exp/template/html: moved error docs out of package docs onto error codes

This replaces the errStr & errLine members of context with a single err
*Error, and introduces a number of const error codes, one per
escape-time failure mode, that can be separately documented.

The changes to the error documentation moved from doc.go to error.go
are cosmetic.

R=r, nigeltao
CC=golang-dev
https://golang.org/cl/5026041

7 files changed, 235 insertions, 218 deletions

diff --git a/src/pkg/exp/template/html/Makefile b/src/pkg/exp/template/html/Makefile
index e53270c9c8..9032aead87 100644
--- a/src/pkg/exp/template/html/Makefile
+++ b/src/pkg/exp/template/html/Makefile
@@ -11,6 +11,7 @@ GOFILES=\
 	context.go\
 	css.go\
 	doc.go\
+	error.go\
 	escape.go\
 	html.go\
 	js.go\
diff --git a/src/pkg/exp/template/html/context.go b/src/pkg/exp/template/html/context.go
index bfe168f646..e8812cf865 100644
--- a/src/pkg/exp/template/html/context.go
+++ b/src/pkg/exp/template/html/context.go
@@ -21,8 +21,7 @@ type context struct {
 	urlPart urlPart
 	jsCtx   jsCtx
 	element element
-	errLine int
-	errStr  string
+	err     *Error
 }
 
 // eq returns whether two contexts are equal.
@@ -32,8 +31,7 @@ func (c context) eq(d context) bool {
 		c.urlPart == d.urlPart &&
 		c.jsCtx == d.jsCtx &&
 		c.element == d.element &&
-		c.errLine == d.errLine &&
-		c.errStr == d.errStr
+		c.err == d.err
 }
 
 // mangle produces an identifier that includes a suffix that distinguishes it
diff --git a/src/pkg/exp/template/html/doc.go b/src/pkg/exp/template/html/doc.go
index 2751ce834b..a9b78ca515 100644
--- a/src/pkg/exp/template/html/doc.go
+++ b/src/pkg/exp/template/html/doc.go
@@ -69,178 +69,8 @@ in this case,
 
 Errors
 
-This section describes the errors returned by EscapeSet. Each error is
-illustrated by an example that triggers the error, followed by an explanation
-of the problem.
-
-Error: "... appears in an ambiguous URL context"
-Example:
-  <a href="
-     {{if .C}}
-       /path/
-     {{else}}
-       /search?q=
-     {{end}}
-     {{.X}}
-  ">
-Discussion:
-  {{.X}} is in an ambiguous URL context since, depending on {{.C}}, it may be
-  either a URL suffix or a query parameter.
-  Moving {{.X}} into the condition removes the ambiguity:
-  <a href="{{if .C}}/path/{{.X}}{{else}}/search?q={{.X}}">
-
-
-Error: "... appears inside a comment"
-Example:
-*/
-//  <!-- {{.X}} -->
-//  <script>/* {{.X}} */</script>
-//  <style>/* {{.X}} */</style>
-/*
-Discussion:
-  {{.X}} appears inside a comment. There is no escaping convention for
-  comments. To use IE conditional comments, inject the
-  whole comment as a type string (see below).
-  To comment out code, break the {{...}}.
+See the documentation of ErrorCode for details.
 
