Warn on all erroneous constants

src/librustc_lint/builtin.rs

@@ -1441,3 +1441,71 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TypeAliasBounds {
         }
     }
 }
+
+/// Lint constants that are erroneous.
+/// Without this lint, we might not get any diagnostic if the constant is
+/// unused within this crate, even though downstream crates can't use it
+/// without producing an error.
+pub struct UnusedBrokenConst;
+
+impl LintPass for UnusedBrokenConst {
+    fn get_lints(&self) -> LintArray {
+        lint_array!()
+    }
+}
+
+fn check_const(cx: &LateContext, body_id: hir::BodyId, what: &str) {
+    let def_id = cx.tcx.hir.body_owner_def_id(body_id);
+    let param_env = cx.tcx.param_env(def_id);
+    let cid = ::rustc::mir::interpret::GlobalId {
+        instance: ty::Instance::mono(cx.tcx, def_id),
+        promoted: None
+    };
+    if let Err(err) = cx.tcx.const_eval(param_env.and(cid)) {
+        let span = cx.tcx.def_span(def_id);
+        let mut diag = cx.struct_span_lint(
+            CONST_ERR,
+            span,
+            &format!("this {} cannot be used", what),
+        );
+        use rustc::middle::const_val::ConstEvalErrDescription;
+        match err.description() {
+            ConstEvalErrDescription::Simple(message) => {
+                diag.span_label(span, message);
+            }
+            ConstEvalErrDescription::Backtrace(miri, frames) => {
+                diag.span_label(span, format!("{}", miri));
+                for frame in frames {
+                    diag.span_label(frame.span, format!("inside call to `{}`", frame.location));
+                }
+            }
+        }
+        diag.emit()
+    }
+}
+
+struct UnusedBrokenConstVisitor<'a, 'tcx: 'a>(&'a LateContext<'a, 'tcx>);
+
+impl<'a, 'tcx, 'v> hir::intravisit::Visitor<'v> for UnusedBrokenConstVisitor<'a, 'tcx> {
+    fn visit_nested_body(&mut self, id: hir::BodyId) {
+        check_const(self.0, id, "array length");
+    }
+    fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'v> {
+        hir::intravisit::NestedVisitorMap::None
+    }
+}
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedBrokenConst {
+    fn check_item(&mut self, cx: &LateContext, it: &hir::Item) {
+        match it.node {
+            hir::ItemConst(_, body_id) => {
+                check_const(cx, body_id, "constant");
+            },
+            hir::ItemTy(ref ty, _) => hir::intravisit::walk_ty(
+                &mut UnusedBrokenConstVisitor(cx),
+                ty
+            ),
+            _ => {},
+        }
+    }
+}

src/librustc_lint/lib.rs

@@ -136,6 +136,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                  UnionsWithDropFields,
                  UnreachablePub,
                  TypeAliasBounds,
+                 UnusedBrokenConst,
                  );
 
     add_builtin_with_new!(sess,

src/test/compile-fail/array_const_index-0.rs

@@ -12,6 +12,7 @@ const A: &'static [i32] = &[];
 const B: i32 = (&A)[1];
 //~^ ERROR constant evaluation error
 //~| index out of bounds: the len is 0 but the index is 1
+//~| WARN this constant cannot be used
 
 fn main() {
     let _ = B;

src/test/compile-fail/array_const_index-1.rs

@@ -12,6 +12,7 @@ const A: [i32; 0] = [];
 const B: i32 = A[1];
 //~^ ERROR constant evaluation error
 //~| index out of bounds: the len is 0 but the index is 1
+//~| WARN this constant cannot be used
 
 fn main() {
     let _ = B;

src/test/compile-fail/const-err-early.rs

@@ -10,17 +10,17 @@
 
 #![deny(const_err)]
 
