issue a future-compat lint for constants of invalid type

This is a [breaking-change]: according to RFC #1445, constants used as
patterns must be of a type that *derives* `Eq`. If you encounter a
problem, you are most likely using a constant in an expression where the
type of the constant is some struct that does not currently implement
`Eq`. Something like the following:

```rust
struct SomeType { ... }
const SOME_CONST: SomeType = ...;

match foo {
    SOME_CONST => ...
}
```

The easiest and most future compatible fix is to annotate the type in
question with `#[derive(Eq)]` (note that merely *implementing* `Eq` is
not enough, it must be *derived*):

```rust
struct SomeType { ... }
const SOME_CONST: SomeType = ...;

match foo {
    SOME_CONST => ...
}
```

Another good option is to rewrite the match arm to use an `if`
condition (this is also particularly good for floating point types,
which implement `PartialEq` but not `Eq`):

```rust
match foo {
    c if c == SOME_CONST => ...
}
```

Finally, a third alternative is to tag the type with
`#[structural_match]`; but this is not recommended, as the attribute is
never expected to be stabilized. Please see RFC #1445 for more details.

src/librustc/lint/builtin.rs

@@ -136,6 +136,19 @@ declare_lint! {
     "type parameter default erroneously allowed in invalid location"
 }
 
+declare_lint! {
+    pub ILLEGAL_FLOATING_POINT_CONSTANT_PATTERN,
+    Warn,
+    "floating-point constants cannot be used in patterns"
+}
+
+declare_lint! {
+    pub ILLEGAL_STRUCT_OR_ENUM_CONSTANT_PATTERN,
+    Deny,
+    "constants of struct or enum type can only be used in a pattern if \
+     the struct or enum has `#[derive(Eq)]`"
+}
+
 declare_lint! {
     pub MATCH_OF_UNIT_VARIANT_VIA_PAREN_DOTDOT,
     Deny,
@@ -193,6 +206,8 @@ impl LintPass for HardwiredLints {
             PRIVATE_IN_PUBLIC,
             INACCESSIBLE_EXTERN_CRATE,
             INVALID_TYPE_PARAM_DEFAULT,
+            ILLEGAL_FLOATING_POINT_CONSTANT_PATTERN,
+            ILLEGAL_STRUCT_OR_ENUM_CONSTANT_PATTERN,
             MATCH_OF_UNIT_VARIANT_VIA_PAREN_DOTDOT,
             CONST_ERR,
             RAW_POINTER_DERIVE,

src/librustc/middle/check_match.rs

@@ -478,7 +478,7 @@ impl<'a, 'tcx> Folder for StaticInliner<'a, 'tcx> {
                     Some(Def::Const(did)) => {
                         let substs = Some(self.tcx.node_id_item_substs(pat.id).substs);
                         if let Some((const_expr, _)) = lookup_const_by_id(self.tcx, did, substs) {
-                            match const_expr_to_pat(self.tcx, const_expr, pat.span) {
+                            match const_expr_to_pat(self.tcx, const_expr, pat.id, pat.span) {
                                 Ok(new_pat) => {
                                     if let Some(ref mut map) = self.renaming_map {
                                         // Record any renamings we do here
@@ -487,7 +487,6 @@ impl<'a, 'tcx> Folder for StaticInliner<'a, 'tcx> {
                                     new_pat
                                 }
                                 Err(def_id) => {
-                                    // TODO back-compat
                                     self.failed = true;
                                     self.tcx.sess.span_err(
                                         pat.span,

src/librustc/middle/const_eval.rs

@@ -16,6 +16,7 @@ use self::EvalHint::*;
 
 use front::map as ast_map;
 use front::map::blocks::FnLikeNode;
+use lint;
 use middle::cstore::{self, CrateStore, InlinedItem};
 use middle::{infer, subst, traits};
 use middle::def::Def;
@@ -323,13 +324,41 @@ impl ConstVal {
     }
 }
 
