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

refactor match guard

This is the first step to implement RFC 2294: if-let-guard. Tracking issue: https://github.com/rust-lang/rust/issues/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

5 files changed, 32 insertions, 9 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index bd0e0d277ee..72f1791ef7c 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -857,11 +857,16 @@ pub struct Local {
 pub struct Arm {
     pub attrs: Vec<Attribute>,
     pub pats: Vec<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 ident: Ident,
     pub expr: P<Expr>,
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 660056e15e0..dff408d2339 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -118,6 +118,10 @@ pub trait Folder : Sized {
         noop_fold_arm(a, self)
     }
 
+    fn fold_guard(&mut self, g: Guard) -> Guard {
+        noop_fold_guard(g, self)
+    }
+
     fn fold_pat(&mut self, p: P<Pat>) -> P<Pat> {
         noop_fold_pat(p, self)
     }
@@ -354,11 +358,17 @@ pub fn noop_fold_arm<T: Folder>(Arm {attrs, pats, guard, body}: Arm,
     Arm {
         attrs: fold_attrs(attrs, fld),
         pats: pats.move_map(|x| fld.fold_pat(x)),
-        guard: guard.map(|x| fld.fold_expr(x)),
+        guard: guard.map(|x| fld.fold_guard(x)),
         body: fld.fold_expr(body),
     }
 }
 
+pub fn noop_fold_guard<T: Folder>(g: Guard, fld: &mut T) -> Guard {
+    match g {
+        Guard::If(e) => Guard::If(fld.fold_expr(e)),
+    }
+}
+
 pub fn noop_fold_ty_binding<T: Folder>(b: TypeBinding, fld: &mut T) -> TypeBinding {
     TypeBinding {
         id: fld.new_id(b.id),
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 6c50b9c82f2..c741bde7c5f 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -12,7 +12,7 @@ use rustc_target::spec::abi::{self, Abi};
 use ast::{AngleBracketedArgs, ParenthesisedArgs, AttrStyle, BareFnTy};
 use ast::{GenericBound, TraitBoundModifier};
 use ast::Unsafety;
-use ast::{Mod, AnonConst, Arg, Arm, Attribute, BindingMode, TraitItemKind};
+use ast::{Mod, AnonConst, Arg, Arm, Guard, Attribute, BindingMode, TraitItemKind};
 use ast::Block;
 use ast::{BlockCheckMode, CaptureBy, Movability};
 use ast::{Constness, Crate};
@@ -3533,7 +3533,7 @@ impl<'a> Parser<'a> {
         self.eat(&token::BinOp(token::Or));
         let pats = self.parse_pats()?;
         let guard = if self.eat_keyword(keywords::If) {
-            Some(self.parse_expr()?)
+            Some(Guard::If(self.parse_expr()?))
         } else {
             None
         };
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index e78e1afe3a4..85d29a5be89 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -2702,10 +2702,14 @@ impl<'a> State<'a> {
         self.print_outer_attributes(&arm.attrs)?;
         self.print_pats(&arm.pats)?;
         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 {
+                ast::Guard::If(ref e) => {
+                    self.word_space("if")?;
+                    self.print_expr(e)?;
+                    self.s.space()?;
+                }
+            }
         }
         self.word_space("=>")?;
 
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index e57d692faae..77311bf53fd 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -819,7 +819,11 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
 
 pub fn walk_arm<'a, V: Visitor<'a>>(visitor: &mut V, arm: &'a 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);
 }