Rollup merge of #54261 - varkor:dyn-keyword-2018, r=petrochenkov

Make `dyn` a keyword in the 2018 edition

Proposed in https://github.com/rust-lang/rust/issues/44662#issuecomment-421596088.

src/test/ui/rust-2018/dyn-keyword.fixed

@@ -0,0 +1,10 @@
+// edition:2015
+// run-rustfix
+
+#![allow(unused_variables)]
+#![deny(keyword_idents)]
+
+fn main() {
+    let r#dyn = (); //~ ERROR dyn
+    //~^ WARN hard error in the 2018 edition
+}

src/test/ui/rust-2018/dyn-keyword.rs

@@ -0,0 +1,10 @@
+// edition:2015
+// run-rustfix
+
+#![allow(unused_variables)]
+#![deny(keyword_idents)]
+
+fn main() {
+    let dyn = (); //~ ERROR dyn
+    //~^ WARN hard error in the 2018 edition
+}

src/test/ui/rust-2018/dyn-keyword.stderr

@@ -0,0 +1,16 @@
+error: `dyn` is a keyword in the 2018 edition
+  --> $DIR/dyn-keyword.rs:8:9
+   |
+LL |     let dyn = (); //~ ERROR dyn
+   |         ^^^ help: you can use a raw identifier to stay compatible: `r#dyn`
+   |
+note: lint level defined here
+  --> $DIR/dyn-keyword.rs:5:9
+   |
+LL | #![deny(keyword_idents)]
+   |         ^^^^^^^^^^^^^^
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
+   = note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>
+
+error: aborting due to previous error
+

src/test/ui/rust-2018/dyn-trait-compatibility.rs

@@ -0,0 +1,8 @@
+// edition:2018
+
+type A0 = dyn;
+type A1 = dyn::dyn; //~ERROR expected identifier, found reserved keyword
+type A2 = dyn<dyn, dyn>; //~ERROR expected identifier, found `<`
+type A3 = dyn<<dyn as dyn>::dyn>;
+
+fn main() {}

src/test/ui/rust-2018/dyn-trait-compatibility.stderr

@@ -0,0 +1,14 @@
+error: expected identifier, found reserved keyword `dyn`
+  --> $DIR/dyn-trait-compatibility.rs:4:16
+   |
+LL | type A1 = dyn::dyn; //~ERROR expected identifier, found reserved keyword
+   |                ^^^ expected identifier, found reserved keyword
+
+error: expected identifier, found `<`
+  --> $DIR/dyn-trait-compatibility.rs:5:14
+   |
+LL | type A2 = dyn<dyn, dyn>; //~ERROR expected identifier, found `<`
+   |              ^ expected identifier
+
+error: aborting due to 2 previous errors
+