-Error: "{{if}} branches end in different contexts"
-Example:
-  {{if .C}}<a href="{{end}}{{.X}}
-Discussion:
-  EscapeSet statically examines each possible path when it encounters a {{if}},
-  {{range}}, or {{with}} to escape any following pipelines. The example is
-  ambiguous since {{.X}} might be an HTML text node, or a URL prefix in an
-  HTML attribute. EscapeSet needs to understand the context of {{.X}} to escape
-  it, but that depends on the run-time value of {{.C}}.
-
-  The problem is usually something like missing quotes or angle brackets, or
-  can be avoided by refactoring to put the two contexts into different
-  branches of an if, range or with. Adding an {{else}} might help.
-
-  First, look for a bug in your template. Missing quotes or '>' can trigger
-  this error.
-
-     {{if .C}}<div ... class="foo>{{end}}  <- No quote after foo
-
-  Second, try refactoring your template.
-
-     {{if .C}}<script>alert({{end}}{{.X}}{{if .C}})</script>{{end}}
-
-     ->
-
-     {{if .C}}<script>alert({{.X}})</script>{{else}}{{.X}}{{end}}
-
-  Third, check for {{range}}s that have no {{else}}
-
-    <a href="/search
-      {{range $i, $v := .}}
-        {{if $i}}&{{else}}?{{end}}
-        v={{$v}}
-      {{end}}
-      &x={{.X}}
-      ">
-
-  looks good, but if {{.}} is empty then the URL is /search&x=...
-  where {{.X}} is not guaranteed to be in a URL query.
-  EscapeSet cannot prove which {{range}} collections are never non-empty, so
-  add an {{else}}
-
-    <a href="{{range ...}}...{{end}}&x={{X}}">
-
-    ->
-
-    <a href="{{range ...}}...{{else}}?{{end}}&x={{.X}}">
-
-  Fourth, contact the mailing list. You may have a useful pattern that
-  EscapeSet does not yet support, and we can work with you.
-
-
-Error: "... ends in a non-text context: ..."
-Examples:
-  <div
-  <div title="no close quote>
-  <script>f()
-Discussion:
-  EscapeSet assumes the ouput is a DocumentFragment of HTML.
-  Templates that end without closing tags will trigger this warning.
-  Templates that produce incomplete Fragments should not be named
-  in the call to EscapeSet.
-
-
-If you have a helper template in your set that is not meant to produce a
-  document fragment, then do not pass its name to EscapeSet(set, ...names).
-
-    {{define "main"}} <script>{{template "helper"}}</script> {{end}}
-    {{define "helper"}} document.write(' <div title=" ') {{end}}
-
-  "helper" does not produce a valid document fragment, though it does
-  produce a valid JavaScript Program.
-
-"must specify names of top level templates"
-
-  EscapeSet does not assume that all templates in a set produce HTML.
-  Some may be helpers that produce snippets of other languages.
-  Passing in no template names is most likely an error, so EscapeSet(set) will
-  panic.
-  If you call EscapeSet with a slice of names, guard it with a len check:
-
-    if len(names) != 0 {
-      set, err := EscapeSet(set, ...names)
-    }
-
-Error: "no such template ..."
-Examples:
-   {{define "main"}}<div {{template "attrs"}}>{{end}}
-   {{define "attrs"}}href="{{.URL}}"{{end}}
-Discussion:
-  EscapeSet looks through template calls to compute the context.
-  Here the {{.URL}} in "attrs" must be treated as a URL when called from "main",
-  but if "attrs" is not in set when EscapeSet(&set, "main") is called, this
-  error will arise.
-
-Error: "on range loop re-entry: ..."
-Example:
-  {{range .}}<p class={{.}}{{end}}
-Discussion:
-  If an iteration through a range would cause it to end in
-  a different context than an earlier pass, there is no single context.
-  In the example, the <p> tag is missing a '>'.
-  EscapeSet cannot tell whether {{.}} is meant to be an HTML class or the
-  content of a broken <p> element and complains because the second iteration
-  would produce something like
-
-    <p class=foo<p class=bar
-
-Error: "unfinished escape sequence in ..."
-Example:
-  <script>alert("\{{.X}}")</script>
-Discussion:
-  EscapeSet does not support actions following a backslash.
-  This is usually an error and there are better solutions; for
-  our example
-    <script>alert("{{.X}}")</script>
-  should work, and if {{.X}} is a partial escape sequence such as
-  "xA0", give it the type ContentTypeJSStr and include the whole
-  sequence, as in
-    {`\xA0`, ContentTypeJSStr}
-
-Error: "unfinished JS regexp charset in ..."
-Example:
-    <script>var pattern = /foo[{{.Chars}}]/</script>
-Discussion:
-  EscapeSet does not support interpolation into regular expression literal
-  character sets.
-
-Error: "ZgotmplZ"
-Example:
-  <img src="{{.X}}">
-  where {{.X}} evaluates to `javascript:...`
-Discussion:
-  "ZgotmplZ" is a special value that indicates that unsafe content reached
-  a CSS or URL context at runtime. The output of the example will be
-    <img src="#ZgotmplZ">
-  If the data can be trusted, giving the string type XXX will exempt
-  it from filtering.
 
