Add support for repetition to proc_macro::quote

library/proc_macro/src/lib.rs

@@ -45,6 +45,7 @@ mod diagnostic;
 mod escape;
 mod to_tokens;
 
+use core::ops::BitOr;
 use std::ffi::CStr;
 use std::ops::{Range, RangeBounds};
 use std::path::PathBuf;
@@ -237,7 +238,7 @@ impl Default for TokenStream {
 }
 
 #[unstable(feature = "proc_macro_quote", issue = "54722")]
-pub use quote::{quote, quote_span};
+pub use quote::{HasIterator, RepInterp, ThereIsNoIteratorInRepetition, ext, quote, quote_span};
 
 fn tree_to_bridge_tree(
     tree: TokenTree,

library/proc_macro/src/quote.rs

@@ -5,9 +5,183 @@
 //! items from `proc_macro`, to build a `proc_macro::TokenStream`.
 
 use crate::{
-    Delimiter, Group, Ident, Literal, Punct, Spacing, Span, ToTokens, TokenStream, TokenTree,
+    BitOr, Delimiter, Group, Ident, Literal, Punct, Spacing, Span, ToTokens, TokenStream, TokenTree,
 };
 
+#[doc(hidden)]
+pub struct HasIterator; // True
+#[doc(hidden)]
+pub struct ThereIsNoIteratorInRepetition; // False
+
+impl BitOr<ThereIsNoIteratorInRepetition> for ThereIsNoIteratorInRepetition {
+    type Output = ThereIsNoIteratorInRepetition;
+    fn bitor(self, _rhs: ThereIsNoIteratorInRepetition) -> ThereIsNoIteratorInRepetition {
+        ThereIsNoIteratorInRepetition
+    }
+}
+
+impl BitOr<ThereIsNoIteratorInRepetition> for HasIterator {
+    type Output = HasIterator;
+    fn bitor(self, _rhs: ThereIsNoIteratorInRepetition) -> HasIterator {
+        HasIterator
+    }
+}
+
+impl BitOr<HasIterator> for ThereIsNoIteratorInRepetition {
+    type Output = HasIterator;
+    fn bitor(self, _rhs: HasIterator) -> HasIterator {
+        HasIterator
+    }
+}
+
+impl BitOr<HasIterator> for HasIterator {
+    type Output = HasIterator;
+    fn bitor(self, _rhs: HasIterator) -> HasIterator {
+        HasIterator
+    }
+}
+
+/// Extension traits used by the implementation of `quote!`. These are defined
+/// in separate traits, rather than as a single trait due to ambiguity issues.
+///
+/// These traits expose a `quote_into_iter` method which should allow calling
+/// whichever impl happens to be applicable. Calling that method repeatedly on
+/// the returned value should be idempotent.
+#[doc(hidden)]
+pub mod ext {
+    use core::slice;
+    use std::collections::btree_set::{self, BTreeSet};
+
+    use super::{
+        HasIterator as HasIter, RepInterp, ThereIsNoIteratorInRepetition as DoesNotHaveIter,
+    };
+    use crate::ToTokens;
+
+    /// Extension trait providing the `quote_into_iter` method on iterators.
+    #[doc(hidden)]
+    pub trait RepIteratorExt: Iterator + Sized {
+        fn quote_into_iter(self) -> (Self, HasIter) {
+            (self, HasIter)
+        }
+    }
+
+    impl<T: Iterator> RepIteratorExt for T {}
+
+    /// Extension trait providing the `quote_into_iter` method for
+    /// non-iterable types. These types interpolate the same value in each
+    /// iteration of the repetition.
+    #[doc(hidden)]
+    pub trait RepToTokensExt {
+        /// Pretend to be an iterator for the purposes of `quote_into_iter`.
+        /// This allows repeated calls to `quote_into_iter` to continue
+        /// correctly returning DoesNotHaveIter.
+        fn next(&self) -> Option<&Self> {
+            Some(self)
+        }
+
+        fn quote_into_iter(&self) -> (&Self, DoesNotHaveIter) {
+            (self, DoesNotHaveIter)
+        }
+    }
+
+    impl<T: ToTokens + ?Sized> RepToTokensExt for T {}
+
+    /// Extension trait providing the `quote_into_iter` method for types that
+    /// can be referenced as an iterator.
+    #[doc(hidden)]
+    pub trait RepAsIteratorExt<'q> {
+        type Iter: Iterator;
