bytes, strings: add TrimPrefix and TrimSuffix

Everybody either gets confused and thinks this is
TrimLeft/TrimRight or does this by hand which gets
repetitive looking.

R=rsc, kevlar
CC=golang-dev
https://golang.org/cl/7239044

10 files changed, 13 insertions, 31 deletions

diff --git a/src/cmd/api/goapi.go b/src/cmd/api/goapi.go
index 906cd3a68d..0d76b0cdb6 100644
--- a/src/cmd/api/goapi.go
+++ b/src/cmd/api/goapi.go
@@ -778,8 +778,7 @@ func (w *Walker) walkConst(vs *ast.ValueSpec) {
 				}
 			}
 		}
-		if strings.HasPrefix(litType, constDepPrefix) {
-			dep := litType[len(constDepPrefix):]
+		if dep := strings.TrimPrefix(litType, constDepPrefix); dep != litType {
 			w.constDep[ident.Name] = dep
 			continue
 		}
diff --git a/src/cmd/cgo/gcc.go b/src/cmd/cgo/gcc.go
index 3266abe618..1449a8d6b5 100644
--- a/src/cmd/cgo/gcc.go
+++ b/src/cmd/cgo/gcc.go
@@ -1542,8 +1542,8 @@ func godefsFields(fld []*ast.Field) {
 	npad := 0
 	for _, f := range fld {
 		for _, n := range f.Names {
-			if strings.HasPrefix(n.Name, prefix) && n.Name != prefix {
-				n.Name = n.Name[len(prefix):]
+			if n.Name != prefix {
+				n.Name = strings.TrimPrefix(n.Name, prefix)
 			}
 			if n.Name == "_" {
 				// Use exported name instead.
diff --git a/src/cmd/cgo/godefs.go b/src/cmd/cgo/godefs.go
index fec70a334b..20376170da 100644
--- a/src/cmd/cgo/godefs.go
+++ b/src/cmd/cgo/godefs.go
@@ -180,7 +180,7 @@ func (p *Package) cdefs(f *File, srcfile string) string {
 			for _, line := range lines {
 				line = strings.TrimSpace(line)
 				if strings.HasPrefix(line, "type ") && strings.HasSuffix(line, " struct {") {
-					s := line[len("type ") : len(line)-len(" struct {")]
+					s := strings.TrimSuffix(strings.TrimPrefix(line, "type "), " struct {")
 					printf("typedef struct %s %s;\n", s, s)
 				}
 			}
diff --git a/src/cmd/fix/typecheck.go b/src/cmd/fix/typecheck.go
index d54d375478..d33b69fddc 100644
--- a/src/cmd/fix/typecheck.go
+++ b/src/cmd/fix/typecheck.go
@@ -395,9 +395,7 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string, a
 			// Field or method.
 			name := n.Sel.Name
 			if t := typeof[n.X]; t != "" {
-				if strings.HasPrefix(t, "*") {
-					t = t[1:] // implicit *
-				}
+				t = strings.TrimPrefix(t, "*") // implicit *
 				if typ := cfg.Type[t]; typ != nil {
 					if t := typ.dot(cfg, name); t != "" {
 						typeof[n] = t
diff --git a/src/cmd/go/testflag.go b/src/cmd/go/testflag.go
index 8dd51437d7..b2ca66b094 100644
--- a/src/cmd/go/testflag.go
+++ b/src/cmd/go/testflag.go
@@ -195,9 +195,7 @@ func testFlag(args []string, i int) (f *testFlagSpec, value string, extra bool)
 	}
 	name := arg[1:]
 	// If there's already "test.", drop it for now.
-	if strings.HasPrefix(name, "test.") {
-		name = name[5:]
-	}
+	name = strings.TrimPrefix(name, "test.")
 	equals := strings.Index(name, "=")
 	if equals >= 0 {
 		value = name[equals+1:]
diff --git a/src/cmd/godoc/dirtrees.go b/src/cmd/godoc/dirtrees.go
index 29bd39e6b0..08dbfc2e8b 100644
--- a/src/cmd/godoc/dirtrees.go
+++ b/src/cmd/godoc/dirtrees.go
@@ -229,9 +229,7 @@ func (dir *Directory) lookupLocal(name string) *Directory {
 }
 
