[C#] Improve error messages for Views (#4435)

This is the implementation of a fix for #4425

# Description of Changes

* Clarified C# generator diagnostics for view return types:
1. Updated the comments around `IQuery<T>` handling to describe the
return value as `T?`, matching C# semantics.
2. Adjusted the validation comment to say views must return `List<T>` or
nullable `T` instead of “Vec/Option”.
* Synced the diagnostics fixture comments with the new terminology so
STDB0024 examples talk about `List<T>`/`T?`.
* Checked current documentation for anything C# related to “Vec/Option”
and confirmed everything now references `List<T>`/`T?`.
* Regenerated/verified tests and snapshots.

# API and ABI breaking changes

None

# Expected complexity level and risk

1 - Changes are documentation and diagnostic-comment only.

# Testing

- [X] CLI rebuilt, local `dotnet test` pass and error output tests
validated.

---------

Signed-off-by: Ryan <r.ekhoff@clockworklabs.io>
Co-authored-by: joshua-spacetime <josh@clockworklabs.io>

Author: rekhoff

crates/bindings-csharp/Codegen.Tests/Tests.cs

@@ -4,6 +4,7 @@ namespace SpacetimeDB.Codegen.Tests;
 using System.Runtime.CompilerServices;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.MSBuild;
 using Microsoft.CodeAnalysis.Text;
 
@@ -15,8 +16,19 @@ public static class GeneratorSnapshotTests
 
     record struct StepOutput(string Key, IncrementalStepRunReason Reason, object Value);
 
-    class Fixture(string projectDir, CSharpCompilation sampleCompilation)
+    class Fixture
     {
+        private readonly string projectDir;
+        private readonly CSharpCompilation sampleCompilation;
+
+        public Fixture(string projectDir, CSharpCompilation sampleCompilation)
+        {
+            this.projectDir = projectDir;
+            this.sampleCompilation = sampleCompilation;
+        }
+
+        public CSharpCompilation SampleCompilation => sampleCompilation;
+
         public static async Task<Fixture> Compile(string name)
         {
             var projectDir = Path.Combine(GetProjectDir(), "fixtures", name);
@@ -63,6 +75,12 @@ IIncrementalGenerator generator
             return genResult.GeneratedTrees;
         }
 
+        public GeneratorDriverRunResult RunGeneratorAndGetResult(IIncrementalGenerator generator)
+        {
+            var driver = CreateDriver(generator, sampleCompilation.LanguageVersion);
+            return driver.RunGenerators(sampleCompilation).GetRunResult();
+        }
+
         public async Task<CSharpCompilation> RunAndCheckGenerators(
             params IIncrementalGenerator[] generators
         ) =>
@@ -257,4 +275,50 @@ public static async Task TestDiagnostics()
         AssertRuntimeDoesNotDefineLocal(compilationAfterGen);
         AssertGeneratedCodeDoesNotUseInternalBound(compilationAfterGen);
     }
+
+    [Fact]
+    public static async Task ViewInvalidReturnHighlightsReturnType()
+    {
+        var fixture = await Fixture.Compile("diag");
+
+        var runResult = fixture.RunGeneratorAndGetResult(new SpacetimeDB.Codegen.Module());
+
+        var method = fixture
+            .SampleCompilation.SyntaxTrees.Select(tree => new
+            {
+                Tree = tree,
+                Root = tree.GetRoot(),
+            })
+            .SelectMany(entry =>
+                entry
+                    .Root.DescendantNodes()
+                    .OfType<MethodDeclarationSyntax>()
+                    .Select(method => new
+                    {
+                        entry.Tree,
+                        entry.Root,
+                        Method = method,
+                    })
+            )
+            .Single(entry => entry.Method.Identifier.Text == "ViewDefIEnumerableReturnFromIter");
+
+        var returnTypeSpan = method.Method.ReturnType.Span;
+        var diagnostics = runResult
+            .Results.SelectMany(result => result.Diagnostics)
+            .Where(d => d.Id == "STDB0024")
+            .ToList();
+        var diagnostic = diagnostics.FirstOrDefault(d =>
+            d.GetMessage().Contains("ViewDefIEnumerableReturnFromIter")
+            && d.Location.SourceTree == method.Tree
+        );
+
+        Assert.NotNull(diagnostic);
+
+        Assert.Equal(returnTypeSpan, diagnostic!.Location.SourceSpan);
+
+        var returnTypeText = method
+            .Root.ToFullString()
+            .Substring(returnTypeSpan.Start, returnTypeSpan.Length);
+        Assert.Contains("IEnumerable", returnTypeText);
+    }
 }

