Support Rollup's this.load() method in the plugin context

[dhaig]

Clear and concise description of the problem

When a Rollup plugin needs to inspect the contents of another module, the load() method in the plugin context resolves and transforms the dependency on demand. Vite's plugin container doesn't provide it, so calling this.load() from a plugin hook throws an error. It would be nice to have it available in Vite as I've run into situations where it's necessary and as I understand it there's no workaround for the same functionality.

Suggested solution

In the plugin context, Vite can implement a version of load() that is similar to Rollup's but with a few changes:

• the final parsing step would be skipped as Vite only parses on demand
• the returned module information might not always have the AST available
• it wouldn't trigger moduleParsed

In Rollup, load() returns a promise that finishes after the module is resolved, loaded, and transformed but before its dependencies have been resolved. It supports a flag resolveDependencies that makes the promise finish afterward instead. I think this option would still make sense for Vite, but I'm not 100% clear on when Vite resolves a module's dependencies.

Alternative

No response

Additional context

• Documentation for this.load() including a few example use cases
• I checked WMR's plugin container to see if it supports load() and it doesn't – I unfortunately couldn't find any discussion about it, so I'm not sure why they decided against it.

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[luxaritas]

Just ran into this causing an error using the latest version of rpt2 with vitest (see ezolenko/rollup-plugin-typescript2@0.33.0...0.34.0#diff-a2a171449d862fe29692ce031981047d7ab755ae7f84c707aef80701b3ea0c80R266 where 0.34 introduced a call to this.load()).

I'd be willing to help move this forward given some guidance (this goes deeper into vite than I'm currently comfortable with, so naturally my assistance is predicated on it actually being useful).

[luxaritas]

My guess why this wasn't implemented is just that it was introduced after PluginContainer was written and wasn't noticed/picked up (this.load was added rollup 2.60, released in November '21). Got some initial pointers from Blu via Discord, so I'm starting to look into this.

[luxaritas]

Yeah, this PR seems to lend credence to that: #5846

[micha-lmxt]

My current workaround is to use server.transformRequest in dev mode:

...
   {
    ...,
    buildStart(){
        if (this.load){
            _this.load = this.load
        }
    },
    configureServer(server){
        if(!_this.load){
            _this.load = (resolvedFile) => server.transformRequest(resolvedFile.id)
        }
    }
....

[alexeyyakimovich]

#5846

Can you, please, add more info about where to add this code?

[micha-lmxt]

Sure. buildStart is a rollup/vite hook and configureServer refers to the vite specific hook. The plugin should export an object with these properties.
Example:

// Load 'file.js' early
export const plugin=({file:'file.js'})=>{
    const _this={}
    let isLoaded = false
    return {
        name:'myFileLoader',
        buildStart(){
            if (this.load){
                _this.load = this.load
            }
        },
        configureServer(server){
            if(!_this.load){
                _this.load = (resolvedFile) =>  server.transformRequest(resolvedFile.id)
            }
        },
        async resolveId(){
            if (isLoaded){
               return null;
            }
            const resolved = await this.resolve(file)
            if (resolved){
                await _this.load(resolved)
            }
        }
   
    }
}