 func splitPath(p string) []string {
-	if strings.HasPrefix(p, "/") {
-		p = p[1:]
-	}
+	p = strings.TrimPrefix(p, "/")
 	if p == "" {
 		return nil
 	}
@@ -310,14 +308,9 @@ func (root *Directory) listing(skipRoot bool) *DirList {
 		// the path is relative to root.Path - remove the root.Path
 		// prefix (the prefix should always be present but avoid
 		// crashes and check)
-		path := d.Path
-		if strings.HasPrefix(d.Path, root.Path) {
-			path = d.Path[len(root.Path):]
-		}
+		path := strings.TrimPrefix(d.Path, root.Path)
 		// remove leading separator if any - path must be relative
-		if len(path) > 0 && path[0] == '/' {
-			path = path[1:]
-		}
+		path = strings.TrimPrefix(path, "/")
 		p.Path = path
 		p.Name = d.Name
 		p.HasPkg = d.HasPkg
diff --git a/src/cmd/godoc/filesystem.go b/src/cmd/godoc/filesystem.go
index c4afbed800..0309d7cabe 100644
--- a/src/cmd/godoc/filesystem.go
+++ b/src/cmd/godoc/filesystem.go
@@ -459,9 +459,7 @@ func (ns nameSpace) ReadDir(path string) ([]os.FileInfo, error) {
 		if hasPathPrefix(old, path) && old != path {
 			// Find next element after path in old.
 			elem := old[len(path):]
-			if strings.HasPrefix(elem, "/") {
-				elem = elem[1:]
-			}
+			elem = strings.TrimPrefix(elem, "/")
 			if i := strings.Index(elem, "/"); i >= 0 {
 				elem = elem[:i]
 			}
diff --git a/src/cmd/godoc/godoc.go b/src/cmd/godoc/godoc.go
index 4d66c3011c..887480911a 100644
--- a/src/cmd/godoc/godoc.go
+++ b/src/cmd/godoc/godoc.go
@@ -419,9 +419,7 @@ func pkgLinkFunc(path string) string {
 	relpath := path[1:]
 	// because of the irregular mapping under goroot
 	// we need to correct certain relative paths
-	if strings.HasPrefix(relpath, "src/pkg/") {
-		relpath = relpath[len("src/pkg/"):]
-	}
+	relpath = strings.TrimPrefix(relpath, "src/pkg/")
 	return pkgHandler.pattern[1:] + relpath // remove trailing '/' for relative URL
 }
 
diff --git a/src/cmd/godoc/main.go b/src/cmd/godoc/main.go
index 02891444b0..1344100906 100644
--- a/src/cmd/godoc/main.go
+++ b/src/cmd/godoc/main.go
@@ -347,7 +347,7 @@ func main() {
 		fs.Bind(target, OS(path), "/", bindReplace)
 		abspath = target
 	} else if strings.HasPrefix(path, cmdPrefix) {
-		path = path[len(cmdPrefix):]
+		path = strings.TrimPrefix(path, cmdPrefix)
 		forceCmd = true
 	} else if bp, _ := build.Import(path, "", build.FindOnly); bp.Dir != "" && bp.ImportPath != "" {
 		fs.Bind(target, OS(bp.Dir), "/", bindReplace)
diff --git a/src/cmd/vet/method.go b/src/cmd/vet/method.go
index 41df96cec5..dcfa8a02f3 100644
--- a/src/cmd/vet/method.go
+++ b/src/cmd/vet/method.go
@@ -90,9 +90,7 @@ func (f *File) checkCanonicalMethod(id *ast.Ident, t *ast.FuncType) {
 			fmt.Fprintf(&f.b, "<%s>", err)
 		}
 		actual := f.b.String()
-		if strings.HasPrefix(actual, "func(") {
-			actual = actual[4:]
-		}
+		actual = strings.TrimPrefix(actual, "func(")
 		actual = id.Name + actual
 
 		f.Warnf(id.Pos(), "method %s should have signature %s", actual, expectFmt)