crates/bindings-csharp/Codegen.Tests/fixtures/diag/snapshots/Module.verified.txt

@@ -330,25 +330,17 @@ public partial struct TestScheduleIssues
       }
     },
     {/*
-    // Invalid: Wrong return type is not List<T> or T?
     [SpacetimeDB.View(Accessor = "view_def_wrong_return", Public = true)]
-    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     public static Player ViewDefWrongReturn(ViewContext ctx)
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+                  ^^^^^^
     {
-^^^^^
-        return new Player { Identity = new() };
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-    }
-^^^^^
-
 */
-      Message: View 'ViewDefWrongReturn' must return Vec<T> or Option<T>.,
+      Message: View 'ViewDefWrongReturn' must return T?, List<T>, or IQuery<T>,
       Severity: Error,
       Descriptor: {
         Id: STDB0024,
-        Title: Views must return Vec<T> or Option<T>,
-        MessageFormat: View '{0}' must return Vec<T> or Option<T>.,
+        Title: Views must return T?, List<T>, or IQuery<T>,
+        MessageFormat: View '{0}' must return T?, List<T>, or IQuery<T>,
         Category: SpacetimeDB,
         DefaultSeverity: Error,
         IsEnabledByDefault: true
@@ -372,25 +364,17 @@ public partial struct TestScheduleIssues
       }
     },
     {/*
-    // Invalid: IEnumerable<T> return type (from Iter()) is not List<T> or T?
     [SpacetimeDB.View(Accessor = "view_def_ienumerable_return_from_iter", Public = true)]
-    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     public static IEnumerable<Player> ViewDefIEnumerableReturnFromIter(ViewContext ctx)
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+                  ^^^^^^^^^^^^^^^^^^^
     {
-^^^^^
-        return ctx.Db.Player.Iter();
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-    }
-^^^^^
-
 */
-      Message: View 'ViewDefIEnumerableReturnFromIter' must return Vec<T> or Option<T>.,
+      Message: View 'ViewDefIEnumerableReturnFromIter' must return T?, List<T>, or IQuery<T>,
       Severity: Error,
       Descriptor: {
         Id: STDB0024,
-        Title: Views must return Vec<T> or Option<T>,
-        MessageFormat: View '{0}' must return Vec<T> or Option<T>.,
+        Title: Views must return T?, List<T>, or IQuery<T>,
+        MessageFormat: View '{0}' must return T?, List<T>, or IQuery<T>,
         Category: SpacetimeDB,
         DefaultSeverity: Error,
         IsEnabledByDefault: true
@@ -414,29 +398,17 @@ public partial struct TestScheduleIssues
       }
     },
     {/*
-    // Invalid: IEnumerable<T> return type (from Filter()) is not List<T> or T?
     [SpacetimeDB.View(Accessor = "view_def_ienumerable_return_from_filter", Public = true)]
-    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     public static IEnumerable<TestScheduleIssues> ViewDefIEnumerableReturnFromFilter(
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         ViewContext ctx
-^^^^^^^^^^^^^^^^^^^^^^^
-    )
-^^^^^
-    {
-^^^^^
-        return ctx.Db.TestIndexIssues.TestUnexpectedColumns.Filter(0);
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-    }
-^^^^^
-
 */
-      Message: View 'ViewDefIEnumerableReturnFromFilter' must return Vec<T> or Option<T>.,
+      Message: View 'ViewDefIEnumerableReturnFromFilter' must return T?, List<T>, or IQuery<T>,
       Severity: Error,
       Descriptor: {
         Id: STDB0024,
-        Title: Views must return Vec<T> or Option<T>,
-        MessageFormat: View '{0}' must return Vec<T> or Option<T>.,
+        Title: Views must return T?, List<T>, or IQuery<T>,
+        MessageFormat: View '{0}' must return T?, List<T>, or IQuery<T>,
         Category: SpacetimeDB,
         DefaultSeverity: Error,
         IsEnabledByDefault: true

crates/bindings-csharp/Codegen/Diag.cs

@@ -215,9 +215,9 @@ string typeName
     public static readonly ErrorDescriptor<MethodDeclarationSyntax> ViewInvalidReturn =
         new(
             group,
-            "Views must return Vec<T> or Option<T>",
-            method => $"View '{method.Identifier}' must return Vec<T> or Option<T>.",
-            method => method
+            "Views must return T?, List<T>, or IQuery<T>",
+            method => $"View '{method.Identifier}' must return T?, List<T>, or IQuery<T>",
+            method => method.ReturnType
         );
 
     // TODO: Remove once Views support Private: Views must be Public currently

crates/bindings-csharp/Codegen/Module.cs

@@ -1189,7 +1189,7 @@ method.ReturnType is INamedTypeSymbol
                 ? "SpacetimeDB.BSATN.ValueOption"
                 : "SpacetimeDB.BSATN.RefOption";
             var opt = $"{optType}<{rowType.Name}, {rowType.BSATNName}>";
-            // Match Rust semantics: Query<T> is described as Option<T>.
+            // Match Rust semantics: Query<T> is described as a nullable row (T?).
             ReturnType = new ReferenceUse(opt, opt);
         }
         else
@@ -1210,7 +1210,7 @@ method.ReturnType is INamedTypeSymbol
             diag.Report(ErrorDescriptor.ViewContextParam, methodSyntax);
         }
 
-        // Validate return type: must be Option<T> or Vec<T>
+        // Validate return type: must be List<T> or T?
         if (
             !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.ValueOption")
             && !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.RefOption")