diagnostics: avoid wrong `unused_parens` on `x as (T) < y`

compiler/rustc_lint/src/early.rs

@@ -228,6 +228,7 @@ impl<'a, T: EarlyLintPass> ast_visit::Visitor<'a> for EarlyContextAndPass<'a, T>
             }) => self.check_id(closure_id),
             _ => {}
         }
+        lint_callback!(self, check_expr_post, e);
     }
 
     fn visit_generic_arg(&mut self, arg: &'a ast::GenericArg) {

compiler/rustc_lint/src/passes.rs

@@ -153,6 +153,7 @@ macro_rules! early_lint_methods {
             fn check_pat(a: &ast::Pat);
             fn check_pat_post(a: &ast::Pat);
             fn check_expr(a: &ast::Expr);
+            fn check_expr_post(a: &ast::Expr);
             fn check_ty(a: &ast::Ty);
             fn check_generic_arg(a: &ast::GenericArg);
             fn check_generic_param(a: &ast::GenericParam);

compiler/rustc_lint/src/unused.rs

@@ -955,11 +955,14 @@ declare_lint! {
 
 pub struct UnusedParens {
     with_self_ty_parens: bool,
+    /// `1 as (i32) < 2` parses to ExprKind::Lt
+    /// `1 as i32 < 2` parses to i32::<2[missing angle bracket]
+    parens_in_cast_in_lt: Vec<ast::NodeId>,
 }
 
 impl UnusedParens {
     pub fn new() -> Self {
-        Self { with_self_ty_parens: false }
+        Self { with_self_ty_parens: false, parens_in_cast_in_lt: Vec::new() }
     }
 }
 
@@ -1055,6 +1058,14 @@ impl UnusedParens {
 impl EarlyLintPass for UnusedParens {
     #[inline]
     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
+        if let ExprKind::Binary(op, lhs, _rhs) = &e.kind &&
+            (op.node == ast::BinOpKind::Lt || op.node == ast::BinOpKind::Shl) &&
+            let ExprKind::Cast(_expr, ty) = &lhs.kind &&
+            let ast::TyKind::Paren(_) = &ty.kind
+        {
+            self.parens_in_cast_in_lt.push(ty.id);
+        }
+
         match e.kind {
             ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
                 self.check_unused_parens_pat(cx, pat, false, false, (true, true));
@@ -1101,6 +1112,17 @@ impl EarlyLintPass for UnusedParens {
         <Self as UnusedDelimLint>::check_expr(self, cx, e)
     }
 
+    fn check_expr_post(&mut self, _cx: &EarlyContext<'_>, e: &ast::Expr) {
+        if let ExprKind::Binary(op, lhs, _rhs) = &e.kind &&
+            (op.node == ast::BinOpKind::Lt || op.node == ast::BinOpKind::Shl) &&
+            let ExprKind::Cast(_expr, ty) = &lhs.kind &&
+            let ast::TyKind::Paren(_) = &ty.kind
+        {
+            let id = self.parens_in_cast_in_lt.pop().expect("check_expr and check_expr_post must balance");
+            assert_eq!(id, ty.id, "check_expr and check_expr_post is a depth-first tree traversal");
+        }
+    }
+
     fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
         use ast::{Mutability, PatKind::*};
         let keep_space = (false, false);
@@ -1141,6 +1163,11 @@ impl EarlyLintPass for UnusedParens {
     }
 
     fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
+        if let ast::TyKind::Paren(_) = ty.kind &&
+            Some(&ty.id) == self.parens_in_cast_in_lt.last()
+        {
+            return;
+        }
         match &ty.kind {
             ast::TyKind::Array(_, len) => {
                 self.check_unused_delims_expr(

tests/ui/lint/unused/unused-parens-issue-106413.rs

@@ -0,0 +1,32 @@
+// check-pass
+#![warn(unused_parens)]
+
+fn id<T>(t: T) -> T { t }
+
+fn main() {
+    // This should not warn
+    let _ = 1 as (i32) < 2;
+    let _ = id(1 as (i32) < 2);
+    let _ = id(1 as i32)
+        as (i32) < 2;
+    // These should warn
+    let _ = 1 as ((i32)) < 2; //~WARN unnecessary parentheses
+    let _ = 1 as (i32); //~WARN unnecessary parentheses
+    let _ = 1 as (i32) > 2; //~WARN unnecessary parentheses
+    let _ = id(1 as (i32)) //~WARN unnecessary parentheses
+        as (i32) < 2;
+    let _ = id(1 as (i32)) < 2; //~WARN unnecessary parentheses
+
+    // This should not warn
+    let _ = 1 as (i32) << 2;
+    let _ = id(1 as (i32) << 2);
+    let _ = id(1 as i32)
+        as (i32) << 2;
+    // These should warn
+    let _ = 1 as ((i32)) << 2; //~WARN unnecessary parentheses
+    let _ = 1 as (i32); //~WARN unnecessary parentheses
+    let _ = 1 as (i32) >> 2; //~WARN unnecessary parentheses
+    let _ = id(1 as (i32)) //~WARN unnecessary parentheses
+        as (i32) << 2;
+    let _ = id(1 as (i32)) << 2; //~WARN unnecessary parentheses
+}

tests/ui/lint/unused/unused-parens-issue-106413.stderr

@@ -0,0 +1,127 @@
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:13:19
+   |
+LL |     let _ = 1 as ((i32)) < 2;
+   |                   ^   ^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-parens-issue-106413.rs:2:9
+   |
+LL | #![warn(unused_parens)]
+   |         ^^^^^^^^^^^^^
+help: remove these parentheses
+   |
+LL -     let _ = 1 as ((i32)) < 2;
+LL +     let _ = 1 as (i32) < 2;
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:14:18
+   |
+LL |     let _ = 1 as (i32);
+   |                  ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = 1 as (i32);
+LL +     let _ = 1 as i32;
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:15:18
+   |
+LL |     let _ = 1 as (i32) > 2;
+   |                  ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = 1 as (i32) > 2;
+LL +     let _ = 1 as i32 > 2;
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:16:21
+   |
+LL |     let _ = id(1 as (i32))
+   |                     ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = id(1 as (i32))
+LL +     let _ = id(1 as i32)
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:18:21
+   |
+LL |     let _ = id(1 as (i32)) < 2;
+   |                     ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = id(1 as (i32)) < 2;
+LL +     let _ = id(1 as i32) < 2;
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:26:19
+   |
+LL |     let _ = 1 as ((i32)) << 2;
+   |                   ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = 1 as ((i32)) << 2;
+LL +     let _ = 1 as (i32) << 2;
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:27:18
+   |
+LL |     let _ = 1 as (i32);
+   |                  ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = 1 as (i32);
+LL +     let _ = 1 as i32;
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:28:18
+   |
+LL |     let _ = 1 as (i32) >> 2;
+   |                  ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = 1 as (i32) >> 2;
+LL +     let _ = 1 as i32 >> 2;
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:29:21
+   |
+LL |     let _ = id(1 as (i32))
+   |                     ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = id(1 as (i32))
+LL +     let _ = id(1 as i32)
+   |
+
+warning: unnecessary parentheses around type
+  --> $DIR/unused-parens-issue-106413.rs:31:21
+   |
+LL |     let _ = id(1 as (i32)) << 2;
+   |                     ^   ^
+   |
+help: remove these parentheses
+   |
+LL -     let _ = id(1 as (i32)) << 2;
+LL +     let _ = id(1 as i32) << 2;
+   |
+
+warning: 10 warnings emitted
+