Remove the unstable snafu::display attribute

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

- rust-lang/rust#58899

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/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]

snafu-derive/src/lib.rs

@@ -11,14 +11,7 @@ use syn::parse::{Error as SynError, Result as SynResult};
 /// See the crate-level documentation for SNAFU which contains tested
 /// examples of this macro.
 
-#[cfg_attr(
-    not(feature = "unstable_display_attribute"),
-    proc_macro_derive(Snafu, attributes(snafu_visibility, snafu_display))
-)]
-#[cfg_attr(
-    feature = "unstable_display_attribute",
-    proc_macro_derive(Snafu, attributes(snafu_visibility, snafu::display, snafu_display))
-)]
+#[proc_macro_derive(Snafu, attributes(snafu_visibility, snafu_display))]
 pub fn snafu_derive(input: TokenStream) -> TokenStream {
     let ast = syn::parse(input).expect("Could not parse type to derive Error for");
 
@@ -144,18 +137,6 @@ fn parse_snafu_information(ty: syn::DeriveInput) -> SynResult<EnumInfo> {
     })
 }
 
-fn is_snafu_display(p: &syn::Path) -> bool {
-    is_path(p, &["snafu", "display"])
-}
-
-fn is_path(p: &syn::Path, parts: &[&str]) -> bool {
-    p.segments
-        .iter()
-        .zip(parts)
-        .map(|(a, b)| a.ident == b)
-        .all(|b| b)
-}
-
 fn parse_snafu_visibility(attrs: &[syn::Attribute]) -> SynResult<Option<Box<quote::ToTokens>>> {
     use syn::spanned::Spanned;
     use syn::Meta;
@@ -206,9 +187,7 @@ fn parse_snafu_display(attrs: &[syn::Attribute]) -> SynResult<Option<DisplayForm
     attrs
         .into_iter()
         .flat_map(|attr| {
-            if is_snafu_display(&attr.path) {
-                Some(parse_snafu_display_beautiful(attr))
-            } else if attr.path.is_ident("snafu_display") {
+            if attr.path.is_ident("snafu_display") {
                 let meta = match attr.parse_meta() {
                     Ok(meta) => meta,
                     Err(e) => return Some(Err(e)),
@@ -231,24 +210,6 @@ fn parse_snafu_display(attrs: &[syn::Attribute]) -> SynResult<Option<DisplayForm
         .my_transpose()
 }
 
-fn parse_snafu_display_beautiful(attr: &syn::Attribute) -> SynResult<DisplayFormat> {
-    use syn::spanned::Spanned;
-    use syn::Expr;
-
-    let expr: Expr = syn::parse2(attr.tts.clone())?;
-    let expr: Box<quote::ToTokens> = match expr {
-        Expr::Tuple(expr_tuple) => Box::new(expr_tuple.elems),
-        Expr::Paren(expr_paren) => Box::new(expr_paren.expr),
-        _ => {
-            return Err(SynError::new(
-                expr.span(),
-                "A parenthesized format string with optional values is expected",
-            ));
-        }
-    };
-    Ok(DisplayFormat::Direct(expr))
-}
-
 fn parse_snafu_display_nested(meta: syn::MetaList) -> SynResult<DisplayFormat> {
     use syn::spanned::Spanned;
     use syn::{Expr, Lit, NestedMeta};

src/lib.rs

@@ -146,10 +146,6 @@
 //! you can specify how the `Display` trait will be implemented for
 //! each variant:
 //!
-//! - `#[snafu::display("a format string with arguments: {}", info)]`
-//!
-//!   No special escaping is needed; this looks just like the arguments to a call to `println!`.
-//!
 //! - `#[snafu_display("a format string with arguments: {}", "info")]`
 //!
 //!   Every argument is quoted as a string literal separately.
@@ -215,10 +211,6 @@
 //! - Adds support for re-exporting the `Snafu` macro directly from
 //!   the `snafu` crate.
 //!
-//! ### `unstable_display_attribute` - supports Rust Nightly
-//!
-//! - Adds support for the `snafu::display` attribute.
-//!
 //! ## Other feature flags
 //!
 //! ### `backtraces`