cmd/compile: add -url flag, print URL with error messages if applicable

If the -url flag is provided, when encountering a type checking error,
the compiler will also print a URL to a more detailed description of
the error and an example, if available.

Example uses:

        go tool compile -url filename.go
        go build -gcflags=-url pkg/path

For instance, a duplicate declaration of an identifier will report

        https://pkg.go.dev/internal/types/errors#DuplicateDecl

We may refine the provided URL over time.

Change-Id: Iabe3008a49d9dd88bf690f99e4a4a5432dc08786
Reviewed-on: https://go-review.googlesource.com/c/go/+/476716
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Run-TryBot: Robert Griesemer <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: griesemer

src/cmd/compile/internal/base/flag.go

@@ -124,6 +124,7 @@ type CmdFlags struct {
 	TrimPath           string       "help:\"remove `prefix` from recorded source file paths\""
 	WB                 bool         "help:\"enable write barrier\"" // TODO: remove
 	PgoProfile         string       "help:\"read profile from `file`\""
+	Url                bool         "help:\"print explanatory URL with error message if applicable\""
 
 	// Configuration derived from flags; not a flag itself.
 	Cfg struct {

src/cmd/compile/internal/base/print.go

@@ -18,8 +18,9 @@ import (
 
 // An errorMsg is a queued error message, waiting to be printed.
 type errorMsg struct {
-	pos src.XPos
-	msg string
+	pos  src.XPos
+	msg  string
+	code errors.Code
 }
 
 // Pos is the current source position being processed,
@@ -43,16 +44,17 @@ func SyntaxErrors() int {
 }
 
 // addErrorMsg adds a new errorMsg (which may be a warning) to errorMsgs.
-func addErrorMsg(pos src.XPos, format string, args ...interface{}) {
+func addErrorMsg(pos src.XPos, code errors.Code, format string, args ...interface{}) {
 	msg := fmt.Sprintf(format, args...)
 	// Only add the position if know the position.
 	// See issue golang.org/issue/11361.
 	if pos.IsKnown() {
 		msg = fmt.Sprintf("%v: %s", FmtPos(pos), msg)
 	}
 	errorMsgs = append(errorMsgs, errorMsg{
-		pos: pos,
-		msg: msg + "\n",
+		pos:  pos,
+		msg:  msg + "\n",
+		code: code,
 	})
 }
 
@@ -84,6 +86,10 @@ func FlushErrors() {
 	for i, err := range errorMsgs {
 		if i == 0 || err.msg != errorMsgs[i-1].msg {
 			fmt.Printf("%s", err.msg)
+			if Flag.Url && err.code != 0 {
+				// TODO(gri) we should come up with a better URL eventually
+				fmt.Printf("\thttps://pkg.go.dev/internal/types/errors#%s\n", err.code)
+			}
 		}
 	}
 	errorMsgs = errorMsgs[:0]
@@ -133,7 +139,7 @@ func ErrorfAt(pos src.XPos, code errors.Code, format string, args ...interface{}
 		lasterror.msg = msg
 	}
 
-	addErrorMsg(pos, "%s", msg)
+	addErrorMsg(pos, code, "%s", msg)
 	numErrors++
 
 	hcrash()
@@ -175,7 +181,7 @@ func Warn(format string, args ...interface{}) {
 // so this should be used only when the user has opted in
 // to additional output by setting a particular flag.
 func WarnfAt(pos src.XPos, format string, args ...interface{}) {
-	addErrorMsg(pos, format, args...)
+	addErrorMsg(pos, 0, format, args...)
 	if Flag.LowerM != 0 {
 		FlushErrors()
 	}