Improve error checking and reporting

- Check for valid attributes more reliably
- Don't bloat the error index for boring errors
- Do use real error codes for the interesting ones

Author: scottmcm

src/librustc/diagnostics.rs

@@ -2116,34 +2116,6 @@ struct Foo;
 ```
 "##,
 
-E0713: r##"
-This error indicates that a `#[marker]` attribute was incorrectly placed
-on something other than a trait.
-
-Examples of erroneous code:
-
-```compile_fail,E0713
-# #![feature(marker_trait_attr)]
-
-#[marker]
-struct Foo { }
-```
-"##,
-
-E0714: r##"
-This error indicates that a `#[marker]` attribute had a value. The
-`#[marker]` should be empty.
-
-Examples of erroneous code:
-
-```compile_fail,E0714
-# #![feature(marker_trait_attr)]
-
-#[marker(anything)]
-trait Foo {}
-```
-"##,
-
 }
 
 

src/librustc/hir/check_attr.rs

@@ -123,22 +123,17 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
         match target {
             Target::Trait => { /* Valid */ },
             _ => {
-                struct_span_err!(self.tcx.sess,
-                                 attr.span,
-                                 E0713,
-                                 "attribute can only be applied to a trait")
+                self.tcx.sess
+                    .struct_span_err(attr.span, "attribute can only be applied to a trait")
                     .span_label(item.span, "not a trait")
                     .emit();
                 return;
             }
         }
 
-        if attr.meta_item_list().is_some() || attr.value_str().is_some() {
-            struct_span_err!(self.tcx.sess,
-                             attr.span,
-                             E0714,
-                             "attribute should be empty")
-                .span_label(item.span, "not empty")
+        if !attr.is_word() {
+            self.tcx.sess
+                .struct_span_err(attr.span, "attribute should be empty")
                 .emit();
         }
     }

src/librustc_typeck/check/wfcheck.rs

@@ -308,10 +308,12 @@ fn check_trait<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item: &hir::Item) {
     let trait_def = tcx.trait_def(trait_def_id);
     if trait_def.is_marker {
         for associated_def_id in &*tcx.associated_item_def_ids(trait_def_id) {
-            tcx.sess.struct_span_err(
-                    tcx.def_span(*associated_def_id),
-                    "marker traits cannot have associated items",
-                ).emit();
+            struct_span_err!(
+                tcx.sess,
+                tcx.def_span(*associated_def_id),
+                E0714,
+                "marker traits cannot have associated items",
+            ).emit();
         }
     }
 

src/librustc_typeck/diagnostics.rs

@@ -4750,6 +4750,22 @@ ambiguity for some types, we disallow calling methods on raw pointers when
 the type is unknown.
 "##,
 
+E0714: r##"
+A `#[marker]` trait contained an associated item.
+
+The items of marker traits cannot be overridden, so there's no need to have them
+when they cannot be changed per-type anyway.  If you wanted them for ergonomic
+reasons, consider making an extension trait instead.
+"##,
+
+E0715: r##"
+An `impl` for a `#[marker]` trait tried to override an associated item.
+
+Because marker traits are allowed to have multiple implementations for the same
+type, it's not allowed to override anything in those implementations, as it
+would be ambiguous which override should actually be used.
+"##,
+
 }
 
 register_diagnostics! {
@@ -4833,5 +4849,4 @@ register_diagnostics! {
     E0641, // cannot cast to/from a pointer with an unknown kind
     E0645, // trait aliases not finished
     E0698, // type inside generator must be known in this context
-    E0715, // impls for marker traits cannot contain items
 }

src/test/ui/marker_trait_attr/marker-attribute-on-non-trait.rs

@@ -0,0 +1,33 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(marker_trait_attr)]
+
+#[marker] //~ ERROR attribute can only be applied to a trait
+struct Struct {}
+
+#[marker] //~ ERROR attribute can only be applied to a trait
+impl Struct {}
+
+#[marker] //~ ERROR attribute can only be applied to a trait
+union Union {
+    x: i32,
+}
+
+#[marker] //~ ERROR attribute can only be applied to a trait
+const CONST: usize = 10;
+
+#[marker] //~ ERROR attribute can only be applied to a trait
