6 files changed, 76 insertions, 9 deletions
diff --git a/src/tools/rustfmt/config_proc_macro/Cargo.lock b/src/tools/rustfmt/config_proc_macro/Cargo.lock
index ecf561f28..49f2f72a8 100644
--- a/src/tools/rustfmt/config_proc_macro/Cargo.lock
+++ b/src/tools/rustfmt/config_proc_macro/Cargo.lock
@@ -22,7 +22,7 @@ dependencies = [
 
 [[package]]
 name = "rustfmt-config_proc_macro"
-version = "0.2.0"
+version = "0.3.0"
 dependencies = [
  "proc-macro2",
  "quote",
diff --git a/src/tools/rustfmt/config_proc_macro/Cargo.toml b/src/tools/rustfmt/config_proc_macro/Cargo.toml
index a41b3a5e6..d10d0469c 100644
--- a/src/tools/rustfmt/config_proc_macro/Cargo.toml
+++ b/src/tools/rustfmt/config_proc_macro/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "rustfmt-config_proc_macro"
-version = "0.2.0"
+version = "0.3.0"
 edition = "2018"
 description = "A collection of procedural macros for rustfmt"
 license = "Apache-2.0/MIT"
diff --git a/src/tools/rustfmt/config_proc_macro/src/attrs.rs b/src/tools/rustfmt/config_proc_macro/src/attrs.rs
index 0baba046f..dd18ff572 100644
--- a/src/tools/rustfmt/config_proc_macro/src/attrs.rs
+++ b/src/tools/rustfmt/config_proc_macro/src/attrs.rs
@@ -1,8 +1,10 @@
 //! This module provides utilities for handling attributes on variants
-//! of `config_type` enum. Currently there are two types of attributes
-//! that could appear on the variants of `config_type` enum: `doc_hint`
-//! and `value`. Both comes in the form of name-value pair whose value
-//! is string literal.
+//! of `config_type` enum. Currently there are the following attributes
+//! that could appear on the variants of `config_type` enum:
+//!
+//! - `doc_hint`: name-value pair whose value is string literal
+//! - `value`: name-value pair whose value is string literal
+//! - `unstable_variant`: name only
 
 /// Returns the value of the first `doc_hint` attribute in the given slice or
 /// `None` if `doc_hint` attribute is not available.
@@ -27,6 +29,11 @@ pub fn find_config_value(attrs: &[syn::Attribute]) -> Option<String> {
     attrs.iter().filter_map(config_value).next()
 }
 
+/// Returns `true` if the there is at least one `unstable` attribute in the given slice.
+pub fn any_unstable_variant(attrs: &[syn::Attribute]) -> bool {
+    attrs.iter().any(is_unstable_variant)
+}
+
 /// Returns a string literal value if the given attribute is `value`
 /// attribute or `None` otherwise.
 pub fn config_value(attr: &syn::Attribute) -> Option<String> {
@@ -38,6 +45,11 @@ pub fn is_config_value(attr: &syn::Attribute) -> bool {
     is_attr_name_value(attr, "value")
 }
 
+/// Returns `true` if the given attribute is an `unstable` attribute.
+pub fn is_unstable_variant(attr: &syn::Attribute) -> bool {
+    is_attr_path(attr, "unstable_variant")
+}
+
 fn is_attr_name_value(attr: &syn::Attribute, name: &str) -> bool {
     attr.parse_meta().ok().map_or(false, |meta| match meta {
         syn::Meta::NameValue(syn::MetaNameValue { ref path, .. }) if path.is_ident(name) => true,
@@ -45,6 +57,13 @@ fn is_attr_name_value(attr: &syn::Attribute, name: &str) -> bool {
     })
 }
 
+fn is_attr_path(attr: &syn::Attribute, name: &str) -> bool {
+    attr.parse_meta().ok().map_or(false, |meta| match meta {
+        syn::Meta::Path(path) if path.is_ident(name) => true,
+        _ => false,
+    })
+}
+
 fn get_name_value_str_lit(attr: &syn::Attribute, name: &str) -> Option<String> {
     attr.parse_meta().ok().and_then(|meta| match meta {
         syn::Meta::NameValue(syn::MetaNameValue {
diff --git a/src/tools/rustfmt/config_proc_macro/src/item_enum.rs b/src/tools/rustfmt/config_proc_macro/src/item_enum.rs
index dcee77a85..731a7ea06 100644
--- a/src/tools/rustfmt/config_proc_macro/src/item_enum.rs
+++ b/src/tools/rustfmt/config_proc_macro/src/item_enum.rs
@@ -1,5 +1,6 @@
 use proc_macro2::TokenStream;
-use quote::quote;
+use quote::{quote, quote_spanned};
+use syn::spanned::Spanned;
 
 use crate::attrs::*;
 use crate::utils::*;
@@ -47,12 +48,23 @@ fn process_variant(variant: &syn::Variant) -> TokenStream {
     let metas = variant
         .attrs
         .iter()
-        .filter(|attr| !is_doc_hint(attr) && !is_config_value(attr));
+        .filter(|attr| !is_doc_hint(attr) && !is_config_value(attr) && !is_unstable_variant(attr));
     let attrs = fold_quote(metas, |meta| quote!(#meta));
     let syn::Variant { ident, fields, .. } = variant;
     quote!(#attrs #ident #fields)
 }
 
