Fully annotate JsonNode for trimmability

Follow up to #52934.

- Using JsonNode in dynamic statements is not trim compatible. Add a LibraryBuild warning since there isn't a direct API to put the warning on.
- Mark JsonValueNotTrimmable's ctor as unsafe
- Fix up a few warning messages
- minor doc fixup

Contributes to #45623

src/libraries/System.Text.Json/ref/System.Text.Json.cs

@@ -518,7 +518,7 @@ public JsonArray(params System.Text.Json.Nodes.JsonNode?[] items) { }
         public int Count { get { throw null; } }
         bool System.Collections.Generic.ICollection<System.Text.Json.Nodes.JsonNode?>.IsReadOnly { get { throw null; } }
         public void Add(System.Text.Json.Nodes.JsonNode? item) { }
-        [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("JSON serialization and deserialization might require types that cannot be statically analyzed. Use the overload that takes a JsonTypeInfo or JsonSerializerContext, or make sure all of the required types are preserved.")]
+        [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("Creating JsonValue instances with non-primitive types is not compatible with trimming. It can result in non-primitive types being serialized, which may have their members trimmed.")]
         public void Add<T>(T? value) { }
         public void Clear() { }
         public bool Contains(System.Text.Json.Nodes.JsonNode? item) { throw null; }
@@ -711,7 +711,7 @@ public abstract partial class JsonValue : System.Text.Json.Nodes.JsonNode
         [System.CLSCompliantAttribute(false)]
         public static System.Text.Json.Nodes.JsonValue Create(ulong value, System.Text.Json.Nodes.JsonNodeOptions? options = default(System.Text.Json.Nodes.JsonNodeOptions?)) { throw null; }
         public static System.Text.Json.Nodes.JsonValue? Create<T>(T? value, System.Text.Json.Serialization.Metadata.JsonTypeInfo<T> jsonTypeInfo, System.Text.Json.Nodes.JsonNodeOptions? options = default(System.Text.Json.Nodes.JsonNodeOptions?)) { throw null; }
-        [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("JSON serialization and deserialization might require types that cannot be statically analyzed. Use the overload that takes a JsonTypeInfo or JsonSerializerContext, or make sure all of the required types are preserved.")]
+        [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("Creating JsonValue instances with non-primitive types is not compatible with trimming. It can result in non-primitive types being serialized, which may have their members trimmed. Use the overload that takes a JsonTypeInfo, or make sure all of the required types are preserved.")]
         public static System.Text.Json.Nodes.JsonValue? Create<T>(T? value, System.Text.Json.Nodes.JsonNodeOptions? options = default(System.Text.Json.Nodes.JsonNodeOptions?)) { throw null; }
         public abstract bool TryGetValue<T>([System.Diagnostics.CodeAnalysis.NotNullWhenAttribute(true)] out T? value);
     }

src/libraries/System.Text.Json/src/ILLink/ILLink.Suppressions.LibraryBuild.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<linker>
+  <assembly fullname="System.Text.Json">
+    <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
+      <argument>ILLink</argument>
+      <argument>IL2026</argument>
+      <property name="Scope">member</property>
+      <property name="Target">M:System.Text.Json.Nodes.JsonNode.System#Dynamic#IDynamicMetaObjectProvider#GetMetaObject(System.Linq.Expressions.Expression)</property>
+      <property name="Justification">System.Text.Json's integration with dynamic is not trim compatible. However, there isn't a direct API developers call. Instead they use the 'dynamic' keyword, which gets compiled into calls to Microsoft.CSharp. Microsoft.CSharp looks for IDynamicMetaObjectProvider implementations. Leaving this warning in the product so developers get a warning stating that using dynamic JsonValue code may be broken in trimmed apps.</property>
+    </attribute>
+  </assembly>
+</linker>

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonArray.cs

@@ -98,7 +98,7 @@ internal JsonArray (JsonElement element, JsonNodeOptions? options = null) : base
         /// <param name="value">
         ///   The object to be added to the end of the <see cref="JsonArray"/>.
         /// </param>
-        [RequiresUnreferencedCode(JsonSerializer.SerializationUnreferencedCodeMessage)]
+        [RequiresUnreferencedCode(JsonValue.CreateUnreferencedCodeMessage)]
         public void Add<T>(T? value)
         {
             if (value == null)

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonNode.Dynamic.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Dynamic;
 using System.Linq.Expressions;
 using System.Reflection;
@@ -10,9 +11,17 @@ namespace System.Text.Json.Nodes
     public partial class JsonNode : IDynamicMetaObjectProvider
     {
         internal virtual MethodInfo? TryGetMemberMethodInfo => null;
-        internal virtual MethodInfo? TrySetMemberMethodInfo => null;
+        internal virtual MethodInfo? TrySetMemberMethodInfo
+        {
+            [RequiresUnreferencedCode(JsonSerializer.SerializationUnreferencedCodeMessage)]
+            get => null;
+        }
 
         DynamicMetaObject IDynamicMetaObjectProvider.GetMetaObject(Expression parameter) =>
-            new MetaDynamic(parameter, this);
+            CreateDynamicObject(parameter, this);
+
+        [RequiresUnreferencedCode("Using JsonNode instances as dynamic types is not compatible with trimming. It can result in non-primitive types being serialized, which may have their members trimmed.")]
+        private static DynamicMetaObject CreateDynamicObject(Expression parameter, JsonNode node) =>
+            new MetaDynamic(parameter, node);
     }
 }

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.Dynamic.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Dynamic;
 using System.Reflection;
 
@@ -21,6 +22,7 @@ private bool TryGetMemberCallback(GetMemberBinder binder, out object? result)
             return true;
         }
 
+        [RequiresUnreferencedCode(JsonSerializer.SerializationUnreferencedCodeMessage)]
         private bool TrySetMemberCallback(SetMemberBinder binder, object? value)
         {
             JsonNode? node = null;
@@ -37,17 +39,17 @@ private bool TrySetMemberCallback(SetMemberBinder binder, object? value)
             return true;
         }
 
-        private static MethodInfo GetMethod(string name) => typeof(JsonObject).GetMethod(
-            name, BindingFlags.Instance | BindingFlags.NonPublic)!;
+        private const BindingFlags MemberInfoBindingFlags = BindingFlags.Instance | BindingFlags.NonPublic;
 
         private static MethodInfo? s_TryGetMember;
         internal override MethodInfo? TryGetMemberMethodInfo =>
-            s_TryGetMember ??
-            (s_TryGetMember = GetMethod(nameof(TryGetMemberCallback)));
+            s_TryGetMember ??= typeof(JsonObject).GetMethod(nameof(TryGetMemberCallback), MemberInfoBindingFlags);
 
         private static MethodInfo? s_TrySetMember;
-        internal override MethodInfo? TrySetMemberMethodInfo =>
-            s_TrySetMember ??
-            (s_TrySetMember = GetMethod(nameof(TrySetMemberCallback)));
+        internal override MethodInfo? TrySetMemberMethodInfo
+        {
+            [RequiresUnreferencedCode(JsonSerializer.SerializationUnreferencedCodeMessage)]
+            get => s_TrySetMember ??= typeof(JsonObject).GetMethod(nameof(TrySetMemberCallback), MemberInfoBindingFlags);
+        }
     }
 }