annotate-snippet emitter: Include suggestions in output

[phansch]

Part of #59346

In order for the new AnnotateSnippetEmitterWriter to include suggestions in the output, we essentially have to pass &db.suggestions to emit_messages_default and deal with a couple of edge-cases.

Relevant FIXME:

rust/src/librustc_errors/annotate_snippet_emitter_writer.rs

Lines 35 to 36 in 57a3300

	// FIXME(#59346): Collect suggestions (see emitter.rs)
	let suggestions: &[_] = &[];

emitter.rs equivalent:

rust/src/librustc_errors/emitter.rs

Lines 84 to 115 in 0e4a56b

	if let Some((sugg, rest)) = db.suggestions.split_first() {
	if rest.is_empty() &&
	// don't display multi-suggestions as labels
	sugg.substitutions.len() == 1 &&
	// don't display multipart suggestions as labels
	sugg.substitutions[0].parts.len() == 1 &&
	// don't display long messages as labels
	sugg.msg.split_whitespace().count() < 10 &&
	// don't display multiline suggestions as labels
	!sugg.substitutions[0].parts[0].snippet.contains('\n') &&
	// when this style is set we want the suggestion to be a message, not inline
	sugg.style != SuggestionStyle::HideCodeAlways &&
	// trivial suggestion for tooling's sake, never shown
	sugg.style != SuggestionStyle::CompletelyHidden
	{
	let substitution = &sugg.substitutions[0].parts[0].snippet.trim();
	let msg = if substitution.len() == 0 || sugg.style.hide_inline() {
	// This substitution is only removal or we explicitly don't want to show the
	// code inline, don't show it
	format!("help: {}", sugg.msg)
	} else {
	format!("help: {}: `{}`", sugg.msg, substitution)
	};
	primary_span.push_span_label(sugg.substitutions[0].parts[0].span, msg);
	} else {
	// if there are multiple suggestions, print them all in full
	// to be consistent. We could try to figure out if we can
	// make one (or the first one) inline, but that would give
	// undue importance to a semi-random suggestion
	suggestions = &db.suggestions;
	}
	}

• The tricky part is figuring out the first half of the conditional. We probably need it in the new emitter, too. Is it enough to just copy it over? Maybe extract that code so that the code is shared in both emitters?
• Otherwise it's just passing &db.suggestions through
• Should take into account the MAX_SUGGESTIONS value somewhere (add a UI test for this)

This issue has been assigned to @phansch via this comment.

[phansch]

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