[release/dev17.13] Preserve encoding during DocumentState updates (#77362)

Backport of #77354 to release/dev17.13

/cc @sharwell

## Customer Impact

## Regression

- [ ] Yes
- [ ] No

[If yes, specify when the regression was introduced. Provide the PR or
commit if known.]

## Testing

[How was the fix verified? How was the issue missed previously? What
tests were added?]

## Risk

[High/Medium/Low. Justify the indication by mentioning how risks were
measured and addressed.]

Author: sharwell

src/Workspaces/Core/Portable/Workspace/Solution/DocumentState.cs

@@ -497,6 +497,20 @@ internal DocumentState UpdateTree(SyntaxNode newRoot, PreservationMode mode)
 
         // use the encoding that we get from the new root
         var encoding = newRoot.SyntaxTree.Encoding;
+        if (encoding is null)
+        {
+            // The new tree doesn't specify an encoding. For these cases, continue to use the previous encoding of the
+            // document.
+            if (TryGetSyntaxTree(out var priorTree))
+            {
+                // this is most likely available since UpdateTree is normally called after modifying the existing tree.
+                encoding = priorTree.Encoding;
+            }
+            else if (TryGetText(out var priorText))
+            {
+                encoding = priorText.Encoding;
+            }
+        }
 
         var syntaxTreeFactory = LanguageServices.GetRequiredService<ISyntaxTreeFactoryService>();
 

src/Workspaces/MSBuildTest/VisualStudioMSBuildWorkspaceTests.cs

@@ -2893,13 +2893,14 @@ class C { }";
             Assert.Equal(encoding.EncodingName, text.Encoding.EncodingName);
             Assert.Equal(fileContent, text.ToString());
 
-            // update root blindly again, after observing encoding, see that encoding is overridden to null
+            // update root blindly again, after observing encoding, see that encoding is preserved
+            // 🐉 Tools rely on encoding preservation; see https://github.com/dotnet/sdk/issues/46780
             var doc3 = document.WithSyntaxRoot(gen.CompilationUnit()); // empty CU
             var doc3text = await doc3.GetTextAsync();
-            Assert.Null(doc3text.Encoding);
+            Assert.Same(text.Encoding, doc3text.Encoding);
             var doc3tree = await doc3.GetSyntaxTreeAsync();
-            Assert.Null(doc3tree.Encoding);
-            Assert.Null(doc3tree.GetText().Encoding);
+            Assert.Same(text.Encoding, doc3tree.Encoding);
+            Assert.Same(text.Encoding, doc3tree.GetText().Encoding);
 
             // change doc to have no encoding, still succeeds at writing to disk with old encoding
             var root = await document.GetSyntaxRootAsync();