Auto merge of #61988 - Centril:there-is-only-loop, r=matthewjasper

[let_chains, 3/6] And then there was only Loop

Here we remove `hir::ExprKind::While`.
Instead, we desugar: `'label: while $cond $body` into:

```rust
'label: loop {
    match DropTemps($cond) {
        true => $body,
        _ => break,
    }
}
```

Per #53667 (comment).
This is a follow up to #59288 which did the same for `if` expressions.

r? @matthewjasper

Author: bors

src/librustc/cfg/construct.rs

@@ -165,48 +165,6 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
                 self.add_ast_node(expr.hir_id.local_id, &[blk_exit])
             }
 
-            hir::ExprKind::While(ref cond, ref body, _) => {
-                //
-                //         [pred]
-                //           |
-                //           v 1
-                //       [loopback] <--+ 5
-                //           |         |
-                //           v 2       |
-                //   +-----[cond]      |
-                //   |       |         |
-                //   |       v 4       |
-                //   |     [body] -----+
-                //   v 3
-                // [expr]
-                //
-                // Note that `break` and `continue` statements
-                // may cause additional edges.
-
-                let loopback = self.add_dummy_node(&[pred]);              // 1
-
-                // Create expr_exit without pred (cond_exit)
-                let expr_exit = self.add_ast_node(expr.hir_id.local_id, &[]);         // 3
-
-                // The LoopScope needs to be on the loop_scopes stack while evaluating the
-                // condition and the body of the loop (both can break out of the loop)
-                self.loop_scopes.push(LoopScope {
-                    loop_id: expr.hir_id.local_id,
-                    continue_index: loopback,
-                    break_index: expr_exit
-                });
-
-                let cond_exit = self.expr(&cond, loopback);             // 2
-
-                // Add pred (cond_exit) to expr_exit
-                self.add_contained_edge(cond_exit, expr_exit);
-
-                let body_exit = self.block(&body, cond_exit);          // 4
-                self.add_contained_edge(body_exit, loopback);            // 5
-                self.loop_scopes.pop();
-                expr_exit
-            }
-
             hir::ExprKind::Loop(ref body, _, _) => {
                 //
                 //     [pred]

src/librustc/hir/intravisit.rs

@@ -1026,11 +1026,6 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr) {
         ExprKind::DropTemps(ref subexpression) => {
             visitor.visit_expr(subexpression);
         }
-        ExprKind::While(ref subexpression, ref block, ref opt_label) => {
-            walk_list!(visitor, visit_label, opt_label);
-            visitor.visit_expr(subexpression);
-            visitor.visit_block(block);
-        }
         ExprKind::Loop(ref block, ref opt_label, _) => {
             walk_list!(visitor, visit_label, opt_label);
             visitor.visit_block(block);

src/librustc/hir/lowering.rs

@@ -63,7 +63,7 @@ use syntax::errors;
 use syntax::ext::hygiene::{Mark, SyntaxContext};
 use syntax::print::pprust;
 use syntax::source_map::{self, respan, ExpnInfo, CompilerDesugaringKind, Spanned};
-use syntax::source_map::CompilerDesugaringKind::IfTemporary;
+use syntax::source_map::CompilerDesugaringKind::CondTemporary;
 use syntax::std_inject;
 use syntax::symbol::{kw, sym, Symbol};
 use syntax::tokenstream::{TokenStream, TokenTree};
@@ -4394,21 +4394,18 @@ impl<'a> LoweringContext<'a> {
                 let then_blk = self.lower_block(then, false);
                 let then_expr = self.expr_block(then_blk, ThinVec::new());
                 let (then_pats, scrutinee, desugar) = match cond.node {
-                    // `<pat> => <then>`
+                    // `<pat> => <then>`:
                     ExprKind::Let(ref pats, ref scrutinee) => {
                         let scrutinee = self.lower_expr(scrutinee);
                         let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
                         let desugar = hir::MatchSource::IfLetDesugar { contains_else_clause };
                         (pats, scrutinee, desugar)
                     }
-                    // `true => then`:
+                    // `true => <then>`:
                     _ => {
                         // Lower condition:
                         let cond = self.lower_expr(cond);
-                        // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
-                        // to preserve drop semantics since `if cond { ... }`
-                        // don't let temporaries live outside of `cond`.
-                        let span_block = self.mark_span_with_reason(IfTemporary, cond.span, None);
+                        let span_block = self.mark_span_with_reason(CondTemporary, cond.span, None);
                         // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
                         // to preserve drop semantics since `if cond { ... }` does not
                         // let temporaries live outside of `cond`.
@@ -4424,69 +4421,78 @@ impl<'a> LoweringContext<'a> {
                 hir::ExprKind::Match(P(scrutinee), vec![then_arm, else_arm].into(), desugar)
             }
             // FIXME(#53667): handle lowering of && and parens.
-            ExprKind::While(ref cond, ref body, opt_label) => {
-                // Desugar `ExprWhileLet`
-                // from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
-                if let ExprKind::Let(ref pats, ref sub_expr) = cond.node {
-                    // to:
-                    //
-                    //   [opt_ident]: loop {
-                    //     match <sub_expr> {
-                    //       <pat> => <body>,
-                    //       _ => break
-                    //     }
-                    //   }
-
-                    // Note that the block AND the condition are evaluated in the loop scope.
-                    // This is done to allow `break` from inside the condition of the loop.
-                    let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
-                        (
-                            this.lower_block(body, false),
-                            this.expr_break(e.span, ThinVec::new()),
-                            this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
-                        )
-                    });
+            ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
+                // Note that the block AND the condition are evaluated in the loop scope.
+                // This is done to allow `break` from inside the condition of the loop.
 
