Snap main to 17.13 P3

.gitignore

@@ -49,7 +49,7 @@ UnitTestResults.html
 *.nuget.props
 *.nuget.targets
 project.lock.json
-msbuild.binlog
+*.binlog
 *.project.lock.json
 
 *_i.c

Roslyn.sln

@@ -539,8 +539,6 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.CodeAnalysis.Exte
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.CodeAnalysis.ExternalAccess.EditorConfigGenerator", "src\VisualStudio\ExternalAccess\EditorConfigGenerator\Microsoft.CodeAnalysis.ExternalAccess.EditorConfigGenerator.csproj", "{09AEDEE4-6358-47C9-8022-3BD37A518070}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "IntegrationTestBuildProject", "src\VisualStudio\IntegrationTest\IntegrationTestBuildProject.csproj", "{4D9D7A28-BB44-4F3F-81DA-14F39B853718}"
-EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.CodeAnalysis.Features.DiagnosticsTests.Utilities", "src\Features\DiagnosticsTestUtilities\Microsoft.CodeAnalysis.Features.DiagnosticsTests.Utilities.csproj", "{5BABC440-4F1B-46E8-9068-DD7F02ED25D3}"
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.CodeAnalysis.Features.Test.Utilities", "src\Features\TestUtilities\Microsoft.CodeAnalysis.Features.Test.Utilities.csproj", "{5762E483-75CE-4328-A410-511F30737712}"
@@ -1365,10 +1363,6 @@ Global
 		{09AEDEE4-6358-47C9-8022-3BD37A518070}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{09AEDEE4-6358-47C9-8022-3BD37A518070}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{09AEDEE4-6358-47C9-8022-3BD37A518070}.Release|Any CPU.Build.0 = Release|Any CPU
-		{4D9D7A28-BB44-4F3F-81DA-14F39B853718}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{4D9D7A28-BB44-4F3F-81DA-14F39B853718}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{4D9D7A28-BB44-4F3F-81DA-14F39B853718}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{4D9D7A28-BB44-4F3F-81DA-14F39B853718}.Release|Any CPU.Build.0 = Release|Any CPU
 		{5BABC440-4F1B-46E8-9068-DD7F02ED25D3}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{5BABC440-4F1B-46E8-9068-DD7F02ED25D3}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{5BABC440-4F1B-46E8-9068-DD7F02ED25D3}.Release|Any CPU.ActiveCfg = Release|Any CPU
@@ -1652,7 +1646,6 @@ Global
 		{B1481D94-682E-46EC-ADBE-A16EB46FEEE9} = {55A62CFA-1155-46F1-ADF3-BEEE51B58AB5}
 		{5E8FB6D6-6C5C-42E6-9220-1EAA7ED9BCAD} = {1AE9182D-B03E-4B00-B32E-37AE01715F57}
 		{09AEDEE4-6358-47C9-8022-3BD37A518070} = {5880FECB-91F1-4AB8-8726-75EAFA8A918E}
-		{4D9D7A28-BB44-4F3F-81DA-14F39B853718} = {CC126D03-7EAC-493F-B187-DCDEE1EF6A70}
 		{5BABC440-4F1B-46E8-9068-DD7F02ED25D3} = {3E5FE3DB-45F7-4D83-9097-8F05D3B3AEC6}
 		{5762E483-75CE-4328-A410-511F30737712} = {3E5FE3DB-45F7-4D83-9097-8F05D3B3AEC6}
 		{6D819E80-BA2F-4317-8368-37F8F4434D3A} = {806F0C6F-3640-4C92-8D55-6767B1535467}

azure-pipelines-pr-validation.yml

@@ -33,6 +33,8 @@ variables:
     value: false
   - name: Codeql.SkipTaskAutoInjection
     value: true
+  - name: SignType
+    value: ''
 
   # To retrieve OptProf data we need to authenticate to the VS drop storage.
   # If the pipeline is running in DevDiv, the account has access to the VS drop storage.
@@ -339,5 +341,6 @@ extends:
         # Symbol validation is not entirely reliable as of yet, so should be turned off until
         # https://github.com/dotnet/arcade/issues/2871 is resolved.
         enableSymbolValidation: false
+        enableSigningValidation: false
         enableSourceLinkValidation: false
         SDLValidationParameters: false

docs/Language Feature Status.md

@@ -10,20 +10,22 @@ efforts behind them.
 
