Support lenient legalization of header field values

Sources/HTTPTypes/HTTPField.swift

@@ -68,6 +68,16 @@ public struct HTTPField: Sendable, Hashable {
         self.rawValue = Self.legalizeValue(ISOLatin1String(value))
     }
 
+    /// Create an HTTP field from a name and a value. Leniently legalize the value.
+    /// - Parameters:
+    ///   - name: The HTTP field name.
+    ///   - lenientValue: The HTTP field value. Newlines and NULs are converted into space
+    ///                   characters.
+    public init(name: Name, lenientValue: some Collection<UInt8>) {
+        self.name = name
+        self.rawValue = Self.lenientLegalizeValue(ISOLatin1String(lenientValue))
+    }
+
     init(name: Name, uncheckedValue: ISOLatin1String) {
         self.name = name
         self.rawValue = uncheckedValue
@@ -162,6 +172,22 @@ public struct HTTPField: Sendable, Hashable {
         }
     }
 
+    static func lenientLegalizeValue(_ value: ISOLatin1String) -> ISOLatin1String {
+        if value._storage.utf8.allSatisfy({ $0 != 0x00 && $0 != 0x0A && $0 != 0x0D }) {
+            return value
+        } else {
+            let bytes = value._storage.utf8.lazy.map { byte -> UInt8 in
+                switch byte {
+                case 0x00, 0x0A, 0x0D:
+                    return 0x20
+                default:
+                    return byte
+                }
+            }
+            return ISOLatin1String(unchecked: String(decoding: bytes, as: UTF8.self))
+        }
+    }
+
     /// Whether the string is valid for an HTTP field value based on RFC 9110.
     ///
     /// https://www.rfc-editor.org/rfc/rfc9110.html#name-field-values

Tests/HTTPTypesTests/HTTPTypesTests.swift

@@ -38,6 +38,7 @@ final class HTTPTypesTests: XCTestCase {
         XCTAssertEqual(HTTPField(name: .accept, value: " a 😀 \t\n b \t \r ").value, "a 😀 \t  b")
         XCTAssertEqual(HTTPField(name: .accept, value: "").value, "")
         XCTAssertFalse(HTTPField.isValidValue(" "))
+        XCTAssertEqual(HTTPField(name: .accept, lenientValue: "  \r\n\0\t ".utf8).value, "     \t ")
     }
 
     func testRequest() {