Toplevel module M = await (P: S) compiles to a static import #8815

Found by adversarial experiments on semantic analysts.

What is broken

The parsetree and Ast_await describe await (M: S) with a module-type path S as a dynamic import:

• https://github.com/rescript-lang/rescript/blob/8d51b9ce7/compiler/ml/parsetree.ml#L770-L772
• https://github.com/rescript-lang/rescript/blob/8d51b9ce7/compiler/ml/ast_await.ml#L39-L44

let module handles that form (bs_builtin_ppx.ml#L417-L439). The toplevel Pstr_module case rewrites only the unconstrained form (L763-L794). module_expr_mapper then strips the await (L854-L856), so the module is imported statically.

Reproduction

P.res: let x = 1

Main.res:

module type S = {let x: int}
module A = await P
module B = await (P: S)
let f = async () => {
  module C = await (P: S)
  C.x
}

Commands (with Primitive_promise.res copied from the runtime and a one-line Primitive_module.res containing external import: 'a => promise<'a> = "%import", each compiled first with bsc -nopervasives):

bsc -nopervasives P.res > P.js
bsc -nopervasives -I . Main.res

Output at master:

let P = require("./p.js");
let A = await import("./p.js");
async function f() {
  return (await import("./p.js")).x;
}
let B = P;

Expected: B is a dynamic import like A and the local C, for example let B = await import("./p.js");, with no static require of P.

Introduced

ece8b14 already compiles it this way. The comments describing it as a dynamic import come from #8738 (23a4197).

Impact

Writing module M = await (P: S) at toplevel puts P in the static import graph, so it is not code-split, and nothing reports it. The same constrained form inside a function is a dynamic import. Either the toplevel case should handle a module-type path like let module does, or the comments should state the restriction.