Adjust inline-arrays.md based on LDM decisions (#7177)
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@ -253,6 +253,119 @@ of inline array types.
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Regular definite assignment rules are applicable to variables that have an inline array type.
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### Collection literals
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An inline array type is a valid *constructible collection* target type for a [collection literal](https://github.com/dotnet/csharplang/blob/main/proposals/collection-literals.md).
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For example:
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``` C#
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int[5] a = [1, 2, 3, 4, 5]; // initializes anonymous inline array of length 5
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Buffer10<int> b = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; // initializes user-defined inline array
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var d = new int[][2] {[11, 12], [21, 22], [31, 32]}; // regular array with an element type of an anonymous inline array type with element type int and length 2
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```
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The length of the collection literal must match the length of the target inline array type. If the length of the literal
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is known at compile time and it doesn't match the target length, an error is reported. Otherwise, an exception is going
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to be thrown at runtime once the mismatch is encountered. The exact exception type is TBD. Some candidates are:
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System.NotSupportedException, System.InvalidOperationException.
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An instance of an inline array type is a valid expression in a [*spread_element*](https://github.com/dotnet/csharplang/blob/main/proposals/collection-literals.md#detailed-design).
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### Validation of the InlineArrayAttribute applications
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Compiler will validate the following aspects of the InlineArrayAttribute applications:
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- The target type is a struct
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- The target type has only one field
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- Specified length > 0
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- The target struct doesn't have an explicit layout specified
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### Inline Array elements in an object initializer
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By default, element initialization will not be supported via *initializer_target* of form ```'[' argument_list ']'```
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(see https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/expressions.md#117153-object-initializers):
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``` C#
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static C M2() => new C() { F = {[0] = 111} }; // error CS1913: Member '[0]' cannot be initialized. It is not a field or property.
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class C
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{
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public Buffer10<int> F;
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}
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```
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However, if the inline array type explicitly defines suitable indexer, object initializer will use it:
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``` C#
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static C M2() => new C() { F = {[0] = 111} }; // Ok, indexer is invoked
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class C
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{
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public Buffer10<int> F;
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}
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[System.Runtime.CompilerServices.InlineArray(10)]
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public struct Buffer10<T>
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{
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private T _element0;
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public T this[int i]
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{
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get => this[i];
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set => this[i] = value;
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}
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}
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```
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### The foreach statement
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[The foreach statement](https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/statements.md#1295-the-foreach-statement) will be adjusted
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to allow usage of an inline array type as a collection in a foreach statement.
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For example:
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``` C#
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foreach (var a in getBufferAsValue())
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{
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WriteLine(a);
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}
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foreach (var b in getBufferAsWritableVariable())
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{
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WriteLine(b);
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}
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foreach (var c in getBufferAsReadonlyVariable())
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{
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WriteLine(c);
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}
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Buffer10<int> getBufferAsValue() => default;
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ref Buffer10<int> getBufferAsWritableVariable() => default;
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ref readonly Buffer10<int> getBufferAsReadonlyVariable() => default;
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```
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is equivalent to:
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``` C#
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Buffer10<int> temp = getBufferAsValue();
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foreach (var a in (System.ReadOnlySpan<int>)temp)
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{
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WriteLine(a);
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}
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foreach (var b in (System.Span<int>)getBufferAsWritableVariable())
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{
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WriteLine(b);
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}
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foreach (var c in (System.ReadOnlySpan<int>)getBufferAsReadonlyVariable())
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{
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WriteLine(c);
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}
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```
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## Open design questions
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## Alternatives
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### Inline array type syntax
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The grammar at https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/types.md#821-general will be adjusted as follows:
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@ -365,94 +478,6 @@ var c = new int[][] {{11, 12}, {21, 22}, {31, 32}}; // An error for the nested a
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var d = new int[][2] {{11, 12}, {21, 22}, {31, 32}}; // An error for the nested array initializer
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```
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#### Collection literals
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An inline array type is a valid *constructible collection* target type for a [collection literal](https://github.com/dotnet/csharplang/blob/main/proposals/collection-literals.md).
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For example:
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``` C#
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int[5] a = [1, 2, 3, 4, 5]; // initializes anonymous inline array of length 5
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Buffer10<int> b = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; // initializes user-defined inline array
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var d = new int[][2] {[11, 12], [21, 22], [31, 32]}; // regular array with an element type of an anonymous inline array type with element type int and length 2
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```
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The length of the collection literal must match the length of the target inline array type. If the length of the literal
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is known at compile time and it doesn't match the target length, an error is reported. Otherwise, an exception is going
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to be thrown at runtime once the mismatch is encountered. The exact exception type is TBD. Some candidates are:
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System.NotSupportedException, System.InvalidOperationException.
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An instance of an inline array type is a valid expression in a [*spread_element*](https://github.com/dotnet/csharplang/blob/main/proposals/collection-literals.md#detailed-design).
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### The foreach statement
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[The foreach statement](https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/statements.md#1295-the-foreach-statement) will be adjusted
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to allow usage of an inline array type as a collection in a foreach statement.
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For example:
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``` C#
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foreach (var a in getBufferAsValue())
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{
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WriteLine(a);
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}
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foreach (var b in getBufferAsWritableVariable())
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{
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WriteLine(b);
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}
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foreach (var c in getBufferAsReadonlyVariable())
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{
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WriteLine(c);
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}
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Buffer10<int> getBufferAsValue() => default;
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ref Buffer10<int> getBufferAsWritableVariable() => default;
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ref readonly Buffer10<int> getBufferAsReadonlyVariable() => default;
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```
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is equivalent to:
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``` C#
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Buffer10<int> temp = getBufferAsValue();
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foreach (var a in (System.ReadOnlySpan<int>)temp)
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{
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WriteLine(a);
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}
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foreach (var b in (System.Span<int>)getBufferAsWritableVariable())
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{
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WriteLine(b);
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}
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foreach (var c in (System.ReadOnlySpan<int>)getBufferAsReadonlyVariable())
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{
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WriteLine(c);
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}
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```
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## Open design questions
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### Should compiler validate applications of the InlineArrayAttribute?
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Possible things we can check:
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- The type is a struct (probably just a warning)
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- Has only one field
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- Specified length > 0
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- The struct doesn't have an explicit layout specified
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### Should we support initialization of Inline Array elements in an object initializer?
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``` C#
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static C M2() => new C() { F = {[0] = 111} };
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class C
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{
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public Buffer10<int> F;
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}
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```
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## Alternatives
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### Detailed Design (Option 2)
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Note, that for the purpose of this proposal a term "fixed-size buffer" refers to a the proposed "safe fixed-size buffer" feature rather than to a buffer described at https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/unsafe-code.md#228-fixed-size-buffers.
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@ -680,3 +705,11 @@ but can be made into one if necessary. Here’s how that would work:
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C/C++ has a different notion of fixed-size buffers. For example, there is a notion of "zero-length fixed sized buffers",
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which is often used as a way to indicate that the data is "variable length". It is not a goal of this proposal to be
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able to interop with that.
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## LDM meetings
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- https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-04-03.md#fixed-size-buffers
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- https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-04-10.md#fixed-size-buffers
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- https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-05-01.md#fixed-size-buffers
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- https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-05-03.md#inline-arrays
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