resolve: Refactor away MacroBinding

`fn resolve_legacy_scope` can now resolve only to `macro_rules!` items,
`fn resolve_lexical_macro_path_segment` is for everything else - modularized macros, preludes

src/test/ui/imports/issue-53269.rs

@@ -0,0 +1,21 @@
+// 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.
+
+// Ambiguity between a `macro_rules` macro and a non-existent import recovered as `Def::Err`
+
+macro_rules! mac { () => () }
+
+mod m {
+    use nonexistent_module::mac; //~ ERROR unresolved import `nonexistent_module`
+
+    mac!(); //~ ERROR `mac` is ambiguous
+}
+
+fn main() {}

src/test/ui/imports/issue-53269.stderr

@@ -0,0 +1,26 @@
+error[E0432]: unresolved import `nonexistent_module`
+  --> $DIR/issue-53269.rs:16:9
+   |
+LL |     use nonexistent_module::mac; //~ ERROR unresolved import `nonexistent_module`
+   |         ^^^^^^^^^^^^^^^^^^ Maybe a missing `extern crate nonexistent_module;`?
+
+error: `mac` is ambiguous
+  --> $DIR/issue-53269.rs:18:5
+   |
+LL |     mac!(); //~ ERROR `mac` is ambiguous
+   |     ^^^
+   |
+note: `mac` could refer to the macro defined here
+  --> $DIR/issue-53269.rs:13:1
+   |
+LL | macro_rules! mac { () => () }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: `mac` could also refer to the macro imported here
+  --> $DIR/issue-53269.rs:16:9
+   |
+LL |     use nonexistent_module::mac; //~ ERROR unresolved import `nonexistent_module`
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0432`.

src/test/ui/imports/issue-53512.rs

@@ -0,0 +1,17 @@
+// 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.
+
+// Macro from prelude is shadowed by non-existent import recovered as `Def::Err`.
+
+use std::assert; //~ ERROR unresolved import `std::assert`
+
+fn main() {
+    assert!(true);
+}

src/test/ui/imports/issue-53512.stderr

@@ -0,0 +1,9 @@
+error[E0432]: unresolved import `std::assert`
+  --> $DIR/issue-53512.rs:13:5
+   |
+LL | use std::assert; //~ ERROR unresolved import `std::assert`
+   |     ^^^^^^^^^^^ no `assert` in the root
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0432`.

src/test/ui/imports/shadow_builtin_macros.rs

@@ -37,10 +37,10 @@ mod m4 {
 
 mod m5 {
     macro_rules! m { () => {
-        macro_rules! panic { () => {} } //~ ERROR `panic` is already in scope
+        macro_rules! panic { () => {} }
     } }
     m!();
-    panic!();
+    panic!(); //~ ERROR `panic` is ambiguous
 }
 
 #[macro_use(n)]

src/test/ui/imports/shadow_builtin_macros.stderr

@@ -1,13 +1,18 @@
-error: `panic` is already in scope
+error: `panic` is ambiguous
+  --> $DIR/shadow_builtin_macros.rs:43:5
+   |
+LL |     panic!(); //~ ERROR `panic` is ambiguous
+   |     ^^^^^
+   |
+note: `panic` could refer to the macro defined here
   --> $DIR/shadow_builtin_macros.rs:40:9
    |
-LL |         macro_rules! panic { () => {} } //~ ERROR `panic` is already in scope
+LL |         macro_rules! panic { () => {} }
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 LL |     } }
 LL |     m!();
    |     ----- in this macro invocation
-   |
-   = note: macro-expanded `macro_rules!`s may not shadow existing macros (see RFC 1560)
+note: `panic` could also refer to the macro imported here
 
 error[E0659]: `panic` is ambiguous
   --> $DIR/shadow_builtin_macros.rs:25:14