Parenthesize AssocOp::Cast in suggestion when replacement operator is < to avoid parse error (#16848)

Fix int_plus_one suggestion causing parse error when LHS is an
`AssocOp::Cast`

When int_plus_one rewrites an expression like `x as usize + 1 <= y`, it
  suggests `x as usize < y`. This doesn't compile because the parser
  interprets `usize<y>` as the start of generic arguments rather than a
  lt comparison.

The fix parenthesizes the LHS if it's an `AssocOp::Cast` and the
replacement
  operator is `<`: `(x as usize) < y`.

  ---
changelog: [int_plus_one]: Parenthesize `AssocOp::Cast` in suggestion
when replacement operator is < to avoid parse error

Author: Jarcho

clippy_lints/src/int_plus_one.rs

@@ -1,6 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::sugg;
 use rustc_ast::ast::{BinOpKind, Expr, ExprKind, LitKind, UnOp};
+use rustc_ast::util::parser::AssocOp;
 use rustc_data_structures::packed::Pu128;
 use rustc_errors::Applicability;
 use rustc_lint::{EarlyContext, EarlyLintPass};
@@ -134,12 +135,19 @@ impl IntPlusOne {
             |diag| {
                 let mut app = Applicability::MachineApplicable;
                 let ctxt = expr.span.ctxt();
-                let new_lhs = sugg::Sugg::ast(cx, new_lhs, "_", ctxt, &mut app);
+                let mut new_lhs = sugg::Sugg::ast(cx, new_lhs, "_", ctxt, &mut app);
                 let new_rhs = sugg::Sugg::ast(cx, new_rhs, "_", ctxt, &mut app);
                 let new_binop = match le_or_ge {
                     LeOrGe::Ge => BinOpKind::Gt,
                     LeOrGe::Le => BinOpKind::Lt,
                 };
+                // When the replacement operator is `<`, an `as` cast on the LHS
+                // must be parenthesized. Otherwise, the parser interprets the `<`
+                // as the start of generic arguments on the cast type
+                // (e.g., `x as usize < y` is parsed as `x as usize<y>`).
+                if matches!(new_lhs, sugg::Sugg::BinOp(AssocOp::Cast, ..)) && new_binop == BinOpKind::Lt {
+                    new_lhs = new_lhs.maybe_paren();
+                }
                 let rec = sugg::make_binop(new_binop, &new_lhs, &new_rhs);
                 diag.span_suggestion(expr.span, "change it to", rec, app);
             },

tests/ui/int_plus_one.fixed

@@ -16,4 +16,18 @@ fn main() {
 
     let _ = x > y; // should be ok
     let _ = y < x; // should be ok
+
+    // When the suggestion replaces `<=`/`>=` with `<`, an `as` cast on
+    // the LHS must be parenthesized to avoid parser ambiguity
+    // (e.g., `x as usize < y` is parsed as `x as usize<y>`).
+    let z = 0usize;
+    let _ = (x as usize) < z; //~ int_plus_one
+    let _ = z > x as usize; //~ int_plus_one
+    // No parentheses needed when the replacement operator is `>`.
+    let _ = x as usize > z; //~ int_plus_one
+    let _ = z < x as usize; //~ int_plus_one
+
+    // Nested and parenthesized casts on the LHS.
+    let _ = ((x as usize) as u8) < 5u8; //~ int_plus_one
+    let _ = (x as usize) < z; //~ int_plus_one
 }

tests/ui/int_plus_one.rs

@@ -16,4 +16,18 @@ fn main() {
 
     let _ = x > y; // should be ok
     let _ = y < x; // should be ok
+
+    // When the suggestion replaces `<=`/`>=` with `<`, an `as` cast on
+    // the LHS must be parenthesized to avoid parser ambiguity
+    // (e.g., `x as usize < y` is parsed as `x as usize<y>`).
+    let z = 0usize;
+    let _ = x as usize + 1 <= z; //~ int_plus_one
+    let _ = z >= x as usize + 1; //~ int_plus_one
+    // No parentheses needed when the replacement operator is `>`.
+    let _ = x as usize - 1 >= z; //~ int_plus_one
+    let _ = z <= x as usize - 1; //~ int_plus_one
+
+    // Nested and parenthesized casts on the LHS.
+    let _ = ((x as usize) as u8) + 1 <= 5u8; //~ int_plus_one
+    let _ = (x as usize) + 1 <= z; //~ int_plus_one
 }

tests/ui/int_plus_one.stderr

@@ -49,5 +49,41 @@ error: unnecessary `>= y + 1` or `x - 1 >=`
 LL |     let _ = y <= -1 + x;
    |             ^^^^^^^^^^^ help: change it to: `y < x`
 
-error: aborting due to 8 previous errors
+error: unnecessary `>= y + 1` or `x - 1 >=`
+  --> tests/ui/int_plus_one.rs:24:13
+   |
+LL |     let _ = x as usize + 1 <= z;
+   |             ^^^^^^^^^^^^^^^^^^^ help: change it to: `(x as usize) < z`
+
+error: unnecessary `>= y + 1` or `x - 1 >=`
+  --> tests/ui/int_plus_one.rs:25:13
+   |
+LL |     let _ = z >= x as usize + 1;
+   |             ^^^^^^^^^^^^^^^^^^^ help: change it to: `z > x as usize`
+
+error: unnecessary `>= y + 1` or `x - 1 >=`
+  --> tests/ui/int_plus_one.rs:27:13
+   |
+LL |     let _ = x as usize - 1 >= z;
+   |             ^^^^^^^^^^^^^^^^^^^ help: change it to: `x as usize > z`
+
+error: unnecessary `>= y + 1` or `x - 1 >=`
+  --> tests/ui/int_plus_one.rs:28:13
+   |
+LL |     let _ = z <= x as usize - 1;
+   |             ^^^^^^^^^^^^^^^^^^^ help: change it to: `z < x as usize`
+
+error: unnecessary `>= y + 1` or `x - 1 >=`
+  --> tests/ui/int_plus_one.rs:31:13
+   |
+LL |     let _ = ((x as usize) as u8) + 1 <= 5u8;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: change it to: `((x as usize) as u8) < 5u8`
+
+error: unnecessary `>= y + 1` or `x - 1 >=`
+  --> tests/ui/int_plus_one.rs:32:13
+   |
+LL |     let _ = (x as usize) + 1 <= z;
+   |             ^^^^^^^^^^^^^^^^^^^^^ help: change it to: `(x as usize) < z`
+
+error: aborting due to 14 previous errors