RFC: Enable depending on NPM packages

text/008-npm-dependencies.md

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+- Start Date: 2018-02-14
+- RFC PR: (leave this empty)
+- Tracking Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Enable Rust crates to transparently depend on packages in the npm ecosystem.
+These dependencies will, like normal Rust dependencies through Cargo, work
+seamlessly when consumed by other crates.
+
+# Motivation
+[motivation]: #motivation
+
+The primary goal of `wasm-bindgen` and `wasm-pack` is to enable seamless
+integration of Rust with JS. A massive portion of the JS ecosystem, npm, however
+currently has little support in `wasm-bindgen` and `wasm-pack`, making it
+difficult to access this rich resource that JS offers!
+
+The goal of this RFC is to enable these dependencies to exist. Rust crates
+should be able to require functionality from NPM, just like how NPM can require
+Rust crates compiled to wasm. Any workflow which currently uses NPM packages
+(such as packaging WebAssembly with a bundler) should continue to work but also
+allow pulling in "custom" NPM packages as well as requested by Rust
+dependencies.
+
+# Stakeholders
+[stakeholders]: #stakeholders
+
+This RFC primarily affects uses of `wasm-pack` and `wasm-bindgen` who are also
+currently using bundlers like Webpack. This also affects, however, developers of
+core foundational crates in the Rust ecosystem who want to be concious of the
+ability to pull in NPM dependencies and such.
+
+# Detailed Explanation
+[detailed-explanation]: #detailed-explanation
+
+Adding an NPM dependency to a Rust project will look very similar to adding an
+NPM dependency to a normal JS project. First the dependency, and its version
+requirement, need to be declare. This RFC proposes doing this in a
+`package.json` file adjacent to the crate's `Cargo.toml` file:
+
+```json
+ {
+  "dependencies": {
+    "foo": "^1.0.1"
+  }
+}
+```
+
+The `package.json` file will initially be a subset of NPM's `package.json`,
+only supporting one `dependencies` top-level key which internally has key/value
+pairs with strings. Beyond this validation though no validation will be
+performed of either key or value pairs within `dependencies`. In the future
+it's intended that more keys of `package.json` in NPM will be supported, but
+this RFC is intended to be an MVP for now to enable dependencies on NPM at all.
+
+After this `package.json` file is created, the package next needs to be
+imported in the Rust crate. Like with other Rust dependencies on JS, this will
+be done with the `#[wasm_bindgen]` attribute:
+
+```rust
+#[wasm_bindgen(module = "foo")]
+extern "C" {
+    fn function_in_foo_package();
+}
+```
+
+> **Note**: in JS the above import would be similar to:
+>
+> ```js
+> import { function_in_foo_package } from "foo";
+> ```
+
+The exiting `module` key in the `#[wasm_bindgen]` attribute can be used to
+indicate which ES module the import is coming from. This affects the `module`
+key in the final output wasm binary, and corresponds to the name of the package
+in `package.json`. This is intended to match how bundler conventions already
+interpret NPM packages as ES modules.
+
+After these two tools are in place, all that's needed is a `wasm-pack build` and
+you should be good to go! The final `package.json` will have the `foo`
+dependency listed in our `package.json` above and be ready for consumption via a
+bundler.
+
+### Technical Implementation
+
+Under the hood there's a few moving parts which enables all of this to happen.
+Let's first take a look at the pieces in `wasm-bindgen`.
+
+The primary goal of this RFC is to enable *tranparent* and *transitive*
+dependencies on NPM. The `#[wasm_bindgen]` macro is the only aspect of a crate's
+build which has access to all transitive dependencies, so this is what we'll be
+using to slurp up `package.json`. When `#[wasm_bindgen]` with a `module` key is
+specified it will look for `package.json` inside the cwd of the procedural macro
+(note that the cwd is set by Cargo to be the directory with the crate's
+`Cargo.toml` that is being compiled, or the crate in which `#[wasm_bindgen]` is
+written). This `package.json`, if found, will have an absolute path to it
+encoded into the custom section that `wasm-bindgen` already emits.
+
+Later, when the `wasm-bindgen` CLI tool executes, it will parse an interpret all
+items in the wasm-bindgen custom section. All `package.json` files listed will
+be loaded, parsed, and validated (aka only `dependencies` allowed for now). If
+any `package.json` is loaded then a `package.json` file will be emitted next to
+the output JS file inside of `--out-dir`.
+
+After `wasm-bindgen` executes, then `wasm-pack` will read the `package.json`
+output, if any, and augment it with metadata and other items which are already
+emitted.
