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proposals/safe-fixed-sized-buffers.md

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+Safe Fixed Size Buffers
+=====
+
+## Summary
+
+Provide a general-purpose and safe mechanism for declaring fixed sized buffers within C# classes, structs and interfaces.
+
+## Motivation
+
+This proposal plans to address the many limitations of https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/unsafe-code.md#228-fixed-size-buffers.
+Specifically it aims to allow the declaration of safe `fixed` buffers for managed and unmanaged types in a `struct`, `class`, or `interface`.
+And provide language safety verification for them.
+
+## Detailed Design 
+
+Grammar for `variable_declarator` in https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/classes.md#145-fields
+will be extended to allow specifying the size of the buffer:
+
+``` antlr
+field_declaration
+    : attributes? field_modifier* type variable_declarators ';'
+    ;
+
+field_modifier
+    : 'new'
+    | 'public'
+    | 'protected'
+    | 'internal'
+    | 'private'
+    | 'static'
+    | 'readonly'
+    | 'volatile'
+    | unsafe_modifier   // unsafe code support
+    ;
+
+variable_declarators
+    : variable_declarator (',' variable_declarator)*
+    ;
+
+variable_declarator
+    : identifier ('=' variable_initializer)?
+    | fixed_size_buffer_declarator
+    ;
+
+fixed_size_buffer_declarator
+    : identifier '[' constant_expression ']'
+    ;    
+```
+
+A `fixed_size_buffer_declarator` introduces a fixed-size buffers of a given element type.
+
+The buffer element type is the `type` specified in `field_declaration`. A fixed-size buffer declarator introduces a new member and consists of an identifier that names the member, followed by a constant expression enclosed in `[` and `]` tokens. The constant expression denotes the number of elements in the member introduced by that fixed-size buffer declarator. The type of the constant expression shall be implicitly convertible to type `int`, and the value shall be a non-zero positive integer.
+
+The elements of a fixed-size buffer shall be laid out sequentially in memory as though they are elements of an array.
+
+A `volatile` modifier cannot be used with `fixed_size_buffer_declarator`. A System.ThreadStaticAttribute cannot be applied to a field declaration like that.
+
+Depending on the situation (details are specified below), an access to a fixes-size buffer member is classified as a value (never a variable) of either
+`System.ReadOnlySpan<S>` or `System.Span<S>`, where S is the element type of the fixed-size buffer. Both types provide indexers returning refernce to a
+specific element with appropriate "readonly-ness". Which prevents direct assignment to the elements when language rules don't permit that.
+
+The resulting span instance will have a length equal to the size declared on the fixed-size buffer.
+Indexing into the span with a constant expression outside of the declared fixed-size buffer bounds is a compile time error.
+
+The `safe-to-escape` scope of the value will be equal to the `safe-to-escape` scope of the container, just as it would if the backing data was accessed as a field.
+
+### Fixed-size buffers in expressions
+
+Member lookup of a fixed-size buffer member proceeds exactly like member lookup of a field.
+
+A fixed-size buffer can be referenced in an expression using a *simple_name* or a *member_access* .
+
+When an instance fixed-size buffer member is referenced as a simple name, the effect is the same as a member access of the form `this.I`, where `I` is the fixed-size buffer member. When a static fixed-size buffer member is referenced as a simple name, the effect is the same as a member access of the form `E.I`, where `I` is the fixed-size buffer member and `E` is the declaring type.
+
+#### Non-readonly fixed-size buffers
+
+In a member access of the form `E.I`, if `E` is of a struct type and a member lookup of `I` in that struct type identifies an instance fixed-size member,
+then `E.I` is evaluated and classified as follows:
+
+- If `E` is classified as a value, the result of the expression is classified as a value of type `System.ReadOnlySpan<S>`, where S is the element type of `I`.
+  The value can be used to access members’ elements.
+- Otherwise, `E` is classified as a variable and the result of the expression is classified as a value of type `System.Span<S>`, where S is the element type of `I`.
+  The value can be used to access members’ elements.
+
+In a member access of the form `E.I`, if `E` is of a class type and a member lookup of `I` in that class type identifies an instance fixed-size member,
+then `E.I` is evaluated and classified as a value of type `System.Span<S>`, where S is the element type of `I`.
+
+In a member access of the form `E.I`, if member lookup of `I` identifies a static fixed-size member,
+then `E.I` is evaluated and classified as a value of type `System.Span<S>`, where S is the element type of `I`.
+
+#### Readonly fixed-size buffers
+
+When a *field_declaration* includes a `readonly` modifier, the member introduced by the fixed_size_buffer_declarator is a ***readony fixed-size buffer***.
+Direct assignments to elements of a readonly fixed-size buffer can only occur in an instance constructor, init member or static constructor in the same type.
+Specifically, direct assignments to an element of readonly fixed-size buffer are permitted only in the following contexts:
+
+- For an instance member, in the instance constructors or init member of the type that contains the member declaration; for a static member,
+  in the static constructor of the type that contains the member declaration. These are also the only contexts in which it is valid to pass
+  an element of readonly fixed-size buffer as an `out` or `ref` parameter.
+
+Attempting to assign to an element of a readonly fixed-size buffer or pass it as an `out` or `ref` parameter in any other context is a compile-time error.
+This is achieved by the following.
+
+A member access for a readonly fixed-size buffer is evaluated and classified as follows:
+
+- If access occurs in a context where direct assignments to an element of readonly fixed-size buffer are permitted, the result of the expression is classified as a value of type `System.Span<S>`, where S is the element type of the fixed-size buffer.
+  The value can be used to access members’ elements.
+- Otherwise, the expression is classified as a value of type `System.ReadOnlySpan<S>`, where S is the element type of the fixed-size buffer.
+  The value can be used to access members’ elements.
+
+### Definite assignment checking
+
+Fixed-size buffers are not subject to definite assignment-checking, and fixed-size buffer members are ignored for purposes of definite-assignment checking of struct type variables.
+
+When a fixed-size buffer member is static or the outermost containing struct variable of a fixed-size buffer member is a static variable, an instance variable of a class instance, or an array element, the elements of the fixed-size buffer are automatically initialized to their default values. In all other cases, the initial content of a fixed-size buffer is undefined.