-pub const A: i8 = -std::i8::MIN; //~ ERROR E0080
-//~^ ERROR attempt to negate with overflow
+pub const A: i8 = -std::i8::MIN; //~ ERROR const_err
+//~^ ERROR this constant cannot be used
 //~| ERROR constant evaluation error
-pub const B: u8 = 200u8 + 200u8; //~ ERROR E0080
-//~^ ERROR attempt to add with overflow
-pub const C: u8 = 200u8 * 4; //~ ERROR E0080
-//~^ ERROR attempt to multiply with overflow
-pub const D: u8 = 42u8 - (42u8 + 1); //~ ERROR E0080
-//~^ ERROR attempt to subtract with overflow
+pub const B: u8 = 200u8 + 200u8; //~ ERROR const_err
+//~^ ERROR this constant cannot be used
+pub const C: u8 = 200u8 * 4; //~ ERROR const_err
+//~^ ERROR this constant cannot be used
+pub const D: u8 = 42u8 - (42u8 + 1); //~ ERROR const_err
+//~^ ERROR this constant cannot be used
 pub const E: u8 = [5u8][1];
-//~^ ERROR E0080
+//~^ ERROR const_err
 
 fn main() {
     let _a = A;

src/test/compile-fail/const-err-multi.rs

@@ -14,12 +14,13 @@ pub const A: i8 = -std::i8::MIN;
 //~^ ERROR E0080
 //~| ERROR attempt to negate with overflow
 //~| ERROR constant evaluation error
+//~| ERROR this constant cannot be used
 pub const B: i8 = A;
-//~^ ERROR E0080
+//~^ ERROR const_err
 pub const C: u8 = A as u8;
-//~^ ERROR E0080
+//~^ ERROR const_err
 pub const D: i8 = 50 - A;
-//~^ ERROR E0080
+//~^ ERROR const_err
 
 fn main() {
     let _ = (A, B, C, D);

src/test/compile-fail/const-eval-overflow2.rs

@@ -22,57 +22,57 @@ use std::{i8, i16, i32, i64, isize};
 use std::{u8, u16, u32, u64, usize};
 
 const VALS_I8: (i8,) =
+     //~^ ERROR this constant cannot be used
     (
      i8::MIN - 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to subtract with overflow
+     //~^ ERROR attempt to subtract with overflow
      );
 
 const VALS_I16: (i16,) =
+     //~^ ERROR this constant cannot be used
     (
      i16::MIN - 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to subtract with overflow
+     //~^ ERROR attempt to subtract with overflow
      );
 
 const VALS_I32: (i32,) =
+     //~^ ERROR this constant cannot be used
     (
      i32::MIN - 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to subtract with overflow
+     //~^ ERROR attempt to subtract with overflow
      );
 
 const VALS_I64: (i64,) =
+     //~^ ERROR this constant cannot be used
     (
      i64::MIN - 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to subtract with overflow
+     //~^ ERROR attempt to subtract with overflow
      );
 
 const VALS_U8: (u8,) =
+     //~^ ERROR this constant cannot be used
     (
      u8::MIN - 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to subtract with overflow
+     //~^ ERROR attempt to subtract with overflow
      );
 
 const VALS_U16: (u16,) = (
+     //~^ ERROR this constant cannot be used
      u16::MIN - 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to subtract with overflow
+     //~^ ERROR attempt to subtract with overflow
      );
 
 const VALS_U32: (u32,) = (
+     //~^ ERROR this constant cannot be used
      u32::MIN - 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to subtract with overflow
+     //~^ ERROR attempt to subtract with overflow
      );
 
 const VALS_U64: (u64,) =
+     //~^ ERROR this constant cannot be used
     (
      u64::MIN - 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to subtract with overflow
+     //~^ ERROR attempt to subtract with overflow
      );
 
 fn main() {

src/test/compile-fail/const-eval-overflow2b.rs

@@ -22,57 +22,57 @@ use std::{i8, i16, i32, i64, isize};
 use std::{u8, u16, u32, u64, usize};
 
 const VALS_I8: (i8,) =
+     //~^ ERROR this constant cannot be used
     (
      i8::MAX + 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to add with overflow
+     //~^ ERROR attempt to add with overflow
      );
 
 const VALS_I16: (i16,) =
+     //~^ ERROR this constant cannot be used
     (
      i16::MAX + 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to add with overflow
+     //~^ ERROR attempt to add with overflow
      );
 
 const VALS_I32: (i32,) =
+     //~^ ERROR this constant cannot be used
     (
      i32::MAX + 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to add with overflow
+     //~^ ERROR attempt to add with overflow
      );
 