+
+If more than one crate in a dependency graph depends on an NPM package then in
+this MVP proposal an error will be generated. In the future we can implement
+some degree of merging version requirements, but for now to remain simple
+`wasm-bindgen` will emit an error.
+
+### Interaction with `--no-modules`
+
+Depending on NPM packages fundamentally requires, well, NPM, in one way or
+another. The `wasm-bindgen` and `wasm-pack` CLI tools have modes of output
+(notably `wasm-bindgen`'s `--no-modules` and `wasm-pack`'s `--target no-modules`
+flags) which are intended to not require NPM and other JS tooling. In these
+situations if a `package.json` in any Rust crate is detected an error will be
+emitted indicating so.
+
+Note that this means that core crates which are intended to work with
+`--no-modules` will not be able add NPM dependencies. Instead they'll have to
+either import Rust dependencies from crates.io or use a feature like [local JS
+snippets][js] to import custom JS code.
+
+[js]: https://github.com/rustwasm/rfcs/pull/6
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+One of the primary drawbacks of this RFC is that it's fundamentally incompatible
+with a major use case of `wasm-bindgen` and `wasm-pack`, the `--no-modules` and
+`--target no-modules` flags. As a short-term band-aid this RFC proposes making
+it a hard error which would hinder the adoption of this feature in crates that
+want to be usable in this mode.
+
+In the long-term, however, it may be possible to get this working. For example
+many NPM packages are available on `unpkg.com` or in other locations. It may be
+possible, if all packages in these locations adhere to well-known conventions,
+to generate code that's compatible with these locations of hosting NPM packages.
+In these situations it may then be possible to "just drop a script tag" in a few
+locations to get `--no-modules` working with NPM packages. It's unclear how
+viable this is, though.
+
+# Rationale and Alternatives
+[alternatives]: #rationale-and-alternatives
+
+When developing this RFC, some guiding values for its design have been
+articulated:
+
+- Development on Rust-generated WebAssembly projects should allow developers to
+  use the development environment they are most comfortable with. Developers
+  writing Rust should get to use Rust, and developers using JavaScript should
+  get to use a JS based runtime environment (Node.js, Chakra, etc).
+
+- JavaScript tooling and workflows should be usable with Rust-generated
+  WebAssembly projects. For example, bundlers like WebPack and Parcel, or
+  dependency management tools such as `npm audit` and GreenKeeper.
+
+- When possible, decisions should be made that allow the solution to be
+  available to developers of not just Rust, but also C, and C++.
+
+- Decisions should be focused on creating workflows that allow developers an
+  easy learning curve and productive development experience.
+
+These principles lead to the above proposal of using `package.json` to declare
+NPM dependencies which is then grouped together by `wasm-bindgen` to be
+published by `wasm-pack`. By using `package.json` we get inherent compatibility
+with existing workflows like GreenKeeper and `npm install`. Additionally
+`package.json` is very well documented and supported throughout the JS ecosystem
+making it very familiar.
+
+Some other alternatives to this RFC which have been ruled out are:
+
+* **Using `Cargo.toml` instead of `package.json`** to declare NPM dependencies.
+  For example we could use:
+
+  ```toml
+  [package.metadata.npm.dependencies]
+  foo = "0.1"
+  ```
+
+  This has the drawback though of being incompatible with all existing workflows
+  around `package.json`. Additionally it also highlights a discrepancy between
+  NPM and Cargo and how `"0.1"` as a version requirement is interpreted (e.g.
+  `^0.1` or `~0.1`).
+
+* **Adding a separate manifest file** instead of using `package.json` is also
+  possibility and might be easier for `wasm-bindgen` to read and later
+  parse/include. This has a possible benefit of being scoped to exactly our use
+  case and not being misleading by disallowing otherwise-valid fields of
+  `package.json`. The downside of this approach is the same as `Cargo.toml`,
+  however, in that it's an unfamiliar format to most and is incompatible with
+  existing tooling without bringing too much benefit.
+
+* **Annotating version dependencies inline** could be used rather than
+  `package.json` as well, such as:
+
+  ```rust
+  #[wasm_bindgen(module = "foo", version = "0.1")]
+  extern "C" {
+      // ...
+  }
+  ```
+
+  As with all other alternatives this is incompatible with existing tooling, but
+  it's also not aligned with Rust's own mechanism for declaring dependencies
+  which separates the location for version information and the code iteslf.
+
+# Unresolved Questions
+[unresolved]: #unresolved-questions
+
+* Is the MVP restriction of only using `dependencies` too limiting? Should more
+  fields be supported in `package.json`?