Rollup merge of #79051 - LeSeulArtichaut:if-let-guard, r=matthewjasper

Implement if-let match guards

Implements rust-lang/rfcs#2294 (tracking issue: #51114).

I probably should do a few more things before this can be merged:
- [x] Add tests (added basic tests, more advanced tests could be done in the future?)
- [x] Add lint for exhaustive if-let guard (comparable to normal if-let statements)
- [x] Fix clippy

However since this is a nightly feature maybe it's fine to land this and do those steps in follow-up PRs.

Thanks a lot `@matthewjasper` ❤️ for helping me with lowering to MIR! Would you be interested in reviewing this?
r? `@ghost` for now

compiler/rustc_passes/src/check_const.rs

@@ -45,6 +45,8 @@ impl NonConstExpr {
                 return None;
             }
 
+            Self::Match(IfLetGuardDesugar) => bug!("if-let guard outside a `match` expression"),
+
             // All other expressions are allowed.
             Self::Loop(Loop | While | WhileLet)
             | Self::Match(

compiler/rustc_passes/src/liveness.rs

@@ -360,6 +360,9 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
 
     fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
         self.add_from_pat(&arm.pat);
+        if let Some(hir::Guard::IfLet(ref pat, _)) = arm.guard {
+            self.add_from_pat(pat);
+        }
         intravisit::walk_arm(self, arm);
     }
 
@@ -866,10 +869,13 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 for arm in arms {
                     let body_succ = self.propagate_through_expr(&arm.body, succ);
 
-                    let guard_succ = self.propagate_through_opt_expr(
-                        arm.guard.as_ref().map(|hir::Guard::If(e)| *e),
-                        body_succ,
-                    );
+                    let guard_succ = arm.guard.as_ref().map_or(body_succ, |g| match g {
+                        hir::Guard::If(e) => self.propagate_through_expr(e, body_succ),
+                        hir::Guard::IfLet(pat, e) => {
+                            let let_bind = self.define_bindings_in_pat(pat, body_succ);
+                            self.propagate_through_expr(e, let_bind)
+                        }
+                    });
                     let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
                     self.merge_from_succ(ln, arm_succ);
                 }