 | Feature | Branch | State | Developer | Reviewer | IDE Buddy | LDM Champ |
 | ------- | ------ | ----- | --------- | -------- | --------- | --------- |
-| [Default in deconstruction](https://github.com/dotnet/roslyn/pull/25562) | [decon-default](https://github.com/dotnet/roslyn/tree/features/decon-default) | [In Progress](https://github.com/dotnet/roslyn/issues/25559) | [jcouv](https://github.com/jcouv) | [gafter](https://github.com/gafter) |  | [jcouv](https://github.com/jcouv) |
-| [Roles/Extensions](https://github.com/dotnet/csharplang/issues/5497) | [roles](https://github.com/dotnet/roslyn/tree/features/roles) | [In Progress](https://github.com/dotnet/roslyn/issues/66722) | [jcouv](https://github.com/jcouv) | [AlekseyTs](https://github.com/AlekseyTs), [jjonescz](https://github.com/jjonescz) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [MadsTorgersen](https://github.com/MadsTorgersen) |
+| String literals in data section as UTF8 | [PR](https://github.com/dotnet/roslyn/pull/76036) | [In Progress](https://github.com/dotnet/roslyn/issues/76234) | [jjonescz](https://github.com/jjonescz) | [AlekseyTs](https://github.com/AlekseyTs), [jcouv](https://github.com/jcouv) | N/A | N/A |
+| Runtime Async | [features/runtime-async](https://github.com/dotnet/roslyn/tree/features/runtime-async) | [In Progress](https://github.com/dotnet/roslyn/issues/75960) | [333fred](https://github.com/333fred) | [jcouv](https://github.com/jcouv), [RikkiGibson](https://github.com/RikkiGibson) |  |  |
+| [Simple lambda parameters with modifiers](https://github.com/dotnet/csharplang/blob/main/proposals/simple-lambda-parameters-with-modifiers.md) | [PR](https://github.com/dotnet/roslyn/pull/75400) | [In Progress](https://github.com/dotnet/roslyn/issues/76298) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [jjonescz](https://github.com/jjonescz), [cston](https://github.com/cston) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) |
 | [Null-conditional assignment](https://github.com/dotnet/csharplang/issues/6045) | [null-conditional-assignment](https://github.com/dotnet/roslyn/tree/features/null-conditional-assignment) | [In Progress](https://github.com/dotnet/roslyn/issues/75554) | [RikkiGibson](https://github.com/RikkiGibson) | [cston](https://github.com/cston), [jjonescz](https://github.com/jjonescz) | TBD | [RikkiGibson](https://github.com/RikkiGibson) |
+| [Extensions](https://github.com/dotnet/csharplang/issues/8697) | [extensions](https://github.com/dotnet/roslyn/tree/features/extensions) | [In Progress](https://github.com/dotnet/roslyn/issues/76130) | [jcouv](https://github.com/jcouv), [AlekseyTs](https://github.com/AlekseyTs) | [jjonescz](https://github.com/jjonescz), TBD | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [MadsTorgersen](https://github.com/MadsTorgersen) |
+| [Dictionary expressions](https://github.com/dotnet/csharplang/issues/8659) | [dictionary-expressions](https://github.com/dotnet/roslyn/tree/features/dictionary-expressions) | [In Progress](https://github.com/dotnet/roslyn/issues/76310) | [cston](https://github.com/cston), [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [333fred](https://github.com/333fred), [jcouv](https://github.com/jcouv) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) |
 | [`field` keyword in properties](https://github.com/dotnet/csharplang/issues/140) | [field-keyword](https://github.com/dotnet/roslyn/tree/features/field-keyword) | [Merged into 17.12p3](https://github.com/dotnet/roslyn/issues/57012) | [Youssef1313](https://github.com/Youssef1313), [cston](https://github.com/cston) | [333fred](https://github.com/333fred), [RikkiGibson](https://github.com/RikkiGibson) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) |
 | [First-class Span Types](https://github.com/dotnet/csharplang/issues/7905) | [FirstClassSpan](https://github.com/dotnet/roslyn/tree/features/FirstClassSpan) | [Merged into 17.13p1](https://github.com/dotnet/roslyn/issues/73445) | [jjonescz](https://github.com/jjonescz) | [cston](https://github.com/cston), [333fred](https://github.com/333fred) | | [333fred](https://github.com/333fred), [stephentoub](https://github.com/stephentoub) |
-| [Unbound generic types in `nameof`](https://github.com/dotnet/csharplang/issues/8480) | [PR](https://github.com/dotnet/roslyn/pull/75368) | [In Progress](https://github.com/dotnet/roslyn/pull/75368) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [jcouv](https://github.com/jcouv) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) |
-| Runtime Async | [features/runtime-async](https://github.com/dotnet/roslyn/tree/features/runtime-async) | [In Progress](https://github.com/dotnet/roslyn/issues/75960) | [333fred](https://github.com/333fred) | [jcouv](https://github.com/jcouv), [RikkiGibson](https://github.com/RikkiGibson) |  |  |
+| [Unbound generic types in `nameof`](https://github.com/dotnet/csharplang/issues/8480) | [PR](https://github.com/dotnet/roslyn/pull/75368) | [Merged into 17.13p2](https://github.com/dotnet/roslyn/pull/75368) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [jcouv](https://github.com/jcouv), [AlekseyTs](https://github.com/AlekseyTs) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) | [CyrusNajmabadi](https://github.com/CyrusNajmabadi) |
 
