Suggest naming types before using explicit type names

[samueltardieu]

missing_transmute_annotations will suggest naming the origin and destination types if they do not have explicit names already.

changelog: [missing_transmute_annotations]: suggest giving origin and destination types a name in order to ascribe the transmute call

Fixes #14984

[rustbot]

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[samueltardieu]

r? clippy

[blyxyas]

What do you think about this improvement? If the transmute argument is something like x, it doesn't make that much sense calling it "the origin type" and not just mentioning x

[blyxyas]

Suggested change

	(true, false) => "origin type",
	(false, true) => "destination type",
	(true, false) => maybe_name_by_expr(cx.tcx.sess, arg, "the origin type"),
	(false, true) => "the destination type",

[blyxyas]

Suggested change

	/// Between the original expression and a default string not as concrete, chooses the shorter.
	fn maybe_name_by_expr(sess: &Session, arg: &Expr<'_>, default: String) -> String {
	let span_data = arg.span.data();
	if span_data.hi - span_data.lo < BytePos(default.len()) {
	format!("`{}`", snippet(sess, arg.span(), &default))
	} else {
	default
	}
	}

[blyxyas]

Suggested change

	"consider giving the {to_name} a name, and adding missing type annotations"
	"consider giving {to_name} a name, and adding missing type annotations"