resolve: Use feature(custom_attribute) fallback only if the feature is enabled

Normally `#![feature(...)]` shouldn't change behavior, but custom attributes in particular are in the process of retirement, and we should not produce a message telling to enable them.
It also helps with unifying diagnostics for unresolved macros.

Author: petrochenkov

src/librustc_resolve/diagnostics.rs

@@ -11,7 +11,6 @@ use rustc::ty::{self, DefIdTree};
 use rustc::util::nodemap::FxHashSet;
 use syntax::ast::{self, Expr, ExprKind, Ident, NodeId, Path, Ty, TyKind};
 use syntax::ext::base::MacroKind;
-use syntax::feature_gate::{feature_err, AttributeGate, GateIssue, Stability, BUILTIN_ATTRIBUTES};
 use syntax::symbol::{Symbol, kw};
 use syntax::util::lev_distance::find_best_match_for_name;
 use syntax_pos::{BytePos, Span};
@@ -903,60 +902,6 @@ impl<'a> Resolver<'a> {
             }
         }
     }
-
-    crate fn report_unknown_attribute(&self, span: Span, name: &str, msg: &str, feature: Symbol) {
-        let mut err = feature_err(
-            &self.session.parse_sess,
-            feature,
-            span,
-            GateIssue::Language,
-            &msg,
-        );
-
-        let features = self.session.features_untracked();
-
-        let attr_candidates = BUILTIN_ATTRIBUTES
-            .iter()
-            .filter_map(|&(name, _, _, ref gate)| {
-                if name.as_str().starts_with("rustc_") && !features.rustc_attrs {
-                    return None;
-                }
-
-                match gate {
-                    AttributeGate::Gated(Stability::Unstable, ..)
-                        if self.session.opts.unstable_features.is_nightly_build() =>
-                    {
-                        Some(name)
-                    }
-                    AttributeGate::Gated(Stability::Deprecated(..), ..) => Some(name),
-                    AttributeGate::Ungated => Some(name),
-                    _ => None,
-                }
-            })
-            .chain(
-                // Add built-in macro attributes as well.
-                self.builtin_macros.iter().filter_map(|(name, binding)| {
-                    match binding.macro_kind() {
-                        Some(MacroKind::Attr) => Some(*name),
-                        _ => None,
-                    }
-                }),
-            )
-            .collect::<Vec<_>>();
-
-        let lev_suggestion = find_best_match_for_name(attr_candidates.iter(), &name, None);
-
-        if let Some(suggestion) = lev_suggestion {
-            err.span_suggestion(
-                span,
-                "a built-in attribute with a similar name exists",
-                suggestion.to_string(),
-                Applicability::MaybeIncorrect,
-            );
-        }
-
-        err.emit();
-    }
 }
 
 impl<'a, 'b> ImportResolver<'a, 'b> {

src/librustc_resolve/macros.rs

@@ -312,23 +312,7 @@ impl<'a> Resolver<'a> {
                     }
                 }
             }
-            Res::NonMacroAttr(attr_kind) => {
-                if attr_kind == NonMacroAttrKind::Custom {
-                    assert!(path.segments.len() == 1);
-                    if !features.custom_attribute {
-                        let msg = format!("The attribute `{}` is currently unknown to the \
-                                            compiler and may have meaning added to it in the \
-                                            future", path);
-                        self.report_unknown_attribute(
-                            path.span,
-                            &path.segments[0].ident.as_str(),
-                            &msg,
-                            sym::custom_attribute,
-                        );
-                    }
-                }
-            }
-            Res::Err => {}
+            Res::NonMacroAttr(..) | Res::Err => {}
             _ => panic!("expected `DefKind::Macro` or `Res::NonMacroAttr`"),
         };
 
@@ -721,7 +705,8 @@ impl<'a> Resolver<'a> {
         }
 
         let determinacy = Determinacy::determined(determinacy == Determinacy::Determined || force);
-        if determinacy == Determinacy::Determined && macro_kind == Some(MacroKind::Attr) {
+        if determinacy == Determinacy::Determined && macro_kind == Some(MacroKind::Attr) &&
+           self.session.features_untracked().custom_attribute {
             // For single-segment attributes interpret determinate "no resolution" as a custom
             // attribute. (Lexical resolution implies the first segment and attr kind should imply
             // the last segment, so we are certainly working with a single-segment attribute here.)

src/test/ui/attributes/obsolete-attr.rs

@@ -1,7 +1,9 @@
 // Obsolete attributes fall back to feature gated custom attributes.
 
