Destabilize 2018 edition

This reverts commit 4d1ec81.

Author: pietroalbini

src/doc/rustdoc/src/command-line-arguments.md

@@ -345,17 +345,3 @@ $ rustdoc src/lib.rs --sysroot /path/to/sysroot
 
 Similar to `rustc --sysroot`, this lets you change the sysroot `rustdoc` uses
 when compiling your code.
-
-### `--edition`: control the edition of docs and doctests
-
-Using this flag looks like this:
-
-```bash
-$ rustdoc src/lib.rs --edition 2018
-$ rustdoc --test src/lib.rs --edition 2018
-```
-
-This flag allows rustdoc to treat your rust code as the given edition. It will compile doctests with
-the given edition as well. As with `rustc`, the default edition that `rustdoc` will use is `2015`
-(the first edition).
-

src/doc/rustdoc/src/unstable-features.md

@@ -346,6 +346,19 @@ details.
 
 [issue-display-warnings]: https://github.com/rust-lang/rust/issues/41574
 
+### `--edition`: control the edition of docs and doctests
+
+Using this flag looks like this:
+
+```bash
+$ rustdoc src/lib.rs -Z unstable-options --edition 2018
+$ rustdoc --test src/lib.rs -Z unstable-options --edition 2018
+```
+
+This flag allows rustdoc to treat your rust code as the given edition. It will compile doctests with
+the given edition as well. As with `rustc`, the default edition that `rustdoc` will use is `2015`
+(the first edition).
+
 ### `--extern-html-root-url`: control how rustdoc links to non-local crates
 
 Using this flag looks like this:

src/librustdoc/lib.rs

@@ -286,7 +286,7 @@ fn opts() -> Vec<RustcOptGroup> {
                       \"light-suffix.css\"",
                      "PATH")
         }),
-        stable("edition", |o| {
+        unstable("edition", |o| {
             o.optopt("", "edition",
                      "edition to use when compiling rust code (default: 2015)",
                      "EDITION")

src/libsyntax_pos/edition.rs

@@ -65,7 +65,7 @@ impl Edition {
     pub fn is_stable(&self) -> bool {
         match *self {
             Edition::Edition2015 => true,
-            Edition::Edition2018 => true,
+            Edition::Edition2018 => false,
         }
     }
 }