Merge pull request #2006 from xushiwei/q

mini spec: size and alignment guarantees

Author: xushiwei

doc/spec-mini.md

@@ -1984,4 +1984,26 @@ The function `StringData` returns a pointer to the underlying bytes of the `str`
 
 ### Size and alignment guarantees
 
+For the [numeric types](#numeric-types), the following sizes are guaranteed:
+
+```go
+type                                 size in bytes
+
+byte, uint8, int8                     1
+uint16, int16                         2
+uint32, int32, float32                4
+uint64, int64, float64, complex64     8
+complex128                           16
+```
+
+The following minimal alignment properties are guaranteed:
+
+* For a variable `x` of any type: `unsafe.Alignof(x)` is at least 1.
+* For a variable `x` of struct type: `unsafe.Alignof(x)` is the largest of all the values `unsafe.Alignof(x.f)` for each field `f` of `x`, but at least 1.
+* For a variable `x` of array type: `unsafe.Alignof(x)` is the same as the alignment of a variable of the array's element type.
+
+A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.
+
+## Appendix
+
 TODO