Use a proper future-compatibility lint

src/librustc/lint/builtin.rs

@@ -192,6 +192,13 @@ declare_lint! {
     "lifetimes or labels named `'_` were erroneously allowed"
 }
 
+declare_lint! {
+    pub RESOLVE_TRAIT_ON_DEFAULTED_UNIT,
+    Warn,
+    "attempt to resolve a trait on an expression whose type cannot be inferred but which \
+     currently defaults to ()"
+}
+
 declare_lint! {
     pub SAFE_EXTERN_STATICS,
     Warn,
@@ -272,6 +279,7 @@ impl LintPass for HardwiredLints {
             SUPER_OR_SELF_IN_GLOBAL_PATH,
             HR_LIFETIME_IN_ASSOC_TYPE,
             LIFETIME_UNDERSCORE,
+            RESOLVE_TRAIT_ON_DEFAULTED_UNIT,
             SAFE_EXTERN_STATICS,
             PATTERNS_IN_FNS_WITHOUT_BODY,
             EXTRA_REQUIREMENT_IN_IMPL,

src/librustc/traits/select.rs

@@ -52,6 +52,7 @@ use std::mem;
 use std::rc::Rc;
 use syntax::abi::Abi;
 use hir;
+use lint;
 use util::nodemap::FxHashMap;
 
 struct InferredObligationsSnapshotVecDelegate<'tcx> {
@@ -455,13 +456,11 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             }
 
             if raise_warning {
-                let sess = tcx.sess;
-                let span = obligation.cause.span;
-                let mut warn = sess.struct_span_warn(span, "code relies on type inference rules \
-                                                            which are likely to change");
-                warn.span_label(span, &"the type of this expression may change from () \
-                                        to ! in a future version of Rust");
-                warn.emit();
+                tcx.sess.add_lint(lint::builtin::RESOLVE_TRAIT_ON_DEFAULTED_UNIT,
+                                  obligation.cause.body_id,
+                                  obligation.cause.span,
+                                  format!("code relies on type inference rules which are likely \
+                                           to change"));
             }
         }
         Ok(ret)

src/librustc_lint/lib.rs

@@ -220,6 +220,10 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(LIFETIME_UNDERSCORE),
             reference: "issue #36892 <https://github.com/rust-lang/rust/issues/36892>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(RESOLVE_TRAIT_ON_DEFAULTED_UNIT),
+            reference: "issue #39216 <https://github.com/rust-lang/rust/issues/39216>",
+        },
         FutureIncompatibleInfo {
             id: LintId::of(SAFE_EXTERN_STATICS),
             reference: "issue #36247 <https://github.com/rust-lang/rust/issues/35112>",

src/test/compile-fail/defaulted-unit-warning.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+#![deny(resolve_trait_on_defaulted_unit)]
+
+trait Deserialize {
+    fn deserialize() -> Result<Self, String>
+}
+
+fn doit() -> Result<(), String> {
+    let _ = Deserialize::deserialize()?;
+    //~^ ERROR attempt to resolve a trait
+    Ok(())
+}
+
+fn main() {
+    doit();
+}
+