Update to nightly-2020-12-14

rust-lang/rust#79956 improved enum's visibility and now we can check
the visibility more precisely.

README.md

@@ -27,13 +27,13 @@ repository and compiled from source or installed from
 of the nightly toolchain is supported at any given time.
 
 <!-- NOTE: Keep in sync with nightly date on rust-toolchain. -->
-It's recommended to use `nightly-2020-12-01` toolchain.
-You can install it by using `rustup install nightly-2020-12-01` if you already have rustup.
+It's recommended to use `nightly-2020-12-14` toolchain.
+You can install it by using `rustup install nightly-2020-12-14` if you already have rustup.
 Then you can do:
 
 ```sh
 $ rustup component add rustc-dev llvm-tools-preview --toolchain nightly-2020-11-19
-$ cargo +nightly-2020-12-01 install --git https://github.com/rust-lang/rust-semverver
+$ cargo +nightly-2020-12-14 install --git https://github.com/rust-lang/rust-semverver
 ```
 
 You'd also need `cmake` for some dependencies, and a few common libraries (if you hit

rust-toolchain

@@ -1,4 +1,4 @@
 # NOTE: Keep in sync with nightly date on README
 [toolchain]
-channel = "nightly-2020-12-01"
+channel = "nightly-2020-12-14"
 components = ["llvm-tools-preview", "rustc-dev"]

tests/cases/enums/stdout

@@ -73,32 +73,32 @@ warning: variant with no public fields changed to a tuple variant (breaking)
    |
 19 |     Abc(u8),
    |     ^^^^^^^
-warning: variant with no public fields changed to a tuple variant (breaking)
+warning: variant with public fields changed to a tuple variant (breaking)
   --> enums/new.rs:20:5
    |
 20 |     Bcd,
    |     ^^^
-warning: variant with no public fields changed to a struct variant (breaking)
+warning: variant with public fields changed to a struct variant (breaking)
   --> enums/new.rs:21:5
    |
 21 |     Cde { f: u8 },
    |     ^^^^^^^^^^^^^
-warning: variant with no public fields changed to a tuple variant (breaking)
+warning: variant with public fields changed to a tuple variant (breaking)
   --> enums/new.rs:22:5
    |
 22 |     Def,
    |     ^^^
-warning: variant with no public fields changed to a struct variant (breaking)
+warning: variant with public fields changed to a struct variant (breaking)
   --> enums/new.rs:23:5
    |
 23 |     Efg { f: u8 },
    |     ^^^^^^^^^^^^^
-warning: private field removed from variant with private fields (breaking)
+warning: public field removed from variant with no private fields (breaking)
   --> enums/old.rs:25:11
    |
 25 |     Ghi { f: u8 },
    |           ^^^^^
-note: private field added to variant with private fields (non-breaking)
+warning: public field added to variant with no private fields (breaking)
   --> enums/new.rs:25:11
    |
 25 |     Ghi { g: u8 },