 A fuller picture
 
@@ -249,8 +79,6 @@ details necessary to understand escaping contexts and error messages. Most users
 will not need to understand these details.
 
 
-
-
 Contexts
 
 Assuming {{.}} is `O'Reilly: How are <i>you</i>?`, the table below shows
diff --git a/src/pkg/exp/template/html/error.go b/src/pkg/exp/template/html/error.go
new file mode 100644
index 0000000000..5fa2357433
--- /dev/null
+++ b/src/pkg/exp/template/html/error.go
@@ -0,0 +1,194 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package html
+
+import (
+	"fmt"
+)
+
+// Error describes a problem encountered during template Escaping.
+type Error struct {
+	// ErrorCode describes the kind of error.
+	ErrorCode ErrorCode
+	// Name is the name of the template in which the error was encountered.
+	Name string
+	// Line is the line number of the error in the template source or 0.
+	Line int
+	// Description is a human-readable description of the problem.
+	Description string
+}
+
+// ErrorCode is a code for a kind of error.
+type ErrorCode int
+
+// We define codes for each error that manifests while escaping templates, but
+// escaped templates may also fail at runtime.
+//
+// Output: "ZgotmplZ"
+// Example:
+//   <img src="{{.X}}">
+//   where {{.X}} evaluates to `javascript:...`
+// Discussion:
+//   "ZgotmplZ" is a special value that indicates that unsafe content reached a
+//   CSS or URL context at runtime. The output of the example will be
+//     <img src="#ZgotmplZ">
+//   If the data comes from a trusted source, use content types to exempt it
+//   from filtering: URL(`javascript:...`).
+const (
+	// OK indicates the lack of an error.
+	OK ErrorCode = iota
+
+	// ErrorAmbigContext: "... appears in an ambiguous URL context"
+	// Example:
+	//   <a href="
+	//      {{if .C}}
+	//        /path/
+	//      {{else}}
+	//        /search?q=
+	//      {{end}}
+	//      {{.X}}
+	//   ">
+	// Discussion:
+	//   {{.X}} is in an ambiguous URL context since, depending on {{.C}},
+	//  it may be either a URL suffix or a query parameter.
+	//   Moving {{.X}} into the condition removes the ambiguity:
+	//   <a href="{{if .C}}/path/{{.X}}{{else}}/search?q={{.X}}">
+	ErrAmbigContext
+
+	// TODO: document
+	ErrBadHTML
+
+	// ErrBranchEnd: "{{if}} branches end in different contexts"
+	// Example:
+	//   {{if .C}}<a href="{{end}}{{.X}}
+	// Discussion:
+	//   EscapeSet statically examines each possible path when it encounters
+	//   a {{if}}, {{range}}, or {{with}} to escape any following pipelines.
+	//   The example is ambiguous since {{.X}} might be an HTML text node,
+	//   or a URL prefix in an HTML attribute. EscapeSet needs to understand
+	//   the context of {{.X}} to escape it, but that depends on the
+	//   run-time value of {{.C}}.
+	//
+	//   The problem is usually something like missing quotes or angle
+	//   brackets, or can be avoided by refactoring to put the two contexts
+	//   into different branches of an if, range or with. If the problem
+	//   is in a {{range}} over a collection that should never be empty,
+	//   adding a dummy {{else}} can help.
+	ErrBranchEnd
+
+	// ErrEndContext: "... ends in a non-text context: ..."
+	// Examples:
+	//   <div
