Turn "unused" errors into warnings

src/cmd/compile/internal/gc/main.go

@@ -1352,9 +1352,9 @@ func pkgnotused(lineno src.XPos, path string, name string) {
 		elem = elem[i+1:]
 	}
 	if name == "" || elem == name {
-		yyerrorl(lineno, "imported and not used: %q", path)
+		yywarningl(lineno, "imported and not used: %q", path)
 	} else {
-		yyerrorl(lineno, "imported and not used: %q as %s", path, name)
+		yywarningl(lineno, "imported and not used: %q as %s", path, name)
 	}
 }
 

src/cmd/compile/internal/gc/subr.go

@@ -26,6 +26,7 @@ type Error struct {
 	msg string
 }
 
+var warnings []Error
 var errors []Error
 
 // largeStack is info about a function whose stack frame is too large (rare).
@@ -59,6 +60,13 @@ func adderrorname(n *Node) {
 	}
 }
 
+func addwarn(pos src.XPos, format string, args ...interface{}) {
+	warnings = append(warnings, Error{
+			pos: pos,
+			msg: fmt.Sprintf("%v: warning: %s\n", linestr(pos), fmt.Sprintf(format, args...)),
+		})
+}
+
 func adderr(pos src.XPos, format string, args ...interface{}) {
 	msg := fmt.Sprintf(format, args...)
 	// Only add the position if know the position.
@@ -95,6 +103,22 @@ func flusherrors() {
 	errors = errors[:0]
 }
 
+// flushwarnings sorts warnings seen so far by line number, prints them to stdout,
+// and empties the warnings array.
+func flushwarnings() {
+	Ctxt.Bso.Flush()
+	if len(warnings) == 0 {
+			return
+		}
+	sort.Stable(byPos(warnings))
+	for i, err := range warnings {
+			if i == 0 || err.msg != warnings[i-1].msg {
+					fmt.Printf("%s", err.msg)
+				}
+		}
+	warnings = warnings[:0]
+}
+
 func hcrash() {
 	if Debug.h != 0 {
 		flusherrors()
@@ -119,13 +143,59 @@ var lasterror struct {
 	msg    string   // error message of last non-syntax error
 }
 
+// lastwarning keeps track of the most recently issued warning.
+// It is used to avoid multiple warning messages on the same
+// line.
+var lastwarning struct {
+	syntax src.XPos // source position of last syntax warning
+	other  src.XPos // source position of last non-syntax warning
+	msg    string   // warning message of last non-syntax warning
+}
+
 // sameline reports whether two positions a, b are on the same line.
 func sameline(a, b src.XPos) bool {
 	p := Ctxt.PosTable.Pos(a)
 	q := Ctxt.PosTable.Pos(b)
 	return p.Base() == q.Base() && p.Line() == q.Line()
 }
 
+func yywarningl(pos src.XPos, format string, args ...interface{}) {
+	msg := fmt.Sprintf(format, args...)
+
+	if strings.HasPrefix(msg, "syntax error") {
+		// only one syntax error per line, no matter what error
+			if sameline(lastwarning.syntax, pos) {
+				return
+			}
+		lastwarning.syntax = pos
+	} else {
+		// only one of multiple equal non-syntax errors per line
+		// (flusherrors shows only one of them, so we filter them
+		// here as best as we can (they may not appear in order)
+		// so that we don't count them here and exit early, and
+		// then have nothing to show for.)
+		if sameline(lastwarning.other, pos) && lastwarning.msg == msg {
+			return
+		}
+		lastwarning.other = pos
+		lastwarning.msg = msg
+	}
+
+	addwarn(pos, "%s", msg)
+
+	hcrash()
+	flushwarnings()
+}
+
+func yywarningv(lang string, format string, args ...interface{}) {
+	what := fmt.Sprintf(format, args...)
+	yywarningl(lineno, "%s requires %s or later (-lang was set to %s; check go.mod)", what, lang, flag_lang)
+}
+
+func yywarning(format string, args ...interface{}) {
+	yywarningl(lineno, format, args...)
+}
+
 func yyerrorl(pos src.XPos, format string, args ...interface{}) {
 	msg := fmt.Sprintf(format, args...)
 
@@ -302,7 +372,7 @@ func importdot(opkg *types.Pkg, pack *Node) {
 
 	if n == 0 {
 		// can't possibly be used - there were no symbols
-		yyerrorl(pack.Pos, "imported and not used: %q", opkg.Path)
+		yywarningl(pack.Pos, "imported and not used: %q", opkg.Path)
 	}
 }
 

src/cmd/compile/internal/gc/swt.go

@@ -32,7 +32,7 @@ func typecheckTypeSwitch(n *Node) {
 	// declaration itself. So if there are no cases, we won't
 	// notice that it went unused.
 	if v := n.Left.Left; v != nil && !v.isBlank() && n.List.Len() == 0 {
-		yyerrorl(v.Pos, "%v declared but not used", v.Sym)
+		yywarningl(v.Pos, "%v declared but not used", v.Sym)
 	}
 
 	var defCase, nilCase *Node

src/cmd/compile/internal/gc/walk.go

@@ -51,10 +51,10 @@ func walk(fn *Node) {
 			if defn.Left.Name.Used() {
 				continue
 			}
-			yyerrorl(defn.Left.Pos, "%v declared but not used", ln.Sym)
+			yywarningl(defn.Left.Pos, "%v declared but not used", ln.Sym)
 			defn.Left.Name.SetUsed(true) // suppress repeats
 		} else {
-			yyerrorl(ln.Pos, "%v declared but not used", ln.Sym)
+			yywarningl(ln.Pos, "%v declared but not used", ln.Sym)
 		}
 	}