Feature request: diagnostic message transformation hook in decorateProgram / LanguagePlugin?

[johanrd]

Not sure what the best approach here, so this is as much a question as a request:

Volar's decorateProgram correctly remaps diagnostic locations from generated code back to source via transformDiagnostic – but is there a hook for language plugins to transform diagnostic messages?

Language plugins that want to augment TS error messages with domain-specific guidance (e.g. augmenting a TS2345: Argument of type 'unknown' is not assignable to parameter of type 'Element' into a more helpful message like An Element must be specified in the component signature in order to use ...attributes) can do so in the tsserver-plugin (IDE) path — but currently not in the CLI (tsc) path?

In Glint we have an augmentDiagnostics function that rewrites certain TS errors into template-aware messages. When trying to get the same augmentation to the cli, it didn't feel right.

Possible API

A transformDiagnostic callback on the LanguagePlugin.typescript interface?

interface LanguagePlugin {                                                                                                                                       
  typescript?: {                                                  
    // ...existing properties...                                                                                                                                 
                                                                                                                                                                 
    /**                                                                                                                                                          
     * Called after Volar remaps a diagnostic's location back to source.                                                                                         
     * Allows the language plugin to rewrite the diagnostic message or                                                                                           
     * add additional context. Return undefined to filter the diagnostic.                                                                                        
     */                                                                                                                                                          
    transformDiagnostic?(diagnostic: ts.Diagnostic, virtualCode: VirtualCode): ts.Diagnostic | undefined;                                                        
  }                                                                                                                                                              
}                                                                                                                                                                

decorateProgram would then call this after transformDiagnostic remaps locations:

// in decorateProgram's getSemanticDiagnostics wrapper:                                                                                                          
return getSemanticDiagnostics(actualSourceFile, cancellationToken)
  .map(d => transformDiagnostic(language, d, program, true))
  .filter(d => !!d)
  .map(d => applyPluginTransform(language, d))  // <-- new step
  .filter(d => !!d);                                                                                                                                           

This could make diagnostic augmentation work consistently across both IDE and CLI paths

Use cases

• Glint (Ember/Glimmer): rewriting TS errors about unknown element types into actionable guidance about component signatures
• Any Volar-based language plugin that wants to provide domain-specific error messages on top of raw TS diagnostics
• Potentially useful for Vue itself if there are ever Vue-specific error rewrites needed