+	//   <div title="no close quote>
+	//   <script>f()
+	// Discussion:
+	//   EscapeSet assumes the ouput is a DocumentFragment of HTML.
+	//   Templates that end without closing tags will trigger this error.
+	//   Templates that produce incomplete Fragments should not be named
+	//   in the call to EscapeSet.
+	//
+	// If you have a helper template in your set that is not meant to
+	// produce a document fragment, then do not pass its name to
+	// EscapeSet(set, ...names).
+	//
+	//   {{define "main"}} <script>{{template "helper"}}</script> {{end}}
+	//   {{define "helper"}} document.write(' <div title=" ') {{end}}
+	// 
+	// "helper" does not produce a valid document fragment, though it does
+	// produce a valid JavaScript Program.
+	ErrEndContext
+
+	// ErrInsideComment: "... appears inside a comment"
+	// Example:
+	//  <!-- {{.X}} -->
+	//  <script>/* {{.X}} */</script>
+	//  <style>/* {{.X}} */</style>
+	//
+	// Discussion:
+	//  {{.X}} appears inside a comment. There is no escaping convention for
+	//  comments. To use IE conditional comments, inject the  whole comment
+	//  as an HTML, JS, or CSS value (see content.go).
+	//  To comment out code, break the {{...}}.
+	ErrInsideComment
+
+	// ErrNoNames: "must specify names of top level templates"
+	// 
+	//   EscapeSet does not assume that all templates in a set produce HTML.
+	//   Some may be helpers that produce snippets of other languages.
+	//   Passing in no template names is most likely an error,
+	//   so EscapeSet(set) will panic.
+	//   If you call EscapeSet with a slice of names, guard it with len:
+	// 
+	//     if len(names) != 0 {
+	//       set, err := EscapeSet(set, ...names)
+	//     }
+	ErrNoNames
+
+	// ErrNoSuchTemplate: "no such template ..."
+	// Examples:
+	//    {{define "main"}}<div {{template "attrs"}}>{{end}}
+	//    {{define "attrs"}}href="{{.URL}}"{{end}}
+	// Discussion:
+	//   EscapeSet looks through template calls to compute the context.
+	//   Here the {{.URL}} in "attrs" must be treated as a URL when called
+	//   from "main", but if "attrs" is not in set when
+	//   EscapeSet(&set, "main") is called, this error will arise.
+	ErrNoSuchTemplate
+
+	// TODO: document
+	ErrOutputContext
+
+	// ErrPartialCharset: "unfinished JS regexp charset in ..."
+	// Example:
+	//     <script>var pattern = /foo[{{.Chars}}]/</script>
+	// Discussion:
+	//   EscapeSet does not support interpolation into regular expression
+	//   literal character sets.
+	ErrPartialCharset
+
+	// ErrPartialEscape: "unfinished escape sequence in ..."
+	// Example:
+	//   <script>alert("\{{.X}}")</script>
+	// Discussion:
+	//   EscapeSet does not support actions following a backslash.
+	//   This is usually an error and there are better solutions; for
+	//   our example
+	//     <script>alert("{{.X}}")</script>
+	//   should work, and if {{.X}} is a partial escape sequence such as
+	//   "xA0", mark the whole sequence as safe content: JSStr(`\xA0`)
+	ErrPartialEscape
+
+	// ErrRangeLoopReentry: "on range loop re-entry: ..."
+	// Example:
+	//   {{range .}}<p class={{.}}{{end}}
+	// Discussion:
+	//   If an iteration through a range would cause it to end in a
