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Use constant indices when populating Lists and arrays in collection expressions #80999

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Merged
merged 12 commits into from
Nov 5, 2025
Merged

Use constant indices when populating Lists and arrays in collection expressions #80999

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536a406
Initial plan
Copilot Nov 3, 2025
fcb6eb3
Use constant indices when populating span for List in collection expr…
Copilot Nov 3, 2025
c210cc9
Update src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewr…
CyrusNajmabadi Nov 3, 2025
738dcea
Update src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewr…
CyrusNajmabadi Nov 3, 2025
7aa7cab
Fix
CyrusNajmabadi Nov 3, 2025
877b12b
Add test for fixed elements before spread in collection expressions
Copilot Nov 4, 2025
bd54c95
Add WorkItem attribute to ListConstruction_FixedElementsBeforeSpread …
Copilot Nov 4, 2025
9ffa846
Apply constant indices optimization to arrays
Copilot Nov 4, 2025
f9b7579
Simplify code by using null state of indexTemp
Copilot Nov 4, 2025
b5ed80e
Improve documentation and inline condition check
Copilot Nov 4, 2025
38fdece
Add comments explaining that spreads always require runtime-tracked i…
Copilot Nov 5, 2025
2fab349
Add WorkItem attributes to all updated tests
Copilot Nov 5, 2025
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117 changes: 117 additions & 0 deletions src/Compilers/CSharp/Test/Emit3/Semantics/CollectionExpressionTests.cs
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Expand Up @@ -22291,6 +22291,123 @@ .maxstack 3
}
}

[Fact]
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@CyrusNajmabadi CyrusNajmabadi Nov 4, 2025

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@copilot add [Fact, WorkItem("...")]

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Copilot AI Nov 4, 2025

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Added WorkItem attribute in commit bd54c95.

public void ListConstruction_FixedElementsBeforeSpread()
{
// Test that when there are fixed elements before a spread element,
// we use an index variable for ALL elements (not constant indices).
string source = """
using System.Collections.Generic;
class Program
{
static void Main()
{
List<int> y = [5, 6];
List<int> x = [1, 2, ..y];
x.Report();
}
}
""";
var verifier = CompileAndVerify(
new[] { source, s_collectionExtensionsWithSpan },
targetFramework: TargetFramework.Net80,
verify: Verification.Skipped,
expectedOutput: IncludeExpectedOutput("[1, 2, 5, 6], "));
verifier.VerifyDiagnostics();

// Verify that we use an index variable (V_6) for all elements, including the fixed elements before the spread
verifier.VerifyIL("Program.Main", """
{
// Code size 167 (0xa7)
.maxstack 5
.locals init (int V_0,
System.Span<int> V_1,
int V_2,
System.Collections.Generic.List<int> V_3,
int V_4,
System.Span<int> V_5,
int V_6,
System.Span<int> V_7)
IL_0000: ldc.i4.2
IL_0001: stloc.0
IL_0002: ldloc.0
IL_0003: newobj "System.Collections.Generic.List<int>..ctor(int)"
IL_0008: dup
IL_0009: ldloc.0
IL_000a: call "void System.Runtime.InteropServices.CollectionsMarshal.SetCount<int>(System.Collections.Generic.List<int>, int)"
IL_000f: dup
IL_0010: call "System.Span<int> System.Runtime.InteropServices.CollectionsMarshal.AsSpan<int>(System.Collections.Generic.List<int>)"
IL_0015: stloc.1
IL_0016: ldloca.s V_1
IL_0018: ldc.i4.0
IL_0019: call "ref int System.Span<int>.this[int].get"
IL_001e: ldc.i4.5
IL_001f: stind.i4
IL_0020: ldloca.s V_1
IL_0022: ldc.i4.1
IL_0023: call "ref int System.Span<int>.this[int].get"
IL_0028: ldc.i4.6
IL_0029: stind.i4
IL_002a: ldc.i4.1
IL_002b: stloc.0
IL_002c: ldc.i4.2
IL_002d: stloc.2
IL_002e: stloc.3
IL_002f: ldc.i4.2
IL_0030: ldloc.3
IL_0031: callvirt "int System.Collections.Generic.List<int>.Count.get"
IL_0036: add
IL_0037: stloc.s V_4
IL_0039: ldloc.s V_4
IL_003b: newobj "System.Collections.Generic.List<int>..ctor(int)"
IL_0040: dup
IL_0041: ldloc.s V_4
IL_0043: call "void System.Runtime.InteropServices.CollectionsMarshal.SetCount<int>(System.Collections.Generic.List<int>, int)"
IL_0048: dup
IL_0049: call "System.Span<int> System.Runtime.InteropServices.CollectionsMarshal.AsSpan<int>(System.Collections.Generic.List<int>)"
IL_004e: stloc.s V_5
IL_0050: ldc.i4.0
IL_0051: stloc.s V_6
IL_0053: ldloca.s V_5
IL_0055: ldloc.s V_6
IL_0057: call "ref int System.Span<int>.this[int].get"
IL_005c: ldloc.0
IL_005d: stind.i4
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@CyrusNajmabadi CyrusNajmabadi Nov 4, 2025

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shows that we're still incrementing.

IL_005e: ldloc.s V_6
IL_0060: ldc.i4.1
IL_0061: add
IL_0062: stloc.s V_6
IL_0064: ldloca.s V_5
IL_0066: ldloc.s V_6
IL_0068: call "ref int System.Span<int>.this[int].get"
IL_006d: ldloc.2
IL_006e: stind.i4
IL_006f: ldloc.s V_6
IL_0071: ldc.i4.1
IL_0072: add
IL_0073: stloc.s V_6
IL_0075: ldloc.3
IL_0076: call "System.Span<int> System.Runtime.InteropServices.CollectionsMarshal.AsSpan<int>(System.Collections.Generic.List<int>)"
IL_007b: stloc.s V_7
IL_007d: ldloca.s V_7
IL_007f: ldloca.s V_5
IL_0081: ldloc.s V_6
IL_0083: ldloca.s V_7
IL_0085: call "int System.Span<int>.Length.get"
IL_008a: call "System.Span<int> System.Span<int>.Slice(int, int)"
IL_008f: call "void System.Span<int>.CopyTo(System.Span<int>)"
IL_0094: ldloc.s V_6
IL_0096: ldloca.s V_7
IL_0098: call "int System.Span<int>.Length.get"
IL_009d: add
IL_009e: stloc.s V_6
IL_00a0: ldc.i4.0
IL_00a1: call "void CollectionExtensions.Report(object, bool)"
IL_00a6: ret
}
""");
}

[CombinatorialData]
[Theory]
public void ListConstruction_02(
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