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_ast_lowering/src/expr.rs

@@ -505,14 +505,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
+        let pat = self.lower_pat(&arm.pat);
+        let guard = arm.guard.as_ref().map(|cond| {
+            if let ExprKind::Let(ref pat, ref scrutinee) = cond.kind {
+                hir::Guard::IfLet(self.lower_pat(pat), self.lower_expr(scrutinee))
+            } else {
+                hir::Guard::If(self.lower_expr(cond))
+            }
+        });
         hir::Arm {
             hir_id: self.next_id(),
             attrs: self.lower_attrs(&arm.attrs),
-            pat: self.lower_pat(&arm.pat),
-            guard: match arm.guard {
-                Some(ref x) => Some(hir::Guard::If(self.lower_expr(x))),
-                _ => None,
-            },
+            pat,
+            guard,
             body: self.lower_expr(&arm.body),
             span: arm.span,
         }