Auto merge of rust-lang#53815 - F001:if-let-guard, r=petrochenkov

refactor match guard

This is the first step to implement RFC 2294: if-let-guard. Tracking issue: rust-lang#51114

The second step should be introducing another variant `IfLet` in the Guard enum. I separated them into 2 PRs for the convenience of reviewers.

r? @petrochenkov

Author: bors

src/librustc/cfg/construct.rs

@@ -488,8 +488,9 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
                     // expression to target
                     let guard_start = self.add_dummy_node(&[pat_exit]);
                     // Visit the guard expression
-                    let guard_exit = self.expr(&guard, guard_start);
-
+                    let guard_exit = match guard {
+                        hir::Guard::If(ref e) => self.expr(e, guard_start),
+                    };
                     // #47295: We used to have very special case code
                     // here for when a pair of arms are both formed
                     // solely from constants, and if so, not add these

src/librustc/hir/intravisit.rs

@@ -1102,7 +1102,11 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr) {
 
 pub fn walk_arm<'v, V: Visitor<'v>>(visitor: &mut V, arm: &'v Arm) {
     walk_list!(visitor, visit_pat, &arm.pats);
-    walk_list!(visitor, visit_expr, &arm.guard);
+    if let Some(ref g) = arm.guard {
+        match g {
+            Guard::If(ref e) => visitor.visit_expr(e),
+        }
+    }
     visitor.visit_expr(&arm.body);
     walk_list!(visitor, visit_attribute, &arm.attrs);
 }

src/librustc/hir/lowering.rs

@@ -1054,7 +1054,10 @@ impl<'a> LoweringContext<'a> {
         hir::Arm {
             attrs: self.lower_attrs(&arm.attrs),
             pats: arm.pats.iter().map(|x| self.lower_pat(x)).collect(),
-            guard: arm.guard.as_ref().map(|ref x| P(self.lower_expr(x))),
+            guard: match arm.guard {
+                Some(Guard::If(ref x)) => Some(hir::Guard::If(P(self.lower_expr(x)))),
+                _ => None,
+            },
             body: P(self.lower_expr(&arm.body)),
         }
     }

src/librustc/hir/mod.rs

@@ -1204,10 +1204,15 @@ impl DeclKind {
 pub struct Arm {
     pub attrs: HirVec<Attribute>,
     pub pats: HirVec<P<Pat>>,
-    pub guard: Option<P<Expr>>,
+    pub guard: Option<Guard>,
     pub body: P<Expr>,
 }
 
+#[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
+pub enum Guard {
+    If(P<Expr>),
+}
+
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct Field {
     pub id: NodeId,

src/librustc/hir/print.rs

@@ -1949,10 +1949,14 @@ impl<'a> State<'a> {
             self.print_pat(&p)?;
         }
         self.s.space()?;
-        if let Some(ref e) = arm.guard {
-            self.word_space("if")?;
-            self.print_expr(&e)?;
-            self.s.space()?;
+        if let Some(ref g) = arm.guard {
+            match g {
+                hir::Guard::If(e) => {
+                    self.word_space("if")?;
+                    self.print_expr(&e)?;
+                    self.s.space()?;
+                }
+            }
         }
         self.word_space("=>")?;
 

src/librustc/ich/impls_hir.rs

@@ -493,6 +493,10 @@ impl_stable_hash_for!(struct hir::Arm {
     body
 });
 
+impl_stable_hash_for!(enum hir::Guard {
+    If(expr),
+});
+
 impl_stable_hash_for!(struct hir::Field {
     id -> _,
     ident,

src/librustc/middle/expr_use_visitor.rs

@@ -792,7 +792,9 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
         }
 
         if let Some(ref guard) = arm.guard {
-            self.consume_expr(&guard);
+            match guard {
+                hir::Guard::If(ref e) => self.consume_expr(e),
+            }
         }
 
         self.consume_expr(&arm.body);

src/librustc/middle/liveness.rs

@@ -1030,7 +1030,12 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 let body_succ =
                     self.propagate_through_expr(&arm.body, succ);
                 let guard_succ =
-                    self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
+                    self.propagate_through_opt_expr(
+                        arm.guard.as_ref().map(|g|
+                            match g {
+                                hir::Guard::If(e) => &**e,
+                            }),
+                        body_succ);
                 // only consider the first pattern; any later patterns must have
                 // the same bindings, and we also consider the first pattern to be
                 // the "authoritative" set of ids

src/librustc/middle/region.rs

@@ -885,8 +885,10 @@ fn resolve_block<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, blk:
 fn resolve_arm<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, arm: &'tcx hir::Arm) {
     visitor.terminating_scopes.insert(arm.body.hir_id.local_id);
 
-    if let Some(ref expr) = arm.guard {
-        visitor.terminating_scopes.insert(expr.hir_id.local_id);
+    if let Some(ref g) = arm.guard {
+        match g {
+            hir::Guard::If(ref expr) => visitor.terminating_scopes.insert(expr.hir_id.local_id),
+        };
     }
 
     intravisit::walk_arm(visitor, arm);

src/librustc_mir/build/matches/mod.rs

@@ -453,7 +453,7 @@ pub struct Candidate<'pat, 'tcx:'pat> {
     bindings: Vec<Binding<'tcx>>,
 
     // ...and the guard must be evaluated...
-    guard: Option<ExprRef<'tcx>>,
+    guard: Option<Guard<'tcx>>,
 
     // ...and then we branch to arm with this index.
     arm_index: usize,
@@ -998,7 +998,9 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
 
             // the block to branch to if the guard fails; if there is no
             // guard, this block is simply unreachable
-            let guard = self.hir.mirror(guard);
+            let guard = match guard {
+                Guard::If(e) => self.hir.mirror(e),
+            };
             let source_info = self.source_info(guard.span);
             let cond = unpack!(block = self.as_local_operand(block, guard));
             if autoref {