 # Working Set VB
 
 | Feature | Branch | State | Developer | Reviewer | IDE Buddy | LDM Champ |
 | ------- | ------ | ----- | --------- | -------- | --------- | --------- |
 | Recognizing 'unmanaged' constraint | main | [Merged into 17.13 P2](https://github.com/dotnet/roslyn/pull/75665) | [AlekseyTs](https://github.com/AlekseyTs) | [cston](https://github.com/cston), [333fred](https://github.com/333fred) | (no IDE impact) | [jaredpar](https://github.com/jaredpar) |
-| Overload Resolution Priority | [features/VBOverloadResolutionPriority](https://github.com/dotnet/roslyn/tree/features/VBOverloadResolutionPriority) | In Progress | [AlekseyTs](https://github.com/AlekseyTs) | [333fred](https://github.com/333fred), [cston](https://github.com/cston) | (no IDE impact) | [333fred](https://github.com/333fred) |
+| Overload Resolution Priority | main | [Merged into 17.13 P3](https://github.com/dotnet/roslyn/pull/76419) | [AlekseyTs](https://github.com/AlekseyTs) | [333fred](https://github.com/333fred), [cston](https://github.com/cston) | (no IDE impact) | [333fred](https://github.com/333fred) |
 
 
 # C# 13.0

docs/compilers/CSharp/Compiler Breaking Changes - DotNet 10.md

@@ -19,7 +19,7 @@ Assert.Equal([2], x); // previously Assert.Equal<T>(T[], T[]), now ambiguous wit
 Assert.Equal([2], x.AsSpan()); // workaround
 
 var y = new int[] { 1, 2 };
-var s = new ArraySegment<int>(x, 1, 1);
+var s = new ArraySegment<int>(y, 1, 1);
 Assert.Equal(y, s); // previously Assert.Equal<T>(T, T), now ambiguous with Assert.Equal<T>(Span<T>, Span<T>)
 Assert.Equal(y.AsSpan(), s); // workaround
 ```
@@ -39,11 +39,26 @@ static class C
     public static void R<T>(IEnumerable<T> e) => Console.Write(1);
     public static void R<T>(Span<T> s) => Console.Write(2);
     // another workaround:
-    [OverloadResolutionPriority(1)]
     public static void R<T>(ReadOnlySpan<T> s) => Console.Write(3);
 }
 ```
 
