Remove the unstable snafu::display attribute

Turns out, this was never supposed to be supported in the first place.

- rust-lang/rust#58899

Author: shepmaster

.cirrus.yml

@@ -37,10 +37,6 @@ nightly_test_task:
     - cargo +nightly update -p backtrace-sys --precise 0.1.20
     - cargo +nightly update -p libc --precise 0.2.7
     - cargo +nightly test
-  compatibility_test_script:
-    - cd compatibility-tests/nightly/
-    - rustc --version
-    - cargo test
   before_cache_script: rm -rf $CARGO_HOME/registry/index
 
 v1_30_test_task:

Cargo.toml

@@ -28,15 +28,11 @@ backtraces = ["snafu-derive/backtraces", "backtrace"]
 # New methods on `Error`; re-export of proc-macro
 rust_1_30 = ["snafu-derive/rust_1_30"]
 
-# The `snafu::display` attribute
-unstable_display_attribute = ["snafu-derive/unstable_display_attribute"]
-
 [workspace]
 # The compatibility tests each set feature flags for the library and
 # cannot be in the same crate graph.
 exclude = [
   "compatibility-tests/compile-fail",
-  "compatibility-tests/nightly",
   "compatibility-tests/v1_30",
   "compatibility-tests/v1_18",
   "compatibility-tests/without-backtrace",

compatibility-tests/compile-fail/Cargo.toml

@@ -6,4 +6,4 @@ edition = "2018"
 
 [dependencies]
 compiletest_rs = { version = "0.3.18", features = ["stable", "tmp"] }
-snafu = { path = "../..", features = ["unstable_display_attribute"] }
+snafu = { path = "../.." }

compatibility-tests/compile-fail/tests/compile-fail/error-reporting.rs

@@ -23,14 +23,6 @@ enum TupleEnumVariant {
     //~^ ERROR Only struct-like and unit enum variants are supported
 }
 
-#[derive(Snafu)]
-enum SnafuDisplayWrongKindOfExpression {
-    #[snafu::display {}]
-    //~^ ERROR A parenthesized format string with optional values is expected
-    //~^^ expected one of `(` or `=`
-    Alpha(i32),
-}
-
 #[derive(Snafu)]
 enum OldSnafuDisplayWithoutArgument {
     #[snafu_display]

compatibility-tests/nightly/.gitignore

@@ -12,7 +12,6 @@ license = "MIT OR Apache-2.0"
 [features]
 backtraces = []
 rust_1_30 = []
-unstable_display_attribute = []
 
 [lib]
 proc-macro = true