Richer Diagnostic Display Meta-Issue

This issue tracks a couple of different useful ideas for making diagnostics "richer" for display purposes. We've wanted some of these for a while, and they're partially inspired by testing tools like Jest and Mocha. We've also seen tools like Rust, Rome, and Flow prove them out to some extent.

Long-term, you could imagine them being interactive with the a richer compiler client.

Printing Messages Across Lines

Types can get unwieldy, and displaying them inline makes it harder to see where things begin/end.

Type

    { x: string, y: number }

is not assignable to

    { x: string, y: string }

Notice no punctuation at the end. Not sure if we can generalize messages that way.

Also not sure how this plays well with

• tools that parse our output.
• diagnostic elaborations

Highlighted Spans in Messages

Imagine being able to highlight individual sections inside of types to indicate where the issue occurs.

                              vvvvvvvvvvv
Type '{ a: string; b: number; c: boolean; }' is not assignable to type '{ a: string; b: number; }'.
                              ^^^^^^^^^^^
  Object literal may only specify known properties, and 'c' does not exist in type '{ a: string; b: number; }'.

Diff Highlighting in Messages

Basically imagine red being the provided color, with green being an expected color.

Type '[string, number, boolean]' is not assignable to type '[string, string, boolean]'.
               ^^^^^^                                                ^^^^^^
               rrrrrr                                                gggggg

No files, tests, or entry points are named. First review the three proposed areas—multi-line messages, highlighted spans, and diff highlighting—along with their compatibility concerns, then identify the relevant diagnostic display code and tests before proposing a scoped implementation. Done requires an agreed design and validation for the selected diagnostic behavior.