 const VALS_I64: (i64,) =
+     //~^ ERROR this constant cannot be used
     (
      i64::MAX + 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to add with overflow
+     //~^ ERROR attempt to add with overflow
      );
 
 const VALS_U8: (u8,) =
+     //~^ ERROR this constant cannot be used
     (
      u8::MAX + 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to add with overflow
+     //~^ ERROR attempt to add with overflow
      );
 
 const VALS_U16: (u16,) = (
+     //~^ ERROR this constant cannot be used
      u16::MAX + 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to add with overflow
+     //~^ ERROR attempt to add with overflow
      );
 
 const VALS_U32: (u32,) = (
+     //~^ ERROR this constant cannot be used
      u32::MAX + 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to add with overflow
+     //~^ ERROR attempt to add with overflow
      );
 
 const VALS_U64: (u64,) =
+     //~^ ERROR this constant cannot be used
     (
      u64::MAX + 1,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to add with overflow
+     //~^ ERROR attempt to add with overflow
      );
 
 fn main() {

src/test/compile-fail/const-eval-overflow2c.rs

@@ -22,57 +22,57 @@ use std::{i8, i16, i32, i64, isize};
 use std::{u8, u16, u32, u64, usize};
 
 const VALS_I8: (i8,) =
+     //~^ ERROR this constant cannot be used
     (
      i8::MIN * 2,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to multiply with overflow
+     //~^ ERROR attempt to multiply with overflow
      );
 
 const VALS_I16: (i16,) =
+     //~^ ERROR this constant cannot be used
     (
      i16::MIN * 2,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to multiply with overflow
+     //~^ ERROR attempt to multiply with overflow
      );
 
 const VALS_I32: (i32,) =
+     //~^ ERROR this constant cannot be used
     (
      i32::MIN * 2,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to multiply with overflow
+     //~^ ERROR attempt to multiply with overflow
      );
 
 const VALS_I64: (i64,) =
+     //~^ ERROR this constant cannot be used
     (
      i64::MIN * 2,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to multiply with overflow
+     //~^ ERROR attempt to multiply with overflow
      );
 
 const VALS_U8: (u8,) =
+     //~^ ERROR this constant cannot be used
     (
      u8::MAX * 2,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to multiply with overflow
+     //~^ ERROR attempt to multiply with overflow
      );
 
 const VALS_U16: (u16,) = (
+     //~^ ERROR this constant cannot be used
      u16::MAX * 2,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to multiply with overflow
+     //~^ ERROR attempt to multiply with overflow
      );
 
 const VALS_U32: (u32,) = (
+     //~^ ERROR this constant cannot be used
      u32::MAX * 2,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to multiply with overflow
+     //~^ ERROR attempt to multiply with overflow
      );
 
 const VALS_U64: (u64,) =
+     //~^ ERROR this constant cannot be used
     (
      u64::MAX * 2,
-     //~^ ERROR constant evaluation error
-     //~| ERROR attempt to multiply with overflow
+     //~^ ERROR attempt to multiply with overflow
      );
 
 fn main() {

src/test/compile-fail/const-slice-oob.rs

@@ -14,6 +14,7 @@ const FOO: &'static[u32] = &[1, 2, 3];
 const BAR: u32 = FOO[5];
 //~^ ERROR constant evaluation error [E0080]
 //~| index out of bounds: the len is 3 but the index is 5
+//~| WARN this constant cannot be used
 
 fn main() {
     let _ = BAR;

src/test/ui/const-eval/conditional_array_execution.rs

@@ -14,6 +14,7 @@ const X: u32 = 5;
 const Y: u32 = 6;
 const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
 //~^ WARN attempt to subtract with overflow
+//~| WARN this constant cannot be used
 
 fn main() {
     println!("{}", FOO);

src/test/ui/const-eval/conditional_array_execution.stderr

@@ -6,8 +6,14 @@ LL | const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
    |
    = note: #[warn(const_err)] on by default
 
+warning: this constant cannot be used
+  --> $DIR/conditional_array_execution.rs:15:1
+   |
+LL | const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to subtract with overflow
+
 warning: constant evaluation error
-  --> $DIR/conditional_array_execution.rs:19:20
+  --> $DIR/conditional_array_execution.rs:20:20
    |
 LL |     println!("{}", FOO);
    |                    ^^^ referenced constant has errors