+For that reason, `ReadOnlySpan<T>` is generally preferred over `Span<T>` by overload resolution in C# 14.
+In some cases, that might lead to compilation breaks,
+for example when there are overloads for both `Span<T>` and `ReadOnlySpan<T>`, both taking and returning the same span type:
+
+```cs
+double[] x = new double[0];
+Span<ulong> y = MemoryMarshal.Cast<double, ulong>(x); // previously worked, now compilation error
+Span<ulong> z = MemoryMarshal.Cast<double, ulong>(x.AsSpan()); // workaround
+
+static class MemoryMarshal
+{
+    public static ReadOnlySpan<TTo> Cast<TFrom, TTo>(ReadOnlySpan<TFrom> span) => default;
+    public static Span<TTo> Cast<TFrom, TTo>(Span<TFrom> span) => default;
+}
+```
+
 When using C# 14 or newer and targeting a .NET older than `net10.0`
 or .NET Framework with `System.Memory` reference,
 there is a breaking change with `Enumerable.Reverse` and arrays:
@@ -89,3 +104,126 @@ class C
     }
 }
 ```
+
+## Set state of enumerator object to "after" during disposal
+
+***Introduced in Visual Studio 2022 version 17.13***
+
+The state machine for enumerators incorrectly allowed resuming execution after the enumerator was disposed.  
+Now, `MoveNext()` on a disposed enumerator properly returns `false` without executing any more user code.
+
+```csharp
+var enumerator = C.GetEnumerator();
+
+Console.Write(enumerator.MoveNext()); // prints True
+Console.Write(enumerator.Current); // prints 1
+
+enumerator.Dispose();
+
+Console.Write(enumerator.MoveNext()); // now prints False
+
+class C
+{
+    public static IEnumerator<int> GetEnumerator()
+    {
+        yield return 1;
+        Console.Write("not executed after disposal")
+        yield return 2;
+    }
+}
+```
+
+## `UnscopedRefAttribute` cannot be used with old ref safety rules
+
+***Introduced in Visual Studio 2022 version 17.13***
+
+The `UnscopedRefAttribute` unintentionally affected code compiled by new Roslyn compiler versions
+even when the code was compiled in the context of the earlier ref safety rules
+(i.e., targeting C# 10 or earlier with net6.0 or earlier).
+However, the attribute should not have an effect in that context, and that is now fixed.
+
+Code that previously did not report any errors in C# 10 or earlier with net6.0 or earlier can now fail to compile:
+
+```cs
+using System.Diagnostics.CodeAnalysis;
+struct S
+{
+    public int F;
+
+    // previously allowed in C# 10 with net6.0
+    // now fails with the same error as if the [UnscopedRef] wasn't there:
+    // error CS8170: Struct members cannot return 'this' or other instance members by reference
+    [UnscopedRef] public ref int Ref() => ref F;
+}
+```
+
+To prevent misunderstanding (thinking the attribute has an effect
+but it actually does not because your code is compiled with the earlier ref safety rules),
+a warning is reported when the attribute is used in C# 10 or earlier with net6.0 or earlier:
+
+```cs
+using System.Diagnostics.CodeAnalysis;
+struct S
+{
+    // both are errors in C# 10 with net6.0:
+    // warning CS9269: UnscopedRefAttribute is only valid in C# 11 or later or when targeting net7.0 or later.
+    [UnscopedRef] public ref int Ref() => throw null!;
+    public static void M([UnscopedRef] ref int x) { }
+}
+```
+
+## Variance of `scoped` and `[UnscopedRef]` is more strict
+
+***Introduced in Visual Studio 2022 version 17.13***
+
+Scope can be changed when overriding a method, implementing an interface, or converting a lambda/method to a delegate under
+[some conditions](https://github.com/dotnet/csharplang/blob/05064c2a9567b7a58a07e526dff403ece1866541/proposals/csharp-11.0/low-level-struct-improvements.md#scoped-mismatch)
+(roughly, `scoped` can be added and `[UnscopedRef]` can be removed).
+Previously, the compiler did not report an error/warning for such mismatch under some circumstances, but it is now always reported.
+Note that the error is downgraded to a warning in `unsafe` contexts and also (in scenarios where it would be a breaking change) with LangVersion 12 or lower.
+
+```cs
+D1 d1 = (ref int i) => { }; // previously no mismatch error reported, now:
+                            // error CS8986: The 'scoped' modifier of parameter 'i' doesn't match target 'D1'.
+
+D2 d2 = (ref int i) => ref i; // an error was and continues to be reported:
+                              // error CS8986: The 'scoped' modifier of parameter 'i' doesn't match target 'D2'.
+
+delegate void D1(scoped ref int x);
+delegate ref int D2(scoped ref int x);
+```
+
+```cs
+using System.Diagnostics.CodeAnalysis;
+
+D1 d1 = ([UnscopedRef] ref int i) => { }; // previously no mismatch error reported, now:
+                                          // error CS8986: The 'scoped' modifier of parameter 'i' doesn't match target 'D1'.
+
+D2 d2 = ([UnscopedRef] ref int i) => ref i; // an error was and continues to be reported:
+                                            // error CS8986: The 'scoped' modifier of parameter 'i' doesn't match target 'D2'.
+
+delegate void D1(ref int x);
+delegate ref int D2(ref int x);
+```
+
+## `Microsoft.CodeAnalysis.EmbeddedAttribute` is validated on declaration
+
+***Introduced in Visual Studio 2022 version 17.13***
+
+The compiler now validates the shape of `Microsoft.CodeAnalysis.EmbeddedAttribute` when declared in source. Previously, the compiler
+would allow user-defined declarations of this attribute, but only when it didn't need to generate one itself. We now validate that:
+
+1. It must be internal
+2. It must be a class
+3. It must be sealed
+4. It must be non-static
+5. It must have an internal or public parameterless constructor
+6. It must inherit from System.Attribute.
+7. It must be allowed on any type declaration (class, struct, interface, enum, or delegate)
+
+```cs
+namespace Microsoft.CodeAnalysis;
+
+// Previously, sometimes allowed. Now, CS9271
+public class EmbeddedAttribute : Attribute {}
+```

docs/compilers/CSharp/Runtime Async Design.md

@@ -40,15 +40,22 @@ public static class RuntimeHelpers
 }
 ```
 
