Auto merge of #28845 - oli-obk:rfc1229, r=pnkfelix

This PR turns statically known erroneous code (e.g. numeric overflow) into a warning and continues normal code-generation to emit the same code that would have been generated without `check_const` detecting that the result can be computed at compile-time.

<del>It's not done yet, as I don't know how to properly emit a lint from trans. I can't seem to extract the real lint level of the item the erroneous expression is in.</del> It's an unconditional warning now.

r? @pnkfelix 

cc @nikomatsakis 

* [RFC 1229 text](https://github.com/rust-lang/rfcs/blob/master/text/1229-compile-time-asserts.md)
* RFC PR: rust-lang/rfcs#1229
* tracking issue: #28238

Author: bors

src/librustc/lint/builtin.rs

@@ -16,6 +16,12 @@
 
 use lint::{LintPass, LateLintPass, LintArray};
 
+declare_lint! {
+    pub CONST_ERR,
+    Warn,
+    "constant evaluation detected erroneous expression"
+}
+
 declare_lint! {
     pub UNUSED_IMPORTS,
     Warn,
@@ -134,7 +140,8 @@ impl LintPass for HardwiredLints {
             VARIANT_SIZE_DIFFERENCES,
             FAT_PTR_TRANSMUTES,
             TRIVIAL_CASTS,
-            TRIVIAL_NUMERIC_CASTS
+            TRIVIAL_NUMERIC_CASTS,
+            CONST_ERR
         )
     }
 }

src/librustc/middle/check_const.rs

@@ -176,8 +176,8 @@ impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
         if mode == Mode::ConstFn {
             for arg in &fd.inputs {
                 match arg.pat.node {
-                    hir::PatIdent(hir::BindByValue(hir::MutImmutable), _, None) => {}
                     hir::PatWild(_) => {}
+                    hir::PatIdent(hir::BindByValue(hir::MutImmutable), _, None) => {}
                     _ => {
                         span_err!(self.tcx.sess, arg.pat.span, E0022,
                                   "arguments of constant functions can only \
@@ -476,9 +476,9 @@ impl<'a, 'tcx, 'v> Visitor<'v> for CheckCrateVisitor<'a, 'tcx> {
                                     self.tcx, ex, ExprTypeChecked) {
                                 Ok(_) => {}
                                 Err(msg) => {
-                                    span_err!(self.tcx.sess, msg.span, E0020,
-                                              "{} in a constant expression",
-                                              msg.description())
+                                    self.tcx.sess.add_lint(::lint::builtin::CONST_ERR, ex.id,
+                                                           msg.span,
+                                                           msg.description().into_owned())
                                 }
                             }
                         }

src/librustc_trans/trans/_match.rs

@@ -275,19 +275,30 @@ impl<'a, 'tcx> Opt<'a, 'tcx> {
     }
 
     fn trans<'blk>(&self, mut bcx: Block<'blk, 'tcx>) -> OptResult<'blk, 'tcx> {
+        use trans::consts::TrueConst::Yes;
         let _icx = push_ctxt("match::trans_opt");
         let ccx = bcx.ccx();
         match *self {
             ConstantValue(ConstantExpr(lit_expr), _) => {
                 let lit_ty = bcx.tcx().node_id_to_type(lit_expr.id);
-                let (llval, _) = consts::const_expr(ccx, &*lit_expr, bcx.fcx.param_substs, None);
+                let expr = consts::const_expr(ccx, &*lit_expr, bcx.fcx.param_substs, None, Yes);
+                let llval = match expr {
+                    Ok((llval, _)) => llval,
+                    Err(err) => bcx.ccx().sess().span_fatal(lit_expr.span, &err.description()),
+                };
                 let lit_datum = immediate_rvalue(llval, lit_ty);
                 let lit_datum = unpack_datum!(bcx, lit_datum.to_appropriate_datum(bcx));
                 SingleResult(Result::new(bcx, lit_datum.val))
             }
             ConstantRange(ConstantExpr(ref l1), ConstantExpr(ref l2), _) => {
-                let (l1, _) = consts::const_expr(ccx, &**l1, bcx.fcx.param_substs, None);
-                let (l2, _) = consts::const_expr(ccx, &**l2, bcx.fcx.param_substs, None);
+                let l1 = match consts::const_expr(ccx, &**l1, bcx.fcx.param_substs, None, Yes) {
+                    Ok((l1, _)) => l1,
+                    Err(err) => bcx.ccx().sess().span_fatal(l1.span, &err.description()),
+                };
+                let l2 = match consts::const_expr(ccx, &**l2, bcx.fcx.param_substs, None, Yes) {
+                    Ok((l2, _)) => l2,
+                    Err(err) => bcx.ccx().sess().span_fatal(l2.span, &err.description()),
+                };
                 RangeResult(Result::new(bcx, l1), Result::new(bcx, l2))
             }
             Variant(disr_val, ref repr, _, _) => {

src/librustc_trans/trans/base.rs

@@ -2125,7 +2125,10 @@ pub fn trans_item(ccx: &CrateContext, item: &hir::Item) {
           let mut v = TransItemVisitor{ ccx: ccx };
           v.visit_expr(&**expr);
 
-          let g = consts::trans_static(ccx, m, expr, item.id, &item.attrs);
+          let g = match consts::trans_static(ccx, m, expr, item.id, &item.attrs) {
+              Ok(g) => g,
+              Err(err) => ccx.tcx().sess.span_fatal(expr.span, &err.description()),
+          };
           set_global_section(ccx, g, item);
           update_linkage(ccx, g, Some(item.id), OriginalTranslation);
       },