MIR building for `match $c { true => $i, _ => $e }` is slow

[Centril]

At the time of writing, if $c { $i } else { $e } builds MIR that is simpler and therefore is ~1-3% better in terms of compile times than match $c { $i } else { $e }. However, after the early-opt MIR optimization, they are basically the same.

In #59288 we start desugaring if expressions to match and so if we optimize for the special case of two match arms we can potentially improve compiles times.

cc @oli-obk

[estebank]

CC #7462

[danii]

May I volunteer in fixing? And can anyone help in pointing to the main file where match MIR is generated?

[oli-obk]

rust/compiler/rustc_mir_build/src/thir/cx/expr.rs

Lines 593 to 601 in a84d1b2

	hir::ExprKind::If(cond, then, else_opt) => ExprKind::If {
	cond: self.mirror_expr(cond),
	then: self.mirror_expr(then),
	else_opt: else_opt.map(|el| self.mirror_expr(el)),
	},
	hir::ExprKind::Match(ref discr, ref arms, _) => ExprKind::Match {
	scrutinee: self.mirror_expr(discr),
	arms: arms.iter().map(|a| self.convert_arm(a)).collect(),
	},

is the entry point for if and match. we could check whether the discr is of bool type and whether both match arms are plain bool literals without a guard.