Wrap Cast expr in parens when the ty ends with empty angle brackets

Fixes 4621

Previously rustfmt would remove empty angle brackets from cast
expressions e.g. `x as i32<>` => `x as i32`. This lead to code that
could not compile when the cast occurred in an if statement.

The advice from the compiler was to wrap the cast in parentheses, which
is now the behavior rustfmt employs.

Author: ytmimi

src/expr.rs

@@ -1,6 +1,7 @@
 use std::borrow::Cow;
 use std::cmp::min;
 use std::collections::HashMap;
+use std::ops::Deref;
 
 use itertools::Itertools;
 use rustc_ast::token::{Delimiter, LitKind};
@@ -282,14 +283,31 @@ fn format_expr_inner(
         ast::ExprKind::AddrOf(borrow_kind, mutability, ref expr) => {
             rewrite_expr_addrof(context, borrow_kind, mutability, expr, shape)
         }
-        ast::ExprKind::Cast(ref expr, ref ty) => rewrite_pair(
-            &**expr,
-            &**ty,
-            PairParts::infix(" as "),
-            context,
-            shape,
-            SeparatorPlace::Front,
-        ),
+        ast::ExprKind::Cast(ref expr, ref ty) => {
+            let mut result = String::new();
+            let empty_angle_brackets = ty_ends_with_empty_angle_brackets(ty);
+            let shape = if empty_angle_brackets {
+                result.push('(');
+                shape.sub_width(2)?
+            } else {
+                shape
+            };
+
+            result.push_str(&rewrite_pair(
+                &**expr,
+                &**ty,
+                PairParts::infix(" as "),
+                context,
+                shape,
+                SeparatorPlace::Front,
+            )?);
+
+            if empty_angle_brackets {
+                result.push(')');
+            }
+
+            Some(result)
+        }
         ast::ExprKind::Type(ref expr, ref ty) => rewrite_pair(
             &**expr,
             &**ty,
@@ -458,6 +476,21 @@ fn format_expr_inner(
         })
 }
 
+fn ty_ends_with_empty_angle_brackets(ty: &ast::Ty) -> bool {
+    if let ast::TyKind::Path(_, path) = &ty.kind {
+        matches!(
+            path.segments.last(),
+            Some(ast::PathSegment {args: Some(generic_args), ..})
+            if matches!(
+                generic_args.deref(),
+                ast::GenericArgs::AngleBracketed(bracket_args) if bracket_args.args.is_empty()
+            )
+        )
+    } else {
+        false
+    }
+}
+
 pub(crate) fn rewrite_array<'a, T: 'a + IntoOverflowableItem<'a>>(
     name: &'a str,
     exprs: impl Iterator<Item = &'a T>,

tests/source/issue-4621/wrap_in_parens.rs

@@ -0,0 +1,6 @@
+pub fn foo() {
+    let x: u32 = 100;
+    if x as i32<> < 0 {
+        // ...
+    }
+}

tests/target/issue-4621/wrap_in_parens.rs

@@ -0,0 +1,6 @@
+pub fn foo() {
+    let x: u32 = 100;
+    if (x as i32) < 0 {
+        // ...
+    }
+}