Increase span of unreachable code error

[ghost]

Fixes #25384

[mhegazy]

this will mark function declarations as well, right?

[ghost]

Right. The quick fix will skip them, but the error message will be a single span rather than several spans separated by functions.

[mhegazy]

do you think it is worth changing it to multiple spans?

[ghost]

I guess this would be ugly for a pattern like:

return { fn1, fn2, fn3, <<etc>> };
doSometing();

function fn1() { ... }
function fn2() { ... }
<<etc>>

that would show hundreds of lines as unreachable when only one line is.

The problem is that our current interface is:

export interface Diagnostic extends DiagnosticRelatedInformation {
    /** May store more in future. For now, this will simply be `true` to indicate when a diagnostic is an unused-identifier diagnostic. */
    reportsUnnecessary?: {};
    source?: string;
    relatedInformation?: DiagnosticRelatedInformation[];
}
export interface DiagnosticRelatedInformation {
    category: DiagnosticCategory;
    code: number;
    file: SourceFile | undefined;
    start: number | undefined;
    length: number | undefined;
    messageText: string | DiagnosticMessageChain;
}
export interface DiagnosticWithLocation extends Diagnostic {
    file: SourceFile;
    start: number;
    length: number;
}

So each additional span would need a new messageText. But it's unreachable for the same reason the first line of code was unreachable, so it doesn't feel right to add an additional span for that.

We could just add a new diagnostic for each group of unreachable statements separated by functions.

[mhegazy]

I was thinking of multiple diagnostics, one for each contiguous span..