+fn function() {}
+
+#[marker] //~ ERROR attribute can only be applied to a trait
+type Type = ();
+
+fn main() {}

src/test/ui/marker_trait_attr/marker-attribute-on-non-trait.stderr

@@ -0,0 +1,52 @@
+error: attribute can only be applied to a trait
+  --> $DIR/marker-attribute-on-non-trait.rs:13:1
+   |
+LL | #[marker] //~ ERROR attribute can only be applied to a trait
+   | ^^^^^^^^^
+LL | struct Struct {}
+   | ---------------- not a trait
+
+error: attribute can only be applied to a trait
+  --> $DIR/marker-attribute-on-non-trait.rs:16:1
+   |
+LL | #[marker] //~ ERROR attribute can only be applied to a trait
+   | ^^^^^^^^^
+LL | impl Struct {}
+   | -------------- not a trait
+
+error: attribute can only be applied to a trait
+  --> $DIR/marker-attribute-on-non-trait.rs:19:1
+   |
+LL |   #[marker] //~ ERROR attribute can only be applied to a trait
+   |   ^^^^^^^^^
+LL | / union Union {
+LL | |     x: i32,
+LL | | }
+   | |_- not a trait
+
+error: attribute can only be applied to a trait
+  --> $DIR/marker-attribute-on-non-trait.rs:24:1
+   |
+LL | #[marker] //~ ERROR attribute can only be applied to a trait
+   | ^^^^^^^^^
+LL | const CONST: usize = 10;
+   | ------------------------ not a trait
+
+error: attribute can only be applied to a trait
+  --> $DIR/marker-attribute-on-non-trait.rs:27:1
+   |
+LL | #[marker] //~ ERROR attribute can only be applied to a trait
+   | ^^^^^^^^^
+LL | fn function() {}
+   | ---------------- not a trait
+
+error: attribute can only be applied to a trait
+  --> $DIR/marker-attribute-on-non-trait.rs:30:1
+   |
+LL | #[marker] //~ ERROR attribute can only be applied to a trait
+   | ^^^^^^^^^
+LL | type Type = ();
+   | --------------- not a trait
+
+error: aborting due to 6 previous errors
+

src/test/ui/marker_trait_attr/marker-attribute-with-values.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(marker_trait_attr)]
+#![feature(unrestricted_attribute_tokens)]
+
+#[marker(always)]
+trait Marker1 {}
+//~^^ ERROR attribute should be empty
+
+#[marker("never")]
+trait Marker2 {}
+//~^^ ERROR attribute should be empty
+
+#[marker(key = value)]
+trait Marker3 {}
+//~^^ ERROR attribute should be empty
+
+fn main() {}

src/test/ui/marker_trait_attr/marker-attribute-with-values.stderr

@@ -0,0 +1,20 @@
+error: attribute should be empty
+  --> $DIR/marker-attribute-with-values.rs:14:1
+   |
+LL | #[marker(always)]
+   | ^^^^^^^^^^^^^^^^^
+
+error: attribute should be empty
+  --> $DIR/marker-attribute-with-values.rs:18:1
+   |
+LL | #[marker("never")]
+   | ^^^^^^^^^^^^^^^^^^
+
+error: attribute should be empty
+  --> $DIR/marker-attribute-with-values.rs:22:1
+   |
+LL | #[marker(key = value)]
+   | ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+

src/test/ui/marker_trait_attr/marker-trait-with-associated-items.stderr

@@ -1,38 +1,39 @@
-error: marker traits cannot have associated items
+error[E0714]: marker traits cannot have associated items
   --> $DIR/marker-trait-with-associated-items.rs:16:5
    |
 LL |     const N: usize;
    |     ^^^^^^^^^^^^^^^
 
-error: marker traits cannot have associated items
+error[E0714]: marker traits cannot have associated items
   --> $DIR/marker-trait-with-associated-items.rs:22:5
    |
 LL |     type Output;
    |     ^^^^^^^^^^^^
 
-error: marker traits cannot have associated items
+error[E0714]: marker traits cannot have associated items
   --> $DIR/marker-trait-with-associated-items.rs:28:5
    |
 LL |     fn foo();
    |     ^^^^^^^^^
 
-error: marker traits cannot have associated items
+error[E0714]: marker traits cannot have associated items
   --> $DIR/marker-trait-with-associated-items.rs:34:5
    |
 LL |     const N: usize = 43;
    |     ^^^^^^^^^^^^^^^^^^^^
 
-error: marker traits cannot have associated items
+error[E0714]: marker traits cannot have associated items
   --> $DIR/marker-trait-with-associated-items.rs:40:5
    |
 LL |     type Output = ();
    |     ^^^^^^^^^^^^^^^^^
 
-error: marker traits cannot have associated items
+error[E0714]: marker traits cannot have associated items
   --> $DIR/marker-trait-with-associated-items.rs:46:5
    |
 LL |     fn foo() {}
    |     ^^^^^^^^^^^
 
 error: aborting due to 6 previous errors
 
+For more information about this error, try `rustc --explain E0714`.