-pub fn const_expr_to_pat(tcx: &ty::TyCtxt, expr: &Expr, span: Span)
+pub fn const_expr_to_pat(tcx: &ty::TyCtxt, expr: &Expr, pat_id: ast::NodeId, span: Span)
                          -> Result<P<hir::Pat>, DefId> {
+    let pat_ty = tcx.expr_ty(expr);
+    debug!("expr={:?} pat_ty={:?} pat_id={}", expr, pat_ty, pat_id);
+    match pat_ty.sty {
+        ty::TyFloat(_) => {
+            tcx.sess.add_lint(
+                lint::builtin::ILLEGAL_FLOATING_POINT_CONSTANT_PATTERN,
+                pat_id,
+                span,
+                format!("floating point constants cannot be used in patterns"));
+        }
+        ty::TyEnum(adt_def, _) |
+        ty::TyStruct(adt_def, _) => {
+            if !tcx.has_attr(adt_def.did, "structural_match") {
+                tcx.sess.add_lint(
+                    lint::builtin::ILLEGAL_STRUCT_OR_ENUM_CONSTANT_PATTERN,
+                    pat_id,
+                    span,
+                    format!("to use a constant of type `{}` \
+                             in a pattern, \
+                             `{}` must be annotated with `#[derive(Eq)]`",
+                            tcx.item_path_str(adt_def.did),
+                            tcx.item_path_str(adt_def.did)));
+            }
+        }
+        _ => { }
+    }
+
     let pat = match expr.node {
         hir::ExprTup(ref exprs) =>
             PatKind::Tup(try!(exprs.iter()
-                                   .map(|expr| const_expr_to_pat(tcx, &expr, span))
-                                   .collect())),
+                                  .map(|expr| const_expr_to_pat(tcx, &expr,
+                                                                pat_id, span))
+                                  .collect())),
 
         hir::ExprCall(ref callee, ref args) => {
             let def = *tcx.def_map.borrow().get(&callee.id).unwrap();
@@ -347,7 +376,8 @@ pub fn const_expr_to_pat(tcx: &ty::TyCtxt, expr: &Expr, span: Span)
                 _ => unreachable!()
             };
             let pats = try!(args.iter()
-                                .map(|expr| const_expr_to_pat(tcx, &**expr, span))
+                                .map(|expr| const_expr_to_pat(tcx, &**expr,
+                                                              pat_id, span))
                                 .collect());
             PatKind::TupleStruct(path, Some(pats))
         }
@@ -359,7 +389,8 @@ pub fn const_expr_to_pat(tcx: &ty::TyCtxt, expr: &Expr, span: Span)
                                span: codemap::DUMMY_SP,
                                node: hir::FieldPat {
                                    name: field.name.node,
-                                   pat: try!(const_expr_to_pat(tcx, &field.expr, span)),
+                                   pat: try!(const_expr_to_pat(tcx, &field.expr,
+                                                               pat_id, span)),
                                    is_shorthand: false,
                                },
                            }))
@@ -369,7 +400,8 @@ pub fn const_expr_to_pat(tcx: &ty::TyCtxt, expr: &Expr, span: Span)
 
         hir::ExprVec(ref exprs) => {
             let pats = try!(exprs.iter()
-                                 .map(|expr| const_expr_to_pat(tcx, &expr, span))
+                                 .map(|expr| const_expr_to_pat(tcx, &expr,
+                                                               pat_id, span))
                                  .collect());
             PatKind::Vec(pats, None, hir::HirVec::new())
         }
@@ -383,7 +415,7 @@ pub fn const_expr_to_pat(tcx: &ty::TyCtxt, expr: &Expr, span: Span)
                 Some(Def::AssociatedConst(def_id)) => {
                     let substs = Some(tcx.node_id_item_substs(expr.id).substs);
                     let (expr, _ty) = lookup_const_by_id(tcx, def_id, substs).unwrap();
-                    return const_expr_to_pat(tcx, expr, span);
+                    return const_expr_to_pat(tcx, expr, pat_id, span);
                 },
                 _ => unreachable!(),
             }

src/librustc_lint/lib.rs

@@ -179,6 +179,14 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(OVERLAPPING_INHERENT_IMPLS),
             reference: "issue #22889 <https://github.com/rust-lang/rust/issues/22889>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(ILLEGAL_FLOATING_POINT_CONSTANT_PATTERN),
+            reference: "RFC 1445 <https://github.com/rust-lang/rfcs/pull/1445>",
+        },
+        FutureIncompatibleInfo {
+            id: LintId::of(ILLEGAL_STRUCT_OR_ENUM_CONSTANT_PATTERN),
+            reference: "RFC 1445 <https://github.com/rust-lang/rfcs/pull/1445>",
+        },
         ]);
 
     // We have one lint pass defined specially

src/librustc_mir/hair/cx/pattern.rs

@@ -92,6 +92,7 @@ impl<'patcx, 'cx, 'tcx> PatCx<'patcx, 'cx, 'tcx> {
                             Some((const_expr, _const_ty)) => {
                                 match const_eval::const_expr_to_pat(self.cx.tcx,
                                                                     const_expr,
+                                                                    pat.id,
                                                                     pat.span) {
                                     Ok(pat) =>
                                         return self.to_pattern(&pat),