-Additionally, we use the following helper attributes to indicate information to the runtime. If these attributes are not present in the reference assemblies, we will generate them; the runtime matches by full
-name, not by type identity, so we do not need to care about using the "canonical" versions.
+We presume the following `MethodImplOptions` bit is present. This is used to indicate to the JIT that it should generate an async state machine for the method.
 
 ```cs
 namespace System.Runtime.CompilerServices;
 
-// Used to tell the runtime to generate the async state machinery for this method
-[AttributeUsage(AttributeTargets.Method)]
-public class RuntimeAsyncMethodAttribute() : Attribute();
+public enum MethodImplOptions
+{
+    Async = 1024
+}
+```
+
+Additionally, we use the following helper type to indicate information to the runtime. If this type is not present in the reference assemblies, we will generate it; the runtime matches by full
+name, not by type identity, so we do not need to care about using the "canonical" version.
+
+```cs
+namespace System.Runtime.CompilerServices;
 
 // Used to mark locals that should be hoisted to the generated async closure. Note that the runtime does not guarantee that all locals marked with this modreq will be hoisted; if it can prove that it
 // doesn't need to hoist a variable, it may avoid doing so.
@@ -67,7 +74,7 @@ public class RuntimeAsyncMethodGenerationAttribute(bool runtimeAsync) : Attribut
 
 ## Transformation strategy
 
-As mentioned previously, we try to expose as little of this to initial binding as possible. The one major exception to this is our handling of the `RuntimeAsyncMethodAttribute`; we do not let this be applied to
+As mentioned previously, we try to expose as little of this to initial binding as possible. The one major exception to this is our handling of the `MethodImplOption.Async`; we do not let this be applied to
 user code, and will issue an error if a user tries to do this by hand.
 
 Compiler generated async state machines and runtime generated async share some of the same building blocks. Both need to have `await`s with in `catch` and `finally` blocks rewritten to pend the exceptions,
@@ -98,7 +105,7 @@ async Task M()
 ```
 
 ```cs
-[RuntimeAsyncMethod, Experimental]
+[MethodImpl(MethodImplOptions.Async), Experimental]
 Task M()
 {
   // ... see lowering strategy for each kind of await below ...