+	//   different context than an earlier pass, there is no single context.
+	//   In the example, the <p> tag is missing a '>'.
+	//   EscapeSet cannot tell whether {{.}} is meant to be an HTML class or
+	//   the content of a broken <p> element and complains because the
+	//   second iteration would produce something like
+	// 
+	//     <p class=foo<p class=bar
+	ErrRangeLoopReentry
+
+	// TODO: document
+	ErrSlashAmbig
+)
+
+func (e *Error) String() string {
+	if e.Line != 0 {
+		return fmt.Sprintf("exp/template/html:%s:%d: %s", e.Name, e.Line, e.Description)
+	} else if e.Name != "" {
+		return fmt.Sprintf("exp/template/html:%s: %s", e.Name, e.Description)
+	}
+	return "exp/template/html: " + e.Description
+}
+
+// errorf creates an error given a format string f and args.
+// The template Name still needs to be supplied.
+func errorf(k ErrorCode, line int, f string, args ...interface{}) *Error {
+	return &Error{k, "", line, fmt.Sprintf(f, args...)}
+}
diff --git a/src/pkg/exp/template/html/escape.go b/src/pkg/exp/template/html/escape.go
index b0acf48df8..3fa92cc98b 100644
--- a/src/pkg/exp/template/html/escape.go
+++ b/src/pkg/exp/template/html/escape.go
@@ -36,7 +36,7 @@ func EscapeSet(s *template.Set, names ...string) (*template.Set, os.Error) {
 	if len(names) == 0 {
 		// TODO: Maybe add a method to Set to enumerate template names
 		// and use those instead.
-		return nil, os.NewError("must specify names of top level templates")
+		return nil, &Error{ErrNoNames, "", 0, "must specify names of top level templates"}
 	}
 	e := escaper{
 		s,
@@ -49,10 +49,10 @@ func EscapeSet(s *template.Set, names ...string) (*template.Set, os.Error) {
 	for _, name := range names {
 		c, _ := e.escapeTree(context{}, name, 0)
 		var err os.Error
-		if c.errStr != "" {
-			err = fmt.Errorf("%s:%d: %s", name, c.errLine, c.errStr)
+		if c.err != nil {
+			err, c.err.Name = c.err, name
 		} else if c.state != stateText {
-			err = fmt.Errorf("%s ends in a non-text context: %v", name, c)
+			err = &Error{ErrEndContext, name, 0, fmt.Sprintf("ends in a non-text context: %v", c)}
 		}
 		if err != nil {
 			// Prevent execution of unsafe templates.
@@ -163,9 +163,8 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 			s = append(s, "exp_template_html_urlescaper")
 		case urlPartUnknown:
 			return context{
-				state:   stateError,
-				errLine: n.Line,
-				errStr:  fmt.Sprintf("%s appears in an ambiguous URL context", n),
+				state: stateError,
+				err:   errorf(ErrAmbigContext, n.Line, "%s appears in an ambiguous URL context", n),
 			}
 		default:
 			panic(c.urlPart.String())
@@ -180,9 +179,8 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		s = append(s, "exp_template_html_jsregexpescaper")
 	case stateComment, stateJSBlockCmt, stateJSLineCmt, stateCSSBlockCmt, stateCSSLineCmt:
 		return context{
-			state:   stateError,
-			errLine: n.Line,
-			errStr:  fmt.Sprintf("%s appears inside a comment", n),
+			state: stateError,
+			err:   errorf(ErrInsideComment, n.Line, "%s appears inside a comment", n),
 		}
 	case stateCSS:
 		s = append(s, "exp_template_html_cssvaluefilter")
@@ -319,9 +317,8 @@ func join(a, b context, line int, nodeName string) context {
 	}
 