-#[ab_isize="stdcall"] extern {} //~ ERROR attribute `ab_isize` is currently unknown
+#[ab_isize="stdcall"] extern {}
+//~^ ERROR cannot find attribute macro `ab_isize` in this scope
 
-#[fixed_stack_segment] fn f() {} //~ ERROR attribute `fixed_stack_segment` is currently unknown
+#[fixed_stack_segment] fn f() {}
+//~^ ERROR cannot find attribute macro `fixed_stack_segment` in this scope
 
 fn main() {}

src/test/ui/attributes/obsolete-attr.stderr

@@ -1,21 +1,14 @@
-error[E0658]: The attribute `fixed_stack_segment` is currently unknown to the compiler and may have meaning added to it in the future
-  --> $DIR/obsolete-attr.rs:5:3
+error: cannot find attribute macro `fixed_stack_segment` in this scope
+  --> $DIR/obsolete-attr.rs:6:3
    |
 LL | #[fixed_stack_segment] fn f() {}
    |   ^^^^^^^^^^^^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
-error[E0658]: The attribute `ab_isize` is currently unknown to the compiler and may have meaning added to it in the future
+error: cannot find attribute macro `ab_isize` in this scope
   --> $DIR/obsolete-attr.rs:3:3
    |
 LL | #[ab_isize="stdcall"] extern {}
    |   ^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0658`.

src/test/ui/attributes/unknown-attr.rs

@@ -2,8 +2,11 @@
 
 #![feature(custom_inner_attributes)]
 
-#![mutable_doc] //~ ERROR attribute `mutable_doc` is currently unknown
+#![mutable_doc]
+//~^ ERROR cannot find attribute macro `mutable_doc` in this scope
 
-#[dance] mod a {} //~ ERROR attribute `dance` is currently unknown
+#[dance] mod a {}
+//~^ ERROR cannot find attribute macro `dance` in this scope
 
-#[dance] fn main() {} //~ ERROR attribute `dance` is currently unknown
+#[dance] fn main() {}
+//~^ ERROR cannot find attribute macro `dance` in this scope

src/test/ui/attributes/unknown-attr.stderr

@@ -1,30 +1,20 @@
-error[E0658]: The attribute `mutable_doc` is currently unknown to the compiler and may have meaning added to it in the future
+error: cannot find attribute macro `mutable_doc` in this scope
   --> $DIR/unknown-attr.rs:5:4
    |
 LL | #![mutable_doc]
    |    ^^^^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
-error[E0658]: The attribute `dance` is currently unknown to the compiler and may have meaning added to it in the future
-  --> $DIR/unknown-attr.rs:7:3
+error: cannot find attribute macro `dance` in this scope
+  --> $DIR/unknown-attr.rs:8:3
    |
 LL | #[dance] mod a {}
    |   ^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
-error[E0658]: The attribute `dance` is currently unknown to the compiler and may have meaning added to it in the future
-  --> $DIR/unknown-attr.rs:9:3
+error: cannot find attribute macro `dance` in this scope
+  --> $DIR/unknown-attr.rs:11:3
    |
 LL | #[dance] fn main() {}
    |   ^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0658`.

src/test/ui/conditional-compilation/cfg-attr-unknown-attribute-macro-expansion.rs

@@ -1,6 +1,7 @@
 macro_rules! foo {
     () => {
-        #[cfg_attr(all(), unknown)] //~ ERROR `unknown` is currently unknown
+        #[cfg_attr(all(), unknown)]
+        //~^ ERROR cannot find attribute macro `unknown` in this scope
         fn foo() {}
     }
 }

src/test/ui/conditional-compilation/cfg-attr-unknown-attribute-macro-expansion.stderr

@@ -1,15 +1,11 @@
-error[E0658]: The attribute `unknown` is currently unknown to the compiler and may have meaning added to it in the future
+error: cannot find attribute macro `unknown` in this scope
   --> $DIR/cfg-attr-unknown-attribute-macro-expansion.rs:3:27
    |
 LL |         #[cfg_attr(all(), unknown)]
    |                           ^^^^^^^
 ...
 LL | foo!();
    | ------- in this macro invocation
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0658`.

src/test/ui/custom_attribute.rs

@@ -1,9 +1,9 @@
 #![feature(stmt_expr_attributes)]
 
-#[foo] //~ ERROR The attribute `foo`
+#[foo] //~ ERROR cannot find attribute macro `foo` in this scope
 fn main() {
-    #[foo] //~ ERROR The attribute `foo`
+    #[foo] //~ ERROR cannot find attribute macro `foo` in this scope
     let x = ();
-    #[foo] //~ ERROR The attribute `foo`
+    #[foo] //~ ERROR cannot find attribute macro `foo` in this scope
     x
 }

src/test/ui/custom_attribute.stderr

@@ -1,30 +1,20 @@
-error[E0658]: The attribute `foo` is currently unknown to the compiler and may have meaning added to it in the future
+error: cannot find attribute macro `foo` in this scope
   --> $DIR/custom_attribute.rs:3:3
    |
 LL | #[foo]
    |   ^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
-error[E0658]: The attribute `foo` is currently unknown to the compiler and may have meaning added to it in the future
+error: cannot find attribute macro `foo` in this scope
   --> $DIR/custom_attribute.rs:5:7
    |
 LL |     #[foo]
    |       ^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
-error[E0658]: The attribute `foo` is currently unknown to the compiler and may have meaning added to it in the future
+error: cannot find attribute macro `foo` in this scope
   --> $DIR/custom_attribute.rs:7:7
    |
 LL |     #[foo]
    |       ^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0658`.