[LibraryImportGenerator] Reduce unnecessary casting/locals in pinning path

[elinor-fung]

Contributes to #69608: Style item (6)

UTF-16 string:

[LibraryImport("lib", StringMarshalling = StringMarshalling.Utf16)]
internal static partial void ManyStrings(string s1, string s2, string s3, string s4, string s5);

// Generated
internal static partial void ManyStrings(string s1, string s2, string s3, string s4, string s5)
{
    // Pin - Pin data in preparation for calling the P/Invoke.
    fixed (void* __s1_native = s1)
        fixed (void* __s2_native = s2)
            fixed (void* __s3_native = s3)
                fixed (void* __s4_native = s4)
                    fixed (void* __s5_native = s5)
                    {
                        __PInvoke(__s1_native, __s2_native, __s3_native, __s4_native, __s5_native);
                    }

    // Local P/Invoke
    [System.Runtime.InteropServices.DllImportAttribute("lib", EntryPoint = "ManyStrings", ExactSpelling = true)]
    static extern unsafe void __PInvoke(void* s1, void* s2, void* s3, void* s4, void* s5);
}

User-defined:

runtime/src/libraries/System.Runtime.InteropServices/tests/TestAssets/SharedTypes/NonBlittable.cs

Lines 98 to 104 in d143e18

	[NativeMarshalling(typeof(IntWrapperMarshaler))]
	public class IntWrapper
	{
	public int i;
	public ref int GetPinnableReference() => ref i;
	}

[LibraryImport("lib")]
public static partial void UserDefinedPinnable(SharedTypes.IntWrapper i1, SharedTypes.IntWrapper i2);

// Generated
public static partial void UserDefinedPinnable(global::SharedTypes.IntWrapper i1, global::SharedTypes.IntWrapper i2)
{
    // Pin - Pin data in preparation for calling the P/Invoke.
    fixed (void* __i1_native = i1)
        fixed (void* __i2_native = i2)
        {
            __PInvoke((int*)__i1_native, (int*)__i2_native);
        }

    // Local P/Invoke
    [System.Runtime.InteropServices.DllImportAttribute("lib", EntryPoint = "UserDefinedPinnable", ExactSpelling = true)]
    static extern unsafe void __PInvoke(int* i1, int* i2);
}

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Issue Details

---

Contributes to #69608: Style item (6)

UTF-16 string:

[LibraryImport("lib", StringMarshalling = StringMarshalling.Utf16)]
internal static partial void ManyStrings(string s1, string s2, string s3, string s4, string s5);

// Generated
internal static partial void ManyStrings(string s1, string s2, string s3, string s4, string s5)
{
    // Pin - Pin data in preparation for calling the P/Invoke.
    fixed (void* __s1_native = s1)
        fixed (void* __s2_native = s2)
            fixed (void* __s3_native = s3)
                fixed (void* __s4_native = s4)
                    fixed (void* __s5_native = s5)
                    {
                        __PInvoke(__s1_native, __s2_native, __s3_native, __s4_native, __s5_native);
                    }

    // Local P/Invoke
    [System.Runtime.InteropServices.DllImportAttribute("lib", EntryPoint = "ManyStrings", ExactSpelling = true)]
    static extern unsafe void __PInvoke(void* s1, void* s2, void* s3, void* s4, void* s5);
}

User-defined:

runtime/src/libraries/System.Runtime.InteropServices/tests/TestAssets/SharedTypes/NonBlittable.cs

Lines 98 to 104 in d143e18

	[NativeMarshalling(typeof(IntWrapperMarshaler))]
	public class IntWrapper
	{
	public int i;
	public ref int GetPinnableReference() => ref i;
	}

[LibraryImport("lib")]
public static partial void UserDefinedPinnable(SharedTypes.IntWrapper i1, SharedTypes.IntWrapper i2);

// Generated
public static partial void UserDefinedPinnable(global::SharedTypes.IntWrapper i1, global::SharedTypes.IntWrapper i2)
{
    // Pin - Pin data in preparation for calling the P/Invoke.
    fixed (void* __i1_native = i1)
        fixed (void* __i2_native = i2)
        {
            __PInvoke((int*)__i1_native, (int*)__i2_native);
        }

    // Local P/Invoke
    [System.Runtime.InteropServices.DllImportAttribute("lib", EntryPoint = "UserDefinedPinnable", ExactSpelling = true)]
    static extern unsafe void __PInvoke(int* i1, int* i2);
}

Author:	elinor-fung
Assignees:	-
Labels:	area-System.Runtime.InteropServices, source-generator
Milestone:	7.0.0

[am11]

Do we have such control over indentation which would make it (conveniently) generate stacked fixed statements like this:

    // Pin - Pin data in preparation for calling the P/Invoke.
    fixed (void* __s1_native = s1)
    fixed (void* __s2_native = s2)
    fixed (void* __s3_native = s3)
    fixed (void* __s4_native = s4)
    fixed (void* __s5_native = s5)
    {
        __PInvoke(__s1_native, __s2_native, __s3_native, __s4_native, __s5_native);
    }

[elinor-fung]

We are relying on Roslyn's NormalizeWhitespace to format / apply indentation accordingly to the entire block of generated code, so we don't fully control the indentation like that. I opened dotnet/roslyn#61518 to track potentially improving the behaviour around cases like this.

[elinor-fung]

Failures are #69792.