 	return context{
-		state:   stateError,
-		errLine: line,
-		errStr:  fmt.Sprintf("{{%s}} branches end in different contexts: %v, %v", nodeName, a, b),
+		state: stateError,
+		err:   errorf(ErrBranchEnd, line, "{{%s}} branches end in different contexts: %v, %v", nodeName, a, b),
 	}
 }
 
@@ -340,8 +337,8 @@ func (e *escaper) escapeBranch(c context, n *parse.BranchNode, nodeName string)
 			// Make clear that this is a problem on loop re-entry
 			// since developers tend to overlook that branch when
 			// debugging templates.
-			c0.errLine = n.Line
-			c0.errStr = "on range loop re-entry: " + c0.errStr
+			c0.err.Line = n.Line
+			c0.err.Description = "on range loop re-entry: " + c0.err.Description
 			return c0
 		}
 	}
@@ -386,9 +383,8 @@ func (e *escaper) escapeTree(c context, name string, line int) (context, string)
 	t := e.template(name)
 	if t == nil {
 		return context{
-			state:   stateError,
-			errStr:  fmt.Sprintf("no such template %s", name),
-			errLine: line,
+			state: stateError,
+			err:   errorf(ErrNoSuchTemplate, line, "no such template %s", name),
 		}, dname
 	}
 	if dname != name {
@@ -428,8 +424,7 @@ func (e *escaper) computeOutCtx(c context, t *template.Template) context {
 		d = context{
 			state: stateError,
 			// TODO: Find the first node with a line in t.Tree.Root
-			errLine: 0,
-			errStr:  fmt.Sprintf("cannot compute output context for template %s", n),
+			err: errorf(ErrOutputContext, 0, "cannot compute output context for template %s", n),
 		}
 		// TODO: If necessary, compute a fixed point by assuming d
 		// as the input context, and recursing to escapeList with a 
diff --git a/src/pkg/exp/template/html/escape_test.go b/src/pkg/exp/template/html/escape_test.go
index 0ab326ceb0..4adf3670ec 100644
--- a/src/pkg/exp/template/html/escape_test.go
+++ b/src/pkg/exp/template/html/escape_test.go
@@ -603,11 +603,11 @@ func TestErrors(t *testing.T) {
 		},
 		{
 			"<a b=1 c={{.H}}",
-			"z ends in a non-text context: {stateAttr delimSpaceOrTagEnd",
+			"z: ends in a non-text context: {stateAttr delimSpaceOrTagEnd",
 		},
 		{
 			"<script>foo();",
-			"z ends in a non-text context: {stateJS",
+			"z: ends in a non-text context: {stateJS",
 		},
 		{
 			`<a href="{{if .F}}/foo?a={{else}}/bar/{{end}}{{.H}}">`,
@@ -656,7 +656,7 @@ func TestErrors(t *testing.T) {
 			// or `/-1\.5/i.test(x)` which is a method call on a
 			// case insensitive regular expression.
 			`<script>{{if false}}var x = 1{{end}}/-{{"1.5"}}/i.test(x)</script>`,
-			`: '/' could start div or regexp: "/-"`,
+			`'/' could start div or regexp: "/-"`,
 		},
 		{
 			`{{template "foo"}}`,
@@ -666,7 +666,7 @@ func TestErrors(t *testing.T) {
 			`{{define "z"}}<div{{template "y"}}>{{end}}` +
 				// Illegal starting in stateTag but not in stateText.
 				`{{define "y"}} foo<b{{end}}`,
-			`z:0: "<" in attribute name: " foo<b"`,
+			`"<" in attribute name: " foo<b"`,
 		},
 		{
 			`{{define "z"}}<script>reverseList = [{{template "t"}}]</script>{{end}}` +
@@ -701,7 +701,7 @@ func TestErrors(t *testing.T) {
 			continue
 		}
 		if strings.Index(got, test.err) == -1 {
-			t.Errorf("input=%q: error %q does not contain expected string %q", test.input, got, test.err)
+			t.Errorf("input=%q: error\n\t%q\ndoes not contain expected string\n\t%q", test.input, got, test.err)
 			continue
 		}
 	}
diff --git a/src/pkg/exp/template/html/transition.go b/src/pkg/exp/template/html/transition.go
index 117b20a5bf..2449a50110 100644
--- a/src/pkg/exp/template/html/transition.go
+++ b/src/pkg/exp/template/html/transition.go
@@ -6,11 +6,12 @@ package html
 
 import (
 	"bytes"
-	"fmt"
-	"os"
 	"strings"
 )
 
