Can't import gorilla/mux (github.com/gorilla/mux@v1.7.4: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt)

In Go, I was trying simple database connection. I need to import gorilla/mux, but I couldn't.

I am using VS Code. After cd ing to my project directory, I created main.go and did run go get -u github.com/gorilla/mux

Here is main.go

package main

import (
    "database/sql"
    "fmt"
    "github.com/gorilla/mux"
    _ "github.com/lib/pq"
)

const (
    host     = "localhost"
    port     = 3000
    user     = "postgres"
    password = "postgres"
    dbname   = "test1"
)

func main() {
    psqlInfo := fmt.Sprintf("host=%s port=%d user=%s "+
        "password=%s dbname=%s sslmode=disable",
        host, port, user, password, dbname)

    db, err := sql.Open("postgres", psqlInfo)
    if err != nil {
        panic(err)
    }
    defer db.Close()

    err = db.Ping()
    if err != nil {
      panic(err)
    }

    fmt.Println("Successfully connected!")
}

[Note that, after executing go get -u github.com/gorilla/mux terminal shows

C:\Go\src\github.com\IamFrost\go-rest-api>go get -v -u github.com/gorilla/mux
go: golang.org/x/text upgrade => v0.3.2
go: golang.org/x/crypto upgrade => v0.0.0-20200429183012-4b2356b1ed79
go: golang.org/x/sys upgrade => v0.0.0-20200430082407-1f5687305801
go: golang.org/x/net upgrade => v0.0.0-20200425230154-ff2c4b7c35a0
go: downloading golang.org/x/sys v0.0.0-20200430082407-1f5687305801
golang.org/x/sys/cpu
golang.org/x/crypto/chacha20poly1305
crypto/tls

]

Look i have no other syntax error in mini map. At red mark, when I put mouse, the hover text, its funny to me:

1)imported but not used

but the next line

2)no package for import github.com/gorilla/mux)

lol didn't that go against 1) ?

Someone please explain why that occurs

However,

After using go build in terminal

Here is terminal:

C:\Go\src\github.com\IamFrost\go-rest-api>go build
go: inconsistent vendoring in C:\Go\src:
        github.com/gorilla/mux@v1.7.4: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
        github.com/lib/pq@v1.4.0: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
        golang.org/x/crypto@v0.0.0-20200429183012-4b2356b1ed79: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
        golang.org/x/net@v0.0.0-20200425230154-ff2c4b7c35a0: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
        golang.org/x/sys@v0.0.0-20200430082407-1f5687305801: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
        golang.org/x/crypto@v0.0.0-20200128174031-69ecbb4d6d5d: is marked as explicit in vendor/modules.txt, but not explicitly required in go.mod
        golang.org/x/net@v0.0.0-20191126235420-ef20fe5d7933: is marked as explicit in vendor/modules.txt, but not explicitly required in go.mod
        golang.org/x/sys@v0.0.0-20200201011859-915c9c3d4ccf: is marked as explicit in vendor/modules.txt, but not explicitly required in go.mod

run 'go mod vendor' to sync, or use -mod=mod or -mod=readonly to ignore the vendor directory

[Note : I did 'go mod vendor' also, but no change]

So someone point me why I can't import gorilla/mux or pq.

What else I have to do?

(and please explain what does that mean? is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt)

I don't know why it works (I am beginner). I found something in net, somehow it worked . Here is how :

1. This is my go directory C:\Go\ . But I create the project in my desktop folder

2. Then I recreated the project under C:\Go\src (this is exact folder C:\Go\src\github.com\[github_username]\[project_name] )

3. Then I pasted all the code at main.go

4. Finally from terminal go mod vendor

5. Then go build

6. Then go run main.go

7. Finally it worked

[I would like to know why it worked. I would really appreciate your comment : about why they are forcing me to go under C:\Go\src. I already have GOPATH C:\Go\ , GOBIN C:\Go\bin set in environment variable]