slog: context

Support putting a Logger into a context.Context and getting it out.

Also, remove concat3 since it wasn't used, and generalize concat.

Change-Id: Ibdac8c163bd8f8f3d3c4d54bbe54f0f80bfdaa98
Reviewed-on: https://go-review.googlesource.com/c/exp/+/429437
Run-TryBot: Jonathan Amsterdam <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: jba

slog/context.go

@@ -0,0 +1,24 @@
+// Copyright 2022 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 slog
+
+import "context"
+
+type contextKey struct{}
+
+// NewContext returns a context that contains the given Logger.
+// Use FromContext to retrieve the Logger.
+func NewContext(ctx context.Context, l *Logger) context.Context {
+	return context.WithValue(ctx, contextKey{}, l)
+}
+
+// FromContext returns the Logger stored in ctx by NewContext, or the default
+// Logger if there is none.
+func FromContext(ctx context.Context) *Logger {
+	if l, ok := ctx.Value(contextKey{}).(*Logger); ok {
+		return l
+	}
+	return Default()
+}

slog/util.go

@@ -4,16 +4,13 @@
 
 package slog
 
-func concat(l1, l2 []Attr) []Attr {
-	return concat3(l1, l2, nil)
-}
-
-func concat3(l1, l2, l3 []Attr) []Attr {
-	l := make([]Attr, len(l1)+len(l2)+len(l3))
-	copy(l, l1)
-	copy(l[len(l1):], l2)
-	copy(l[len(l1)+len(l2):], l3)
-	return l
+// concat returns a new slice with the elements of s1 followed
+// by those of s2. The slice has no additional capacity.
+func concat[T any](s1, s2 []T) []T {
+	s := make([]T, len(s1)+len(s2))
+	copy(s, s1)
+	copy(s[len(s1):], s2)
+	return s
 }
 
 // Cheap integer to fixed-width decimal ASCII. Give a negative width to avoid zero-padding.