Document new implementation-defined limits (#360)

Based on discussion in #335 and subgroup meetings.

document/js-api/index.bs

@@ -1398,6 +1398,8 @@ In practice, an implementation may run out of resources for valid modules below
 <ul>
 <li>The maximum size of a module is 1073741824 bytes (1 GiB).</li>
 <li>The maximum number of types defined in the types section is 1000000.</li>
+<li>The maximum number of recursion groups defined in the types sections is 1000000.</li>
+<li>The maximum depth of a defined subtype hierarchy is 63 (where a type defined with no supertype has depth 0).
 <li>The maximum number of functions defined in a module is 1000000.</li>
 <li>The maximum number of imports declared in a module is 100000.</li>
 <li>The maximum number of exports declared in a module is 100000.</li>
@@ -1413,6 +1415,8 @@ In practice, an implementation may run out of resources for valid modules below
 <li>The maximum number of return values for any function or block is 1000.</li>
 <li>The maximum size of a function body, including locals declarations, is 7654321 bytes.</li>
 <li>The maximum number of locals declared in a function, including implicitly declared as parameters, is 50000.</li>
+<li>The maximum number of fields in a struct is 10000.</li>
+<li>The maximum number of operands to `array.new_fixed` is 10000.</li>
 </ul>
 
 An implementation must throw a {{RuntimeError}} if one of the following limits is exceeded during runtime:

proposals/gc/MVP-JS.md

@@ -69,3 +69,16 @@ internal reference value that is being externalized:
 
 _TODO: avoid having to patch the behavior of `extern.internalize` and
 `extern.internalize` by converting to/from JS numbers separately._
+
+## Implementation-defined Limits
+
+The following limits will be added to the Implementation-defined Limits
+[section](https://webassembly.github.io/spec/js-api/index.html#limits) of the JS
+API.
+
+ - The maximum number of recursion groups is 1000000. (The maximum number of
+   individual types remains unchanged and is also 1000000.)
+ - The maximum number of struct fields is 10000.
+ - The maximum number of operands to `array.new_fixed` is 10000.
+ - The maximum length of a supertype chain is 63. (A type declared with no
+   supertypes has a supertype chain of length 0)

proposals/gc/MVP.md

@@ -108,7 +108,7 @@ New abbreviations are introduced for reference types in binary and text format,
   - `module ::= {..., types vec(<deftype>)}`
   - a `rec` definition defines a group of mutually recursive types that can refer to each other; it thereby defines several type indices at a time
   - a single type definition, as in Wasm before this proposal, is reinterpreted as a short-hand for a recursive group containing just one type
-  - Note that the number of type section entries is now the number of recursion groups rather than the number of individual types. 
+  - Note that the number of type section entries is now the number of recursion groups rather than the number of individual types.
 
 * `subtype` is a new category of type defining a single type, as a subtype of possible other types
   - `subtype ::= sub final? <typeidx>* <strtype>`
@@ -782,8 +782,7 @@ The opcode for heap types is encoded as an `s33`.
 
 ## JS API
 
-See [GC JS API document](MVP-JS.md) .
-
+See [GC JS API document](MVP-JS.md).
 
 ## Questions