Overhaul const-checking diagnostics

compiler/rustc_error_codes/src/error_codes.rs

@@ -18,7 +18,6 @@ E0010: include_str!("./error_codes/E0010.md"),
 E0013: include_str!("./error_codes/E0013.md"),
 E0014: include_str!("./error_codes/E0014.md"),
 E0015: include_str!("./error_codes/E0015.md"),
-E0019: include_str!("./error_codes/E0019.md"),
 E0023: include_str!("./error_codes/E0023.md"),
 E0025: include_str!("./error_codes/E0025.md"),
 E0026: include_str!("./error_codes/E0026.md"),
@@ -461,6 +460,7 @@ E0774: include_str!("./error_codes/E0774.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
+//  E0019, merged into E0015
 //  E0035, merged into E0087/E0089
 //  E0036, merged into E0087/E0089
 //  E0068,

compiler/rustc_middle/src/mir/mod.rs

@@ -670,7 +670,7 @@ impl Atom for Local {
 }
 
 /// Classifies locals into categories. See `Body::local_kind`.
-#[derive(PartialEq, Eq, Debug, HashStable)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug, HashStable)]
 pub enum LocalKind {
     /// User-declared variable binding.
     Var,

compiler/rustc_mir/src/transform/check_consts/mod.rs

@@ -57,6 +57,16 @@ impl ConstCx<'mir, 'tcx> {
             && self.tcx.features().staged_api
             && is_const_stable_const_fn(self.tcx, self.def_id.to_def_id())
     }
+
+    /// Returns the function signature of the item being const-checked if it is a `fn` or `const fn`.
+    pub fn fn_sig(&self) -> Option<&'tcx hir::FnSig<'tcx>> {
+        // Get this from the HIR map instead of a query to avoid cycle errors.
+        //
+        // FIXME: Is this still an issue?
+        let hir_map = self.tcx.hir();
+        let hir_id = hir_map.local_def_id_to_hir_id(self.def_id);
+        hir_map.fn_sig_by_hir_id(hir_id)
+    }
 }
 
 /// Returns `true` if this `DefId` points to one of the official `panic` lang items.