Add trivial bounds lint

src/librustc_lint/builtin.rs

@@ -1591,3 +1591,61 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ExternCrate {
         self.0 += 1;
     }
 }
+
+/// Lint for trait and lifetime bounds that don't depend on type parameters
+/// which either do nothing, or stop the item from being used.
+pub struct TrivialConstraints;
+
+declare_lint! {
+    TRIVIAL_BOUNDS,
+    Warn,
+    "these bounds don't depend on an type parameters"
+}
+
+impl LintPass for TrivialConstraints {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(TRIVIAL_BOUNDS)
+    }
+}
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TrivialConstraints {
+    fn check_item(
+        &mut self,
+        cx: &LateContext<'a, 'tcx>,
+        item: &'tcx hir::Item,
+    ) {
+        use rustc::ty::fold::TypeFoldable;
+        use rustc::ty::Predicate::*;
+
+
+        if cx.tcx.features().trivial_bounds {
+            let def_id = cx.tcx.hir.local_def_id(item.id);
+            let predicates = cx.tcx.predicates_of(def_id);
+            for predicate in &predicates.predicates {
+                let predicate_kind_name = match *predicate {
+                    Trait(..) => "Trait",
+                    TypeOutlives(..) |
+                    RegionOutlives(..) => "Lifetime",
+
+                    // Ignore projections, as they can only be global
+                    // if the trait bound is global
+                    Projection(..) |
+                    // Ignore bounds that a user can't type
+                    WellFormed(..) |
+                    ObjectSafe(..) |
+                    ClosureKind(..) |
+                    Subtype(..) |
+                    ConstEvaluatable(..) => continue,
+                };
+                if !predicate.is_global() {
+                    cx.span_lint(
+                        TRIVIAL_BOUNDS,
+                        item.span,
+                        &format!("{} bound {} does not depend on any type \
+                                or lifetime parameters", predicate_kind_name, predicate),
+                    );
+                }
+            }
+        }
+    }
+}

src/librustc_lint/lib.rs

@@ -137,6 +137,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                  UnreachablePub,
                  TypeAliasBounds,
                  UnusedBrokenConst,
+                 TrivialConstraints,
                  );
 
     add_builtin_with_new!(sess,

src/test/ui/feature-gate-trivial_bounds-lint.rs

@@ -0,0 +1,18 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-pass
+
+#![allow(unused)]
+#![deny(trivial_bounds)] // Ignored without the trivial_bounds feature flag.
+
+struct A where i32: Copy;
+
+fn main() {}

src/test/ui/trivial-bounds-lint.rs

@@ -0,0 +1,50 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(trivial_bounds)]
+#![allow(unused)]
+#![deny(trivial_bounds)]
+
+struct A where i32: Copy; //~ ERROR
+
+trait X<T: Copy> {}
+
+trait Y<T>: Copy {}
+
+trait Z {
+    type S: Copy;
+}
+
+// Check only the bound the user writes trigger the lint
+fn trivial_elaboration<T>() where T: X<i32> + Z<S = i32>, i32: Y<T> {} // OK
+
+fn global_param() where i32: X<()> {} //~ ERROR
+
+// Should only error on the trait bound, not the implicit
+// projection bound <i32 as Z>::S == i32.
+fn global_projection() where i32: Z<S = i32> {} //~ ERROR
+
+impl A {
+    fn new() -> A { A }
+}
+
+// Lifetime bounds should be linted as well
+fn global_lifetimes() where i32: 'static, &'static str: 'static {}
+//~^ ERROR
+//~| ERROR
+
+fn local_lifetimes<'a>() where i32: 'a, &'a str: 'a {} // OK
+
+fn global_outlives() where 'static: 'static {} //~ ERROR
+
+// Check that each bound is checked individually
+fn mixed_bounds<T: Copy>() where i32: X<T> + Copy {} //~ ERROR
+
+fn main() {}

src/test/ui/trivial-bounds-lint.stderr

@@ -0,0 +1,50 @@
+error: Trait bound i32: std::marker::Copy does not depend on any type or lifetime parameters
+  --> $DIR/trivial-bounds-lint.rs:15:1
+   |
+LL | struct A where i32: Copy; //~ ERROR
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/trivial-bounds-lint.rs:13:9
+   |
+LL | #![deny(trivial_bounds)]
+   |         ^^^^^^^^^^^^^^
+
+error: Trait bound i32: X<()> does not depend on any type or lifetime parameters
+  --> $DIR/trivial-bounds-lint.rs:28:1
+   |
+LL | fn global_param() where i32: X<()> {} //~ ERROR
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: Trait bound i32: Z does not depend on any type or lifetime parameters
+  --> $DIR/trivial-bounds-lint.rs:32:1
+   |
+LL | fn global_projection() where i32: Z<S = i32> {} //~ ERROR
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: Lifetime bound i32 : 'static does not depend on any type or lifetime parameters
+  --> $DIR/trivial-bounds-lint.rs:39:1
+   |
+LL | fn global_lifetimes() where i32: 'static, &'static str: 'static {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: Lifetime bound &'static str : 'static does not depend on any type or lifetime parameters
+  --> $DIR/trivial-bounds-lint.rs:39:1
+   |
+LL | fn global_lifetimes() where i32: 'static, &'static str: 'static {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: Lifetime bound 'static : 'static does not depend on any type or lifetime parameters
+  --> $DIR/trivial-bounds-lint.rs:45:1
+   |
+LL | fn global_outlives() where 'static: 'static {} //~ ERROR
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: Trait bound i32: std::marker::Copy does not depend on any type or lifetime parameters
+  --> $DIR/trivial-bounds-lint.rs:48:1
+   |
+LL | fn mixed_bounds<T: Copy>() where i32: X<T> + Copy {} //~ ERROR
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 7 previous errors
+