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_hir/src/hir.rs

@@ -1431,6 +1431,7 @@ impl Expr<'_> {
             ExprKind::Lit(_) => ExprPrecedence::Lit,
             ExprKind::Type(..) | ExprKind::Cast(..) => ExprPrecedence::Cast,
             ExprKind::DropTemps(ref expr, ..) => expr.precedence(),
+            ExprKind::If(..) => ExprPrecedence::If,
             ExprKind::Loop(..) => ExprPrecedence::Loop,
             ExprKind::Match(..) => ExprPrecedence::Match,
             ExprKind::Closure(..) => ExprPrecedence::Closure,
@@ -1492,6 +1493,7 @@ impl Expr<'_> {
             | ExprKind::MethodCall(..)
             | ExprKind::Struct(..)
             | ExprKind::Tup(..)
+            | ExprKind::If(..)
             | ExprKind::Match(..)
             | ExprKind::Closure(..)
             | ExprKind::Block(..)
@@ -1608,6 +1610,10 @@ pub enum ExprKind<'hir> {
     /// This construct only exists to tweak the drop order in HIR lowering.
     /// An example of that is the desugaring of `for` loops.
     DropTemps(&'hir Expr<'hir>),
+    /// An `if` block, with an optional else block.
+    ///
+    /// I.e., `if <expr> { <expr> } else { <expr> }`.
+    If(&'hir Expr<'hir>, &'hir Expr<'hir>, Option<&'hir Expr<'hir>>),
     /// A conditionless loop (can be exited with `break`, `continue`, or `return`).
     ///
     /// I.e., `'label: loop { <block> }`.
@@ -1761,8 +1767,6 @@ pub enum LocalSource {
 pub enum MatchSource {
     /// A `match _ { .. }`.
     Normal,
-    /// An `if _ { .. }` (optionally with `else { .. }`).
-    IfDesugar { contains_else_clause: bool },
     /// An `if let _ = _ { .. }` (optionally with `else { .. }`).
     IfLetDesugar { contains_else_clause: bool },
     /// An `if let _ = _ => { .. }` match guard.
@@ -1785,7 +1789,7 @@ impl MatchSource {
         use MatchSource::*;
         match self {
             Normal => "match",
-            IfDesugar { .. } | IfLetDesugar { .. } | IfLetGuardDesugar => "if",
+            IfLetDesugar { .. } | IfLetGuardDesugar => "if",
             WhileDesugar | WhileLetDesugar => "while",
             ForLoopDesugar => "for",
             TryDesugar => "?",

compiler/rustc_hir/src/intravisit.rs

@@ -1146,6 +1146,11 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
         ExprKind::DropTemps(ref subexpression) => {
             visitor.visit_expr(subexpression);
         }
+        ExprKind::If(ref cond, ref then, ref else_opt) => {
+            visitor.visit_expr(cond);
+            visitor.visit_expr(then);
+            walk_list!(visitor, visit_expr, else_opt);
+        }
         ExprKind::Loop(ref block, ref opt_label, _) => {
             walk_list!(visitor, visit_label, opt_label);
             visitor.visit_block(block);