go.json: review and update snippets

[stamblerre]

See microsoft/vscode-go#2380 for the original discussion.

[hyangah]

Facts to consider (from microsoft/vscode-go#2380 (comment))

• Due to the current limitation in VS Code's snippets, it's hard to selectively disable snippets included in an extension.
• The snippets may interfere with the language servers' auto-completion features. Be conservative in accepting any new snippet contribution.
• There is clear overlap with some autocompletion features language servers can provide. So, if gopls can provide a better option, do not add the snippet.
• Encourage to use personal snippets https://code.visualstudio.com/docs/editor/userdefinedsnippets#_create-your-own-snippets instead of adding to the extension's snippet unless the snippet promotes commonly used idiomatic pattern.

Existing snippets

Μaybe

• single import: im -> import "package"
• multiple imports: ims -> import ( "package" )
• single constant: co -> const name = value
• multiple constants: cos -> const ( name = value )
• type interface declaration: tyi -> type name interface { }
• type struct declaration: tys -> type name struct { }
• package main and main function: pkgm -> package main\n\nfunc main() { }
• function declaration: func -> func $1($2) $3 { $0 }
• variable declaration: var -> var $1 $2
• switch statement: switch -> switch $1 { case $2: }
• select statement: sel -> select { case $1: }
• case clause: cs -> case $1:
• for statement: for -> for i := 0; i < count; i++ { }
• for range statement: forr -> for _, var := range var { }
• channel declaration: ch -> chan type
• map declaration: map -> map[type]type
• empty interface: in -> interface{}
• if statement: if -> if condition { }
• else branch: el -> else { }
• if else statement: ie -> if condition { } else { }
• if err != nil: iferr -> if err != nil { return nil, err }
• defer statement: df -> defer $1($0)
• goroutine anonymous function: go -> go func() { }( ) (Snippets: GoRoutine Function removal microsoft/vscode-go#2717 proposes to remove)
• goroutine function: gf -> go $1($0) (Snippets: GoRoutine Function removal microsoft/vscode-go#2717 proposes to remove)

Can gopls's completion do a reasonable job if not better?

• fmt.Println: fp -> fmt.Println("$1")
• fmt.Printf: ff -> fmt.Printf("$1", $2)
• log.Println: lp -> log.Println("$1")
• log.Printf: lf -> log.Printf("$1", $2)
• log variable content: lv -> log.Printf("$1: %#+v\n", $1)
• t.Logf: tl -> t.Log("$1")
• t.Logf: tlf -> t.Logf("$1", $2)
• t.Logf variable content: tlv -> t.Logf("$1: %#+v\n", $1)
• make(...): make -> make($1, $2)
• new(...): new -> new($1)
• panic(...): pn -> panic("")
• method declaration: meth ->func (${1:receiver} ${2:type}) ${3:method}($4) $5 {\n\t$0\n}

HTTP specific shortcut keys, maybe reject?

• http ResponseWriter *Request: wr -> w http.ResponseWriter, r *http.Request,
• http.HandleFunc: hf -> http.HandlerFunc("/", handler)
• http handler declaration: hand -> func (w http.ResponseWriter, r *http.Request) { }
• http.Redirect: rd -> http.Redirect(w, r, "/", https.StatusFound)
• http.Error: herr -> http.Error(w, err.Error(), http.StatusInternalServerError)
• http.ListenAndServe: las ->http.ListenAdnServe(":8080", nil)
• http.Serve: sv -> http.Serve(":8080", nil)

Test specific templates, maybe?

• test function: tf -> func Test$1(t *testing.T) {}"
• benchmark function: bf -> func Benchmark$1(b *testing.B) { for i := 0; i < b.N; i++ { } }
• example function: ef -> func Example$1() {\n\t$2\n\t//Output:\n\t$3\n}
• table driven test: tdt -> func Test$1(t *testing.T) {\n\ttestCases := []struct {\n\t\tdesc\tstring\n\t\t$2\n\t}{\n\t\t{\n\t\t\tdesc: \"$3\",\n\t\t\t$4\n\t\t},\n\t}\n\tfor _, tC := range testCases {\n\t\tt.Run(tC.desc, func(t *testing.T) {\n\t\t\t$0\n\t\t})\n\t}\n}
• init function: finit -> func init() { }
• main function: fmain -> func main() { }

To delete

• hello world web app: helloweb ->package main\n\nimport (\n\t\"fmt\"\n\t\"net/http\"\n\t\"time\"\n)\n\nfunc greet(w http.ResponseWriter, r *http.Request) {\n\tfmt.Fprintf(w, \"Hello World! %s\", time.Now())\n}\n\nfunc main() {\n\thttp.HandleFunc(\"/\", greet)\n\thttp.ListenAndServe(\":8080\", nil)\n}
• sort implementation: sort -> type ${1:SortBy} []${2:Type}\n\nfunc (a $1) Len() int { return len(a) }\nfunc (a $1) Swap(i, j int) { a[i], a[j] = a[j], a[i] }\nfunc (a $1) Less(i, j int) bool { ${3:return a[i] < a[j]} }
  : recent versions of Go have better APIs than this (sort.Sort, ...)

New things to consider

• Add struct and struct field snippets (Add struct and struct field snippets microsoft/vscode-go#2867)
• added snippets for friendly failing test output (added snippets for friendly failing test output microsoft/vscode-go#2616)
• snippets: add friendly failing test output snippet (snippets: add friendly failing test output snippet microsoft/vscode-go#2615)
• Test snippet should support Parallel execution by default (Test snippet should support Parallel execution by default microsoft/vscode-go#2291)
• Contributing to snippets: mutex lock and unlock support (Contributing to snippets: mutex lock and unlock support microsoft/vscode-go#2273)
• Snippet for TestMain (snippets: add snippet for TestMain #629)