Convert warning about *const _ to a future-compat lint

src/librustc/lint/builtin.rs

@@ -240,6 +240,12 @@ declare_lint! {
    "detects single use lifetimes"
 }
 
+declare_lint! {
+    pub TYVAR_BEHIND_RAW_POINTER,
+    Warn,
+    "raw pointer to an inference variable"
+}
+
 /// Does nothing as a lint pass, but registers some `Lint`s
 /// which are used by other parts of the compiler.
 #[derive(Copy, Clone)]
@@ -284,7 +290,8 @@ impl LintPass for HardwiredLints {
             UNUSED_UNSAFE,
             UNUSED_MUT,
             COERCE_NEVER,
-            SINGLE_USE_LIFETIME
+            SINGLE_USE_LIFETIME,
+            TYVAR_BEHIND_RAW_POINTER
         )
     }
 }

src/librustc_lint/lib.rs

@@ -255,6 +255,10 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(COERCE_NEVER),
             reference: "issue #46325 <https://github.com/rust-lang/rust/issues/46325>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(TYVAR_BEHIND_RAW_POINTER),
+            reference: "issue #46906 <https://github.com/rust-lang/rust/issues/46906>",
+        },
         ]);
 
     // Register renamed and removed lints

src/librustc_typeck/check/method/probe.rs

@@ -27,6 +27,7 @@ use syntax::ast;
 use syntax::util::lev_distance::{lev_distance, find_best_match_for_name};
 use syntax_pos::Span;
 use rustc::hir;
+use rustc::lint;
 use std::mem;
 use std::ops::Deref;
 use std::rc::Rc;
@@ -249,7 +250,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         // think cause spurious errors. Really though this part should
         // take place in the `self.probe` below.
         let steps = if mode == Mode::MethodCall {
-            match self.create_steps(span, self_ty, is_suggestion) {
+            match self.create_steps(span, scope_expr_id, self_ty, is_suggestion) {
                 Some(steps) => steps,
                 None => {
                     return Err(MethodError::NoMatch(NoMatchData::new(Vec::new(),
@@ -291,6 +292,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
     fn create_steps(&self,
                     span: Span,
+                    scope_expr_id: ast::NodeId,
                     self_ty: Ty<'tcx>,
                     is_suggestion: IsSuggestion)
                     -> Option<Vec<CandidateStep<'tcx>>> {
@@ -318,18 +320,19 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         match final_ty.sty {
             ty::TyInfer(ty::TyVar(_)) => {
                 // Ended in an inference variable. If we are doing
-                // a real method lookup, this is a hard error (it's an
-                // ambiguity and we can't make progress).
+                // a real method lookup, this is a hard error because it's
+                // possible that there will be multiple applicable methods.
                 if !is_suggestion.0 {
                     if reached_raw_pointer
                     && !self.tcx.sess.features.borrow().arbitrary_self_types {
-                        // only produce a warning in this case, because inference variables used to
-                        // be allowed here in some cases for raw pointers
-                        struct_span_warn!(self.tcx.sess, span, E0619,
-                            "the type of this value must be known in this context")
-                        .note("this will be made into a hard error in a future version of \
-                               the compiler")
-                        .emit();
+                        // this case used to be allowed by the compiler,
+                        // so we do a future-compat lint here
+                        // (see https://github.com/rust-lang/rust/issues/46906)
+                        self.tcx.lint_node(
+                            lint::builtin::TYVAR_BEHIND_RAW_POINTER,
+                            scope_expr_id,
+                            span,
+                            &format!("the type of this value must be known in this context"));
                     } else {
                         let t = self.structurally_resolved_type(span, final_ty);
                         assert_eq!(t, self.tcx.types.err);

src/test/ui/inference-variable-behind-raw-pointer.stderr

@@ -1,8 +1,10 @@
-warning[E0619]: the type of this value must be known in this context
+warning: the type of this value must be known in this context
   --> $DIR/inference-variable-behind-raw-pointer.rs:18:13
    |
 18 |     if data.is_null() {}
    |             ^^^^^^^
    |
-   = note: this will be made into a hard error in a future version of the compiler
+   = note: #[warn(tyvar_behind_raw_pointer)] on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>