tools/cmd/vet: check only first rune of example suffix

The documentation for the testing package states only that "The suffix
must start with a lower-case letter."

Fixes golang/go#12663

Change-Id: I9b079b105a7c9680325fed442c42adcf3b75055e
Reviewed-on: https://go-review.googlesource.com/14760
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/vet/example.go

@@ -7,6 +7,8 @@ package main
 import (
 	"go/ast"
 	"strings"
+	"unicode"
+	"unicode/utf8"
 
 	"golang.org/x/tools/go/types"
 )
@@ -18,7 +20,10 @@ func init() {
 		funcDecl)
 }
 
-func isExampleSuffix(s string) bool { return strings.ToLower(s) == s }
+func isExampleSuffix(s string) bool {
+	r, size := utf8.DecodeRuneInString(s)
+	return size > 0 && unicode.IsLower(r)
+}
 
 // checkExample walks the documentation example functions checking for common
 // mistakes of misnamed functions, failure to map functions to existing

cmd/vet/testdata/examples_test.go

@@ -17,7 +17,7 @@ var DefaultBuf Buf
 
 func Example() {} // OK because is package-level.
 
-func Example_suffix() // OK because refers to suffix annotation.
+func Example_goodSuffix() // OK because refers to suffix annotation.
 
 func Example_BadSuffix() // ERROR "Example_BadSuffix has malformed example suffix: BadSuffix"