+/// Return the correct syntax to pattern match on the enum variant, discarding all
+/// internal field data.
+fn fields_in_variant(variant: &syn::Variant) -> TokenStream {
+    // With thanks to https://stackoverflow.com/a/65182902
+    match &variant.fields {
+        syn::Fields::Unnamed(_) => quote_spanned! { variant.span() => (..) },
+        syn::Fields::Unit => quote_spanned! { variant.span() => },
+        syn::Fields::Named(_) => quote_spanned! { variant.span() => {..} },
+    }
+}
+
 fn impl_doc_hint(ident: &syn::Ident, variants: &Variants) -> TokenStream {
     let doc_hint = variants
         .iter()
@@ -60,12 +72,26 @@ fn impl_doc_hint(ident: &syn::Ident, variants: &Variants) -> TokenStream {
         .collect::<Vec<_>>()
         .join("|");
     let doc_hint = format!("[{}]", doc_hint);
+
+    let variant_stables = variants
+        .iter()
+        .map(|v| (&v.ident, fields_in_variant(&v), !unstable_of_variant(v)));
+    let match_patterns = fold_quote(variant_stables, |(v, fields, stable)| {
+        quote! {
+            #ident::#v #fields => #stable,
+        }
+    });
     quote! {
         use crate::config::ConfigType;
         impl ConfigType for #ident {
             fn doc_hint() -> String {
                 #doc_hint.to_owned()
             }
+            fn stable_variant(&self) -> bool {
+                match self {
+                    #match_patterns
+                }
+            }
         }
     }
 }
@@ -123,13 +149,21 @@ fn impl_from_str(ident: &syn::Ident, variants: &Variants) -> TokenStream {
 }
 
 fn doc_hint_of_variant(variant: &syn::Variant) -> String {
-    find_doc_hint(&variant.attrs).unwrap_or(variant.ident.to_string())
+    let mut text = find_doc_hint(&variant.attrs).unwrap_or(variant.ident.to_string());
+    if unstable_of_variant(&variant) {
+        text.push_str(" (unstable)")
+    };
+    text
 }
 
 fn config_value_of_variant(variant: &syn::Variant) -> String {
     find_config_value(&variant.attrs).unwrap_or(variant.ident.to_string())
 }
 
+fn unstable_of_variant(variant: &syn::Variant) -> bool {
+    any_unstable_variant(&variant.attrs)
+}
+
 fn impl_serde(ident: &syn::Ident, variants: &Variants) -> TokenStream {
     let arms = fold_quote(variants.iter(), |v| {
         let v_ident = &v.ident;
diff --git a/src/tools/rustfmt/config_proc_macro/src/lib.rs b/src/tools/rustfmt/config_proc_macro/src/lib.rs
index e772c53f4..0c54c132c 100644
--- a/src/tools/rustfmt/config_proc_macro/src/lib.rs
+++ b/src/tools/rustfmt/config_proc_macro/src/lib.rs
@@ -69,3 +69,16 @@ pub fn stable_only_test(_args: TokenStream, input: TokenStream) -> TokenStream {
         TokenStream::from_str("").unwrap()
     }
 }
+
+/// Used to conditionally output the TokenStream for tests that should be run as part of rustfmts
+/// test suite, but should be ignored when running in the rust-lang/rust test suite.
+#[proc_macro_attribute]
+pub fn rustfmt_only_ci_test(_args: TokenStream, input: TokenStream) -> TokenStream {
+    if option_env!("RUSTFMT_CI").is_some() {
+        input
+    } else {
+        let mut token_stream = TokenStream::from_str("#[ignore]").unwrap();
+        token_stream.extend(input);
+        token_stream
+    }
+}
diff --git a/src/tools/rustfmt/config_proc_macro/tests/smoke.rs b/src/tools/rustfmt/config_proc_macro/tests/smoke.rs
index 940a8a0c2..c8a83e39c 100644
--- a/src/tools/rustfmt/config_proc_macro/tests/smoke.rs
+++ b/src/tools/rustfmt/config_proc_macro/tests/smoke.rs
@@ -1,6 +1,7 @@
 pub mod config {
     pub trait ConfigType: Sized {
         fn doc_hint() -> String;
+        fn stable_variant(&self) -> bool;
     }
 }