-                    // `<pat> => <body>`
-                    let pat_arm = {
-                        let body_expr = P(self.expr_block(body, ThinVec::new()));
-                        let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
-                        self.arm(pats, body_expr)
-                    };
+                // `_ => break`:
+                let else_arm = {
+                    let else_pat = this.pat_wild(e.span);
+                    let else_expr = this.expr_break(e.span, ThinVec::new());
+                    this.arm(hir_vec![else_pat], else_expr)
+                };
 
-                    // `_ => break`
-                    let break_arm = {
-                        let pat_under = self.pat_wild(e.span);
-                        self.arm(hir_vec![pat_under], break_expr)
-                    };
+                // Handle then + scrutinee:
+                let then_blk = this.lower_block(body, false);
+                let then_expr = this.expr_block(then_blk, ThinVec::new());
+                let (then_pats, scrutinee, desugar, source) = match cond.node {
+                    ExprKind::Let(ref pats, ref scrutinee) => {
+                        // to:
+                        //
+                        //   [opt_ident]: loop {
+                        //     match <sub_expr> {
+                        //       <pat> => <body>,
+                        //       _ => break
+                        //     }
+                        //   }
+                        let scrutinee = this.with_loop_condition_scope(|t| t.lower_expr(scrutinee));
+                        let pats = pats.iter().map(|pat| this.lower_pat(pat)).collect();
+                        let desugar = hir::MatchSource::WhileLetDesugar;
+                        (pats, scrutinee, desugar, hir::LoopSource::WhileLet)
+                    }
+                    _ => {
+                        // We desugar: `'label: while $cond $body` into:
+                        //
+                        // ```
+                        // 'label: loop {
+                        //     match DropTemps($cond) {
+                        //         true => $body,
+                        //         _ => break,
+                        //     }
+                        // }
+                        // ```
 
-                    // `match <sub_expr> { ... }`
-                    let arms = hir_vec![pat_arm, break_arm];
-                    let match_expr = self.expr(
-                        sub_expr.span,
-                        hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
-                        ThinVec::new(),
-                    );
+                        // Lower condition:
+                        let cond = this.with_loop_condition_scope(|this| this.lower_expr(cond));
+                        let span_block = this.mark_span_with_reason(CondTemporary, cond.span, None);
+                        // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
+                        // to preserve drop semantics since `while cond { ... }` does not
+                        // let temporaries live outside of `cond`.
+                        let cond = this.expr_drop_temps(span_block, P(cond), ThinVec::new());
 
-                    // `[opt_ident]: loop { ... }`
-                    let loop_block = P(self.block_expr(P(match_expr)));
-                    let loop_expr = hir::ExprKind::Loop(
-                        loop_block,
-                        self.lower_label(opt_label),
-                        hir::LoopSource::WhileLet,
-                    );
-                    // Add attributes to the outer returned expr node.
-                    loop_expr
-                } else {
-                    self.with_loop_scope(e.id, |this| {
-                        hir::ExprKind::While(
-                            this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
-                            this.lower_block(body, false),
-                            this.lower_label(opt_label),
-                        )
-                    })
-                }
-            }
+                        let desugar = hir::MatchSource::WhileDesugar;
+                        // `true => <then>`:
+                        let pats = hir_vec![this.pat_bool(e.span, true)];
+                        (pats, cond, desugar, hir::LoopSource::While)
+                    }
+                };
+                let then_arm = this.arm(then_pats, P(then_expr));
+
+                // `match <scrutinee> { ... }`
+                let match_expr = this.expr_match(
+                    scrutinee.span,
+                    P(scrutinee),
+                    hir_vec![then_arm, else_arm],
+                    desugar,
+                );
+
+                // `[opt_ident]: loop { ... }`
+                hir::ExprKind::Loop(
+                    P(this.block_expr(P(match_expr))),
+                    this.lower_label(opt_label),
+                    source
+                )
+            }),
             ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
                 hir::ExprKind::Loop(
                     this.lower_block(body, false),

