Do not try to convert interpolated string format specifiers into virtual chars.

src/EditorFeatures/CSharpTest/Classification/SemanticClassifierTests.cs

@@ -3079,5 +3079,15 @@ public async Task TestDirectiveStringLiteral()
         {
             await TestInMethodAsync(@"#line 1 ""a\b""");
         }
+
+        [WorkItem(30378, "https://github.com/dotnet/roslyn/issues/30378")]
+        [Fact, Trait(Traits.Feature, Traits.Features.Classification)]
+        public async Task TestFormatSpecifierInInterpolation()
+        {
+            await TestInMethodAsync(@"var goo = $""goo{{1:0000}}bar"";",
+                Keyword("var"),
+                Escape(@"{{"),
+                Escape(@"}}"));
+        }
     }
 }

src/EditorFeatures/VisualBasicTest/Classification/SemanticClassifierTests.vb

@@ -721,5 +721,13 @@ End Operator"
             Await TestAsync("#region ""goo""""bar""",
                 Escape(""""""))
         End Function
+
+        <WorkItem(30378, "https://github.com/dotnet/roslyn/issues/30378")>
+        <Fact, Trait(Traits.Feature, Traits.Features.Classification)>
+        Public Async Function TestFormatSpecifierInInterpolation() As Task
+            Await TestInMethodAsync("dim goo = $""goo{{1:0000}}bar""",
+                Escape("{{"),
+                Escape("}}"))
+        End Function
     End Class
 End Namespace

src/Workspaces/CSharp/Portable/EmbeddedLanguages/VirtualChars/CSharpVirtualCharService.cs

@@ -48,17 +48,21 @@ protected override ImmutableArray<VirtualChar> TryConvertToVirtualCharsWorker(Sy
                     ? TryConvertVerbatimStringToVirtualChars(token, "@\"", "\"", escapeBraces: false)
                     : TryConvertStringToVirtualChars(token, "\"", "\"", escapeBraces: false);
             }
-            else if (token.Kind() == SyntaxKind.InterpolatedStringTextToken)
+
+            if (token.Kind() == SyntaxKind.InterpolatedStringTextToken)
+            {
+                // The sections between  `}` and `{` are InterpolatedStringTextToken *as are* the
+                // format specifiers in an interpolated string.  We only want to get the virtual
+                // chars for this first type.
+                if (token.Parent.Parent is InterpolatedStringExpressionSyntax interpolatedString)
                 {
-                var interpolatedString = (InterpolatedStringExpressionSyntax)token.Parent.Parent;
                     return interpolatedString.StringStartToken.Kind() == SyntaxKind.InterpolatedVerbatimStringStartToken
                        ? TryConvertVerbatimStringToVirtualChars(token, "", "", escapeBraces: true)
                        : TryConvertStringToVirtualChars(token, "", "", escapeBraces: true);
                 }
-            else
-            {
-                return default;
             }
+
+            return default;
         }
 
         private bool IsInDirective(SyntaxNode node)

src/Workspaces/VisualBasic/Portable/EmbeddedLanguages/VirtualChars/VisualBasicVirtualCharService.vb

@@ -4,6 +4,7 @@ Imports System.Collections.Immutable
 Imports System.Composition
 Imports Microsoft.CodeAnalysis.Host.Mef
 Imports Microsoft.CodeAnalysis.EmbeddedLanguages.VirtualChars
+Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
 
 Namespace Microsoft.CodeAnalysis.VisualBasic.EmbeddedLanguages.VirtualChars
     <ExportLanguageService(GetType(IVirtualCharService), LanguageNames.VisualBasic), [Shared]>
@@ -21,7 +22,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.EmbeddedLanguages.VirtualChars
 
             If token.Kind() = SyntaxKind.StringLiteralToken Then
                 Return TryConvertSimpleDoubleQuoteString(token, """", """", escapeBraces:=False)
-            ElseIf token.Kind() = SyntaxKind.InterpolatedStringTextToken Then
+            End If
+
+            If token.Kind() = SyntaxKind.InterpolatedStringTextToken AndAlso
+               TypeOf token.Parent.Parent Is InterpolatedStringExpressionSyntax Then
                 Return TryConvertSimpleDoubleQuoteString(token, "", "", escapeBraces:=True)
             End If