Remove constraints on stackalloc (support new locations) (#1211)

* add implicit conversion of T* to Span<T>

* Remove constraints on location of stackalloc expressions

* Support stackalloc expressions anywhere for C# 8.

* Remove incorrect conversions.

* Update standard/expressions.md

Co-authored-by: Bill Wagner <[email protected]>

* Update standard/unsafe-code.md

Co-authored-by: Bill Wagner <[email protected]>

---------

Co-authored-by: Rex Jaeschke <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>

standard/expressions.md

@@ -3275,8 +3275,7 @@ The safe context rules for a stack allocation expression are described in [§16.
 ```ANTLR
 stackalloc_expression
     : 'stackalloc' unmanaged_type '[' expression ']'
-    | 'stackalloc' unmanaged_type? '[' constant_expression? ']'
-      stackalloc_initializer
+    | 'stackalloc' unmanaged_type? '[' constant_expression? ']' stackalloc_initializer
     ;
 
 stackalloc_initializer
@@ -3292,18 +3291,6 @@ stackalloc_element_initializer
     ;
 ```
 
-<!-- The following restrictions apply to C# 7.3, they are relaxed in C# 8 -->
-A *stackalloc_expression* is only permitted in two contexts:
-
-1. The initializing *expression*, `E`, of a *local_variable_declaration* ([§13.6.2](statements.md#1362-local-variable-declarations)); and
-2. The right operand *expression*, `E`, of a simple assignment ([§12.21.2](expressions.md#12212-simple-assignment)) which itself occurs as a *expression_statement* ([§13.7](statements.md#137-expression-statements))
-
-In both contexts the *stackalloc_expression* is only permitted to occur as:
-
-- The whole of `E`; or
-- The second and/or third operands of a *conditional_expression* ([§12.18](expressions.md#1218-conditional-operator)) which is itself the whole of `E`.
-<!-- End of C# 7.3 restrictions -->
-
 The *unmanaged_type* ([§8.8](types.md#88-unmanaged-types)) indicates the type of the items that will be stored in the newly allocated location, and the *expression* indicates the number of these items. Taken together, these specify the required allocation size. The type of *expression* shall be implicitly convertible to the type `int`.
 
 As the size of a stack allocation cannot be negative, it is a compile-time error to specify the number of items as a *constant_expression* that evaluates to a negative value.
@@ -3318,14 +3305,12 @@ When a *stackalloc_initializer* is present:
 
 Each *stackalloc_element_initializer* shall have an implicit conversion to *unmanaged_type* ([§10.2](conversions.md#102-implicit-conversions)). The *stackalloc_element_initializer*s initialize elements in the allocated memory in increasing order, starting with the element at index zero. In the absence of a *stackalloc_initializer*, the content of the newly allocated memory is undefined.
 
-The result of a *stackalloc_expression* is an instance of type `Span<T>`, where `T` is the *unmanaged_type*:
+If a *stackalloc_expression* occurs directly as the initializing expression of a *local_variable_declaration* ([§13.6.2](statements.md#1362-local-variable-declarations)), where the *local_variable_type* is either a pointer type ([§23.3](unsafe-code.md#233-pointer-types)) or inferred (`var`), then the result of the *stackalloc_expression* is a pointer of type `T*` (§23.9). In this case the *stackalloc_expression* must appear in unsafe code. Otherwise the result of a *stackalloc_expression* is an instance of type `Span<T>`, where `T` is the *unmanaged_type*:
 
 - `Span<T>` ([§C.3](standard-library.md#c3-standard-library-types-not-defined-in-isoiec-23271)) is a ref struct type ([§16.2.3](structs.md#1623-ref-modifier)), which presents a block of memory, here the block allocated by the *stackalloc_expression*, as an indexable collection of typed (`T`) items.
 - The result’s `Length` property returns the number of items allocated.
 - The result’s indexer ([§15.9](classes.md#159-indexers)) returns a *variable_reference* ([§9.5](variables.md#95-variable-references)) to an item of the allocated block and is range checked.
 
-> *Note*: When occurring in unsafe code the result of a *stackalloc_expression* may be of a different type, see ([§23.9](unsafe-code.md#239-stack-allocation)). *end note*
-
 Stack allocation initializers are not permitted in `catch` or `finally` blocks ([§13.11](statements.md#1311-the-try-statement)).
 
 > *Note*: There is no way to explicitly free memory allocated using `stackalloc`. *end note*

standard/unsafe-code.md

@@ -1049,9 +1049,7 @@ When the outermost containing struct variable of a fixed-size buffer member is a
 
 See [§12.8.22](expressions.md#12822-stack-allocation) for general information about the operator `stackalloc`. Here, the ability of that operator to result in a pointer is discussed.
 
-In an unsafe context if a *stackalloc_expression* ([§12.8.22](expressions.md#12822-stack-allocation)) occurs as the initializing expression of a *local_variable_declaration* ([§13.6.2](statements.md#1362-local-variable-declarations)), where the *local_variable_type* is either a pointer type ([§23.3](unsafe-code.md#233-pointer-types)) or inferred (`var`), then the result of the *stackalloc_expression* is a pointer of type `T *` to be beginning of the allocated block, where `T` is the *unmanaged_type* of the *stackalloc_expression*.
-
-In all other respects the semantics of *local_variable_declaration*s ([§13.6.2](statements.md#1362-local-variable-declarations)) and *stackalloc_expression*s ([§12.8.22](expressions.md#12822-stack-allocation)) in unsafe contexts follow those defined for safe contexts.
+When a *stackalloc_expression* occurs as the initializing expression of a *local_variable_declaration* ([§13.6.2](statements.md#1362-local-variable-declarations)), where the *local_variable_type* is either a pointer type ([§23.3](unsafe-code.md#233-pointer-types)) or inferred (`var`), the result of the *stackalloc_expression* is a pointer of type `T*`, where `T` is the *unmanaged_type* of the *stackalloc_expression*. In this case the result is a pointer to be beginning of the allocated block.
 
 > *Example*:
 >