src/librustc/hir/map/mod.rs

@@ -731,7 +731,7 @@ impl<'hir> Map<'hir> {
             match *node {
                 Node::Expr(ref expr) => {
                     match expr.node {
-                        ExprKind::While(..) | ExprKind::Loop(..) | ExprKind::Ret(..) => true,
+                        ExprKind::Loop(..) | ExprKind::Ret(..) => true,
                         _ => false,
                     }
                 }

src/librustc/hir/mod.rs

@@ -1405,7 +1405,6 @@ impl Expr {
             ExprKind::Lit(_) => ExprPrecedence::Lit,
             ExprKind::Type(..) | ExprKind::Cast(..) => ExprPrecedence::Cast,
             ExprKind::DropTemps(ref expr, ..) => expr.precedence(),
-            ExprKind::While(..) => ExprPrecedence::While,
             ExprKind::Loop(..) => ExprPrecedence::Loop,
             ExprKind::Match(..) => ExprPrecedence::Match,
             ExprKind::Closure(..) => ExprPrecedence::Closure,
@@ -1464,7 +1463,6 @@ impl Expr {
             ExprKind::Break(..) |
             ExprKind::Continue(..) |
             ExprKind::Ret(..) |
-            ExprKind::While(..) |
             ExprKind::Loop(..) |
             ExprKind::Assign(..) |
             ExprKind::InlineAsm(..) |
@@ -1532,10 +1530,6 @@ pub enum ExprKind {
     /// This construct only exists to tweak the drop order in HIR lowering.
     /// An example of that is the desugaring of `for` loops.
     DropTemps(P<Expr>),
-    /// A while loop, with an optional label
-    ///
-    /// I.e., `'label: while expr { <block> }`.
-    While(P<Expr>, P<Block>, Option<Label>),
     /// A conditionless loop (can be exited with `break`, `continue`, or `return`).
     ///
     /// I.e., `'label: loop { <block> }`.
@@ -1653,6 +1647,8 @@ pub enum MatchSource {
     IfLetDesugar {
         contains_else_clause: bool,
     },
+    /// A `while _ { .. }` (which was desugared to a `loop { match _ { .. } }`).
+    WhileDesugar,
     /// A `while let _ = _ { .. }` (which was desugared to a
     /// `loop { match _ { .. } }`).
     WhileLetDesugar,
@@ -1669,12 +1665,25 @@ pub enum MatchSource {
 pub enum LoopSource {
     /// A `loop { .. }` loop.
     Loop,
+    /// A `while _ { .. }` loop.
+    While,
     /// A `while let _ = _ { .. }` loop.
     WhileLet,
     /// A `for _ in _ { .. }` loop.
     ForLoop,
 }
 
+impl LoopSource {
+    pub fn name(self) -> &'static str {
+        match self {
+            LoopSource::Loop => "loop",
+            LoopSource::While => "while",
+            LoopSource::WhileLet => "while let",
+            LoopSource::ForLoop => "for",
+        }
+    }
+}
+
 #[derive(Copy, Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub enum LoopIdError {
     OutsideLoopScope,

src/librustc/hir/print.rs

@@ -1299,16 +1299,6 @@ impl<'a> State<'a> {
                 // Print `}`:
                 self.bclose_maybe_open(expr.span, indent_unit, true);
             }
-            hir::ExprKind::While(ref test, ref blk, opt_label) => {
-                if let Some(label) = opt_label {
-                    self.print_ident(label.ident);
-                    self.word_space(":");
-                }
-                self.head("while");
-                self.print_expr_as_cond(&test);
-                self.s.space();
-                self.print_block(&blk);
-            }
             hir::ExprKind::Loop(ref blk, opt_label, _) => {
                 if let Some(label) = opt_label {
                     self.print_ident(label.ident);
@@ -2289,7 +2279,6 @@ fn expr_requires_semi_to_be_stmt(e: &hir::Expr) -> bool {
     match e.node {
         hir::ExprKind::Match(..) |
         hir::ExprKind::Block(..) |
-        hir::ExprKind::While(..) |
         hir::ExprKind::Loop(..) => false,
         _ => true,
     }

src/librustc/ich/impls_syntax.rs

@@ -415,7 +415,7 @@ impl_stable_hash_for!(enum ::syntax_pos::hygiene::ExpnFormat {
 });
 
 impl_stable_hash_for!(enum ::syntax_pos::hygiene::CompilerDesugaringKind {
-    IfTemporary,
+    CondTemporary,
     Async,
     Await,
     QuestionMark,

src/librustc/middle/expr_use_visitor.rs

@@ -487,11 +487,6 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                 self.walk_block(&blk);
             }
 
-            hir::ExprKind::While(ref cond_expr, ref blk, _) => {
-                self.consume_expr(&cond_expr);
-                self.walk_block(&blk);
-            }
-
             hir::ExprKind::Unary(_, ref lhs) => {
                 self.consume_expr(&lhs);
             }