Auto merge of #62075 - Centril:guardless-match-arms, r=petrochenkov

Remove `ast::Guard`

With the introduction of `ast::ExprKind::Let` in https://github.com/rust-lang/rust/pull/60861, the `ast::Guard` structure is now redundant in terms of representing [`if let` guards](https://github.com/rust-lang/rust/issues/51114) in AST since it can be represented by `ExprKind::Let` syntactically. Therefore, we remove `ast::Guard` here.

However, we keep `hir::Guard` because the semantic representation is a different matter and this story is more unclear right now (might involve `goto 'arm` in HIR or something...).

r? @petrochenkov

src/librustc/hir/lowering.rs

@@ -1382,7 +1382,7 @@ fn lower_arm(&mut self, arm: &Arm) -> hir::Arm {
             attrs: self.lower_attrs(&arm.attrs),
             pats: arm.pats.iter().map(|x| self.lower_pat(x)).collect(),
             guard: match arm.guard {
-                Some(Guard::If(ref x)) => Some(hir::Guard::If(P(self.lower_expr(x)))),
+                Some(ref x) => Some(hir::Guard::If(P(self.lower_expr(x)))),
                 _ => None,
             },
             body: P(self.lower_expr(&arm.body)),

src/librustc_resolve/lib.rs

@@ -3055,7 +3055,7 @@ fn resolve_arm(&mut self, arm: &Arm) {
 
         self.resolve_pats(&arm.pats, PatternSource::Match);
 
-        if let Some(ast::Guard::If(ref expr)) = arm.guard {
+        if let Some(ref expr) = arm.guard {
             self.visit_expr(expr)
         }
         self.visit_expr(&arm.body);

src/librustc_save_analysis/dump_visitor.rs

@@ -1609,9 +1609,8 @@ fn visit_pat(&mut self, p: &'l ast::Pat) {
 
     fn visit_arm(&mut self, arm: &'l ast::Arm) {
         self.process_var_decl_multi(&arm.pats);
-        match arm.guard {
-            Some(ast::Guard::If(ref expr)) => self.visit_expr(expr),
-            _ => {}
+        if let Some(expr) = &arm.guard {
+            self.visit_expr(expr);
         }
         self.visit_expr(&arm.body);
     }

src/libsyntax/ast.rs

@@ -893,16 +893,11 @@ pub struct Local {
 pub struct Arm {
     pub attrs: Vec<Attribute>,
     pub pats: Vec<P<Pat>>,
-    pub guard: Option<Guard>,
+    pub guard: Option<P<Expr>>,
     pub body: P<Expr>,
     pub span: Span,
 }
 
-#[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
-pub enum Guard {
-    If(P<Expr>),
-}
-
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct Field {
     pub ident: Ident,

src/libsyntax/mut_visit.rs

@@ -131,10 +131,6 @@ fn visit_arm(&mut self, a: &mut Arm) {
         noop_visit_arm(a, self);
     }
 
-    fn visit_guard(&mut self, g: &mut Guard) {
-        noop_visit_guard(g, self);
-    }
-
     fn visit_pat(&mut self, p: &mut P<Pat>) {
         noop_visit_pat(p, self);
     }
@@ -389,17 +385,11 @@ pub fn noop_visit_arm<T: MutVisitor>(
 ) {
     visit_attrs(attrs, vis);
     visit_vec(pats, |pat| vis.visit_pat(pat));
-    visit_opt(guard, |guard| vis.visit_guard(guard));
+    visit_opt(guard, |guard| vis.visit_expr(guard));
     vis.visit_expr(body);
     vis.visit_span(span);
 }
 
-pub fn noop_visit_guard<T: MutVisitor>(g: &mut Guard, vis: &mut T) {
-    match g {
-        Guard::If(e) => vis.visit_expr(e),
-    }
-}
-
 pub fn noop_visit_ty_constraint<T: MutVisitor>(
     AssocTyConstraint { id, ident, kind, span }: &mut AssocTyConstraint,
     vis: &mut T

src/libsyntax/parse/parser.rs

@@ -3,7 +3,7 @@
 use crate::ast::{AngleBracketedArgs, ParenthesizedArgs, AttrStyle, BareFnTy};
 use crate::ast::{GenericBound, TraitBoundModifier};
 use crate::ast::Unsafety;
-use crate::ast::{Mod, AnonConst, Arg, Arm, Guard, Attribute, BindingMode, TraitItemKind};
+use crate::ast::{Mod, AnonConst, Arg, Arm, Attribute, BindingMode, TraitItemKind};
 use crate::ast::Block;
 use crate::ast::{BlockCheckMode, CaptureBy, Movability};
 use crate::ast::{Constness, Crate};
@@ -3396,7 +3396,7 @@ fn parse_match_expr(&mut self, mut attrs: ThinVec<Attribute>) -> PResult<'a, P<E
         let lo = self.token.span;
         let pats = self.parse_pats()?;
         let guard = if self.eat_keyword(kw::If) {
-            Some(Guard::If(self.parse_expr()?))
+            Some(self.parse_expr()?)
         } else {
             None
         };

src/libsyntax/print/pprust.rs

@@ -2653,15 +2653,11 @@ fn print_arm(&mut self, arm: &ast::Arm) -> io::Result<()> {
         self.print_outer_attributes(&arm.attrs)?;
         self.print_pats(&arm.pats)?;
         self.s.space()?;
-        if let Some(ref g) = arm.guard {
-            match g {
-                ast::Guard::If(ref e) => {
+        if let Some(ref e) = arm.guard {
             self.word_space("if")?;
             self.print_expr(e)?;
             self.s.space()?;
         }
-            }
-        }
         self.word_space("=>")?;
 
         match arm.body.node {

src/libsyntax/visit.rs

@@ -826,10 +826,8 @@ 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);
-    if let Some(ref g) = &arm.guard {
-        match g {
-            Guard::If(ref e) => visitor.visit_expr(e),
-        }
+    if let Some(ref e) = &arm.guard {
+        visitor.visit_expr(e);
     }
     visitor.visit_expr(&arm.body);
     walk_list!(visitor, visit_attribute, &arm.attrs);