTime   `url:"due_date,omitempty" json:"due_date,omitempty"`
	Weight           *int       `url:"weight,omitempty" json:"weight,omitempty"`
	DiscussionLocked *bool      `url:"discussion_locked,omitempty" json:"discussion_locked,omitempty"`
}
func (s *IssuesService) UpdateIssue(pid interface{}, issue int, opt *UpdateIssueOptions, options ...OptionFunc) (*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues/%d", pathEscape(project), issue)
	req, err := s.client.NewRequest("PUT", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	i := new(Issue)
	resp, err := s.client.Do(req, i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
func (s *IssuesService) DeleteIssue(pid interface{}, issue int, options ...OptionFunc) (*Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, err
	}
	u := fmt.Sprintf("projects/%s/issues/%d", pathEscape(project), issue)
	req, err := s.client.NewRequest("DELETE", u, nil, options)
	if err != nil {
		return nil, err
	}
	return s.client.Do(req, nil)
}
func (s *IssuesService) SubscribeToIssue(pid interface{}, issue int, options ...OptionFunc) (*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues/%d/subscribe", pathEscape(project), issue)
	req, err := s.client.NewRequest("POST", u, nil, options)
	if err != nil {
		return nil, nil, err
	}
	i := new(Issue)
	resp, err := s.client.Do(req, i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
func (s *IssuesService) UnsubscribeFromIssue(pid interface{}, issue int, options ...OptionFunc) (*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues/%d/unsubscribe", pathEscape(project), issue)
	req, err := s.client.NewRequest("POST", u, nil, options)
	if err != nil {
		return nil, nil, err
	}
	i := new(Issue)
	resp, err := s.client.Do(req, i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type ListMergeRequestsClosingIssueOptions ListOptions
func (s *IssuesService) ListMergeRequestsClosingIssue(pid interface{}, issue int, opt *ListMergeRequestsClosingIssueOptions, options ...OptionFunc) ([]*MergeRequest, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("/projects/%s/issues/%d/closed_by", pathEscape(project), issue)
	req, err := s.client.NewRequest("GET", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var m []*MergeRequest
	resp, err := s.client.Do(req, &m)
	if err != nil {
		return nil, resp, err
	}
	return m, resp, err
}
type ListMergeRequestsRelatedToIssueOptions ListOptions