review: fix stale Reset comment, document dsw self-reference invariant, test cached bind errors

- context: correct newContext comment (Reset clears the store map, no longer nils it);
  document that only json()'s nested-guard may point the response at &c.dsw
- test: deterministic cold-then-warm bind ensures the per-type cache preserves
  field-name conversion errors regardless of suite ordering

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vishr

bind_cache_test.go

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: © 2015 LabStack LLC and Echo contributors
+
+package echo
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestBindCachedMetaPreservesFieldNameError ensures the per-type bind metadata cache preserves the
+// field-name prefix in conversion errors on BOTH the cold (first) and warm (cached) bind of a type.
+// DTO is declared locally so its reflect.Type is independent of suite ordering, making the second
+// bind a deterministic cache hit (the bindMetaFor Load branch).
+func TestBindCachedMetaPreservesFieldNameError(t *testing.T) {
+	type DTO struct {
+		Number int `query:"number"`
+	}
+	bind := func() error {
+		e := New()
+		req := httptest.NewRequest(http.MethodGet, "/?number=10a", nil)
+		var dto DTO
+		return e.NewContext(req, httptest.NewRecorder()).Bind(&dto)
+	}
+
+	assert.ErrorContains(t, bind(), "number", "cold cache: error must carry field name")
+	assert.ErrorContains(t, bind(), "number", "warm cache: error must still carry field name")
+}

context.go

@@ -78,7 +78,9 @@ type Context struct {
 	handler HandlerFunc
 
 	// dsw is reused by json() so that each JSON response does not heap-allocate a delayedStatusWriter.
-	// It lives on the pooled Context; &c.dsw is a stable, allocation-free pointer.
+	// It lives on the pooled Context; &c.dsw is a stable, allocation-free pointer. Only json() may point
+	// the response at &c.dsw, and only via the nested-call guard there — aliasing it to itself (wrapping
+	// &c.dsw around &c.dsw) would make the response writer reference itself.
 	dsw delayedStatusWriter
 
 	store  map[string]any
@@ -105,7 +107,7 @@ func NewContext(r *http.Request, w http.ResponseWriter, opts ...any) *Context {
 }
 
 func newContext(r *http.Request, w http.ResponseWriter, e *Echo) *Context {
-	// store is created lazily by Set (and reset to nil by Reset), so we deliberately do not allocate a map here.
+	// store is created lazily by Set and cleared (not freed) by Reset, so we deliberately do not allocate a map here.
 	c := &Context{
 		pathValues: nil,
 		echo:       e,