+// TODO: ensure transition error messages contain template name and ideally
+// line info.
+
 // transitionFunc is the array of context transition functions for text nodes.
 // A transition function takes a context and template text input, and returns
 // the updated context and any unconsumed text.
@@ -82,8 +83,8 @@ func tTag(c context, s []byte) (context, []byte) {
 	i, err := eatAttrName(s, attrStart)
 	if err != nil {
 		return context{
-			state:  stateError,
-			errStr: err.String(),
+			state: stateError,
+			err:   err,
 		}, nil
 	}
 	if i == len(s) {
@@ -204,8 +205,8 @@ func tJS(c context, s []byte) (context, []byte) {
 			c.jsCtx = jsCtxRegexp
 		default:
 			return context{
-				state:  stateError,
-				errStr: fmt.Sprintf("'/' could start div or regexp: %.32q", s[i:]),
+				state: stateError,
+				err:   errorf(ErrSlashAmbig, 0, "'/' could start div or regexp: %.32q", s[i:]),
 			}, nil
 		}
 	default:
@@ -235,8 +236,8 @@ func tJSStr(c context, s []byte) (context, []byte) {
 			i++
 			if i == len(b) {
 				return context{
-					state:  stateError,
-					errStr: fmt.Sprintf("unfinished escape sequence in JS string: %q", s),
+					state: stateError,
+					err:   errorf(ErrPartialEscape, 0, "unfinished escape sequence in JS string: %q", s),
 				}, nil
 			}
 		} else {
@@ -271,8 +272,8 @@ func tJSRegexp(c context, s []byte) (context, []byte) {
 			i++
 			if i == len(b) {
 				return context{
-					state:  stateError,
-					errStr: fmt.Sprintf("unfinished escape sequence in JS regexp: %q", s),
+					state: stateError,
+					err:   errorf(ErrPartialEscape, 0, "unfinished escape sequence in JS regexp: %q", s),
 				}, nil
 			}
 		case '[':
@@ -289,8 +290,8 @@ func tJSRegexp(c context, s []byte) (context, []byte) {
 		// This can be fixed by making context richer if interpolation
 		// into charsets is desired.
 		return context{
-			state:  stateError,
-			errStr: fmt.Sprintf("unfinished JS regexp charset: %q", s),
+			state: stateError,
+			err:   errorf(ErrPartialCharset, 0, "unfinished JS regexp charset: %q", s),
 		}, nil
 	}
 
@@ -463,8 +464,8 @@ func tCSSStr(c context, s []byte) (context, []byte) {
 			i++
 			if i == len(b) {
 				return context{
-					state:  stateError,
-					errStr: fmt.Sprintf("unfinished escape sequence in CSS string: %q", s),
+					state: stateError,
+					err:   errorf(ErrPartialEscape, 0, "unfinished escape sequence in CSS string: %q", s),
 				}, nil
 			}
 		} else {
@@ -486,7 +487,7 @@ func tError(c context, s []byte) (context, []byte) {
 // It returns an error if s[i:] does not look like it begins with an
 // attribute name, such as encountering a quote mark without a preceding
 // equals sign.
-func eatAttrName(s []byte, i int) (int, os.Error) {
+func eatAttrName(s []byte, i int) (int, *Error) {
 	for j := i; j < len(s); j++ {
 		switch s[j] {
 		case ' ', '\t', '\n', '\f', '\r', '=', '>':
@@ -495,7 +496,7 @@ func eatAttrName(s []byte, i int) (int, os.Error) {
 			// These result in a parse warning in HTML5 and are
 			// indicative of serious problems if seen in an attr
 			// name in a template.
-			return 0, fmt.Errorf("%q in attribute name: %.32q", s[j:j+1], s)
+			return -1, errorf(ErrBadHTML, 0, "%q in attribute name: %.32q", s[j:j+1], s)
 		default:
 			// No-op.
 		}