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_mir_build/src/build/expr/as_place.rs

@@ -531,6 +531,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Borrow { .. }
             | ExprKind::AddressOf { .. }
             | ExprKind::Match { .. }
+            | ExprKind::If { .. }
             | ExprKind::Loop { .. }
             | ExprKind::Block { .. }
             | ExprKind::Assign { .. }

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -251,6 +251,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::StaticRef { .. }
             | ExprKind::Block { .. }
             | ExprKind::Match { .. }
+            | ExprKind::If { .. }
             | ExprKind::NeverToAny { .. }
             | ExprKind::Use { .. }
             | ExprKind::Borrow { .. }

compiler/rustc_mir_build/src/build/expr/category.rs

@@ -45,6 +45,7 @@ impl Category {
 
             ExprKind::LogicalOp { .. }
             | ExprKind::Match { .. }
+            | ExprKind::If { .. }
             | ExprKind::NeverToAny { .. }
             | ExprKind::Use { .. }
             | ExprKind::Adt { .. }

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -69,6 +69,40 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             ExprKind::Match { scrutinee, arms } => {
                 this.match_expr(destination, scope, expr_span, block, scrutinee, arms)
             }
+            ExprKind::If { cond, then, else_opt } => {
+                let place = unpack!(block = this.as_temp(block, Some(this.local_scope()), cond, Mutability::Mut));
+                let operand = Operand::Move(Place::from(place));
+
+                let mut then_block = this.cfg.start_new_block();
+                let mut else_block = this.cfg.start_new_block();
+                let term = TerminatorKind::if_(this.hir.tcx(), operand, then_block, else_block);
+                this.cfg.terminate(block, source_info, term);
+
+                unpack!(then_block = this.into(destination, scope, then_block, then));
+                else_block = if let Some(else_opt) = else_opt {
+                    unpack!(this.into(destination, None, else_block, else_opt))
+                } else {
+                    // Body of the `if` expression without an `else` clause must return `()`, thus
+                    // we implicitly generate a `else {}` if it is not specified.
+                    let correct_si = this.source_info(expr_span.shrink_to_hi());
+                    this.cfg.push_assign_unit(else_block, correct_si, destination, this.hir.tcx());
+                    else_block
+                };
+
+                let join_block = this.cfg.start_new_block();
+                this.cfg.terminate(
+                    then_block,
+                    source_info,
+                    TerminatorKind::Goto { target: join_block },
+                );
+                this.cfg.terminate(
+                    else_block,
+                    source_info,
+                    TerminatorKind::Goto { target: join_block },
+                );
+
+                join_block.unit()
+            },
             ExprKind::NeverToAny { source } => {
                 let source = this.hir.mirror(source);
                 let is_call = matches!(source.kind, ExprKind::Call { .. } | ExprKind::InlineAsm { .. });

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

@@ -537,6 +537,11 @@ fn make_mirror_unadjusted<'a, 'tcx>(
             },
             Err(err) => bug!("invalid loop id for continue: {}", err),
         },
+        hir::ExprKind::If(cond, then, else_opt) => ExprKind::If {
+            cond: cond.to_ref(),
+            then: then.to_ref(),
+            else_opt: else_opt.map(|el| el.to_ref()),
+        },
         hir::ExprKind::Match(ref discr, ref arms, _) => ExprKind::Match {
             scrutinee: discr.to_ref(),
             arms: arms.iter().map(|a| convert_arm(cx, a)).collect(),

compiler/rustc_mir_build/src/thir/mod.rs

@@ -139,6 +139,11 @@ crate enum ExprKind<'tcx> {
     Box {
         value: ExprRef<'tcx>,
     },
+    If {
+        cond: ExprRef<'tcx>,
+        then: ExprRef<'tcx>,
+        else_opt: Option<ExprRef<'tcx>>,
+    },
     Call {
         ty: Ty<'tcx>,
         fun: ExprRef<'tcx>,

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -403,7 +403,7 @@ fn report_arm_reachability<'p, 'tcx>(
         match is_useful {
             NotUseful => {
                 match source {
-                    hir::MatchSource::IfDesugar { .. } | hir::MatchSource::WhileDesugar => bug!(),
+                    hir::MatchSource::WhileDesugar => bug!(),
 
                     hir::MatchSource::IfLetDesugar { .. } | hir::MatchSource::WhileLetDesugar => {
                         // Check which arm we're on.