[type-layout] Document minimum size and alignment

src/type-layout.md

@@ -32,6 +32,28 @@ as [dynamically sized types]. Since all values of a `Sized` type share the same
 size and alignment, we refer to those shared values as the size of the type and
 the alignment of the type respectively.
 
+### Minimum Size and Alignment
+
+Every type, including [dynamically sized types], has a "minimum size" and "minimum
+alignment." Every value of a type must have a size at least as large as the type's
+minimum size and an alignment at least as large as the type's minimum alignment.
+Some notable cases of minimum size and alignment are:
+- For [`Sized`] types, the type's minimum size and minimum alignment
+  are always equal to the type's size and alignment
+- For a [slice type](#slice-layout), `[T]`, the minimum size is 0 bytes (corresponding
+  a slice with 0 elements), and the minimum alignment is the alignment of `T`
+- For a [trait object](#trait-object-layout), the minimum size is 0 and the minimum
+  alignment is 1
+- For a struct type with a dynamically-sized field, the minimum size is taken to be
+  the size of the struct when the dynamically-sized field has *its* minimum size.
+  The struct's minimum alignment is taken to be the alignment of the struct when the
+  dynamically-sized field has *its* minimum alignment. When relying on this property
+  of dynamically-sized structs, be careful not to assume too much! For example,
+  this property provides *no* guarantees about [`repr(Rust)`](#the-rust-representation)
+  types, as such a type can have arbitrarily large size and alignment regardless of
+  the sizes and alignments of its fields
+- Every type's minimum size is less than or equal to `isize::MAX`
+
 ## Primitive Data Layout
 
 The size of most primitives is given in this table.