+
+        fn quote_into_iter(&'q self) -> (Self::Iter, HasIter);
+    }
+
+    impl<'q, T: RepAsIteratorExt<'q> + ?Sized> RepAsIteratorExt<'q> for &T {
+        type Iter = T::Iter;
+
+        fn quote_into_iter(&'q self) -> (Self::Iter, HasIter) {
+            <T as RepAsIteratorExt>::quote_into_iter(*self)
+        }
+    }
+
+    impl<'q, T: RepAsIteratorExt<'q> + ?Sized> RepAsIteratorExt<'q> for &mut T {
+        type Iter = T::Iter;
+
+        fn quote_into_iter(&'q self) -> (Self::Iter, HasIter) {
+            <T as RepAsIteratorExt>::quote_into_iter(*self)
+        }
+    }
+
+    impl<'q, T: 'q> RepAsIteratorExt<'q> for [T] {
+        type Iter = slice::Iter<'q, T>;
+
+        fn quote_into_iter(&'q self) -> (Self::Iter, HasIter) {
+            (self.iter(), HasIter)
+        }
+    }
+
+    impl<'q, T: 'q, const N: usize> RepAsIteratorExt<'q> for [T; N] {
+        type Iter = slice::Iter<'q, T>;
+
+        fn quote_into_iter(&'q self) -> (Self::Iter, HasIter) {
+            (self.iter(), HasIter)
+        }
+    }
+
+    impl<'q, T: 'q> RepAsIteratorExt<'q> for Vec<T> {
+        type Iter = slice::Iter<'q, T>;
+
+        fn quote_into_iter(&'q self) -> (Self::Iter, HasIter) {
+            (self.iter(), HasIter)
+        }
+    }
+
+    impl<'q, T: 'q> RepAsIteratorExt<'q> for BTreeSet<T> {
+        type Iter = btree_set::Iter<'q, T>;
+
+        fn quote_into_iter(&'q self) -> (Self::Iter, HasIter) {
+            (self.iter(), HasIter)
+        }
+    }
+
+    impl<'q, T: RepAsIteratorExt<'q>> RepAsIteratorExt<'q> for RepInterp<T> {
+        type Iter = T::Iter;
+
+        fn quote_into_iter(&'q self) -> (Self::Iter, HasIter) {
+            self.0.quote_into_iter()
+        }
+    }
+}
+
+// Helper type used within interpolations to allow for repeated binding names.
+// Implements the relevant traits, and exports a dummy `next()` method.
+#[derive(Copy, Clone)]
+#[doc(hidden)]
+pub struct RepInterp<T>(pub T);
+
+impl<T> RepInterp<T> {
+    // This method is intended to look like `Iterator::next`, and is called when
+    // a name is bound multiple times, as the previous binding will shadow the
+    // original `Iterator` object. This allows us to avoid advancing the
+    // iterator multiple times per iteration.
+    pub fn next(self) -> Option<T> {
+        Some(self.0)
+    }
+}
+
+impl<T: Iterator> Iterator for RepInterp<T> {
+    type Item = T::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+}
+
+impl<T: ToTokens> ToTokens for RepInterp<T> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        self.0.to_tokens(tokens);
+    }
+}
+
 macro_rules! minimal_quote_tt {
     (($($t:tt)*)) => { Group::new(Delimiter::Parenthesis, minimal_quote!($($t)*)) };
     ([$($t:tt)*]) => { Group::new(Delimiter::Bracket, minimal_quote!($($t)*)) };
@@ -20,7 +194,13 @@ macro_rules! minimal_quote_tt {
     (>) => { Punct::new('>', Spacing::Alone) };
     (&) => { Punct::new('&', Spacing::Alone) };
     (=) => { Punct::new('=', Spacing::Alone) };
+    (#) => { Punct::new('#', Spacing::Alone) };
+    (|) => { Punct::new('|', Spacing::Alone) };
+    (:) => { Punct::new(':', Spacing::Alone) };
+    (*) => { Punct::new('*', Spacing::Alone) };
+    (_) => { Ident::new("_", Span::def_site()) };
     ($i:ident) => { Ident::new(stringify!($i), Span::def_site()) };
+    ($lit:literal) => { stringify!($lit).parse::<Literal>().unwrap() };
 }
 
 macro_rules! minimal_quote_ts {
@@ -36,6 +216,39 @@ macro_rules! minimal_quote_ts {
             [c.0, c.1].into_iter().collect::<TokenStream>()
         }
     };
+    (=>) => {
+        {
+            let mut c = (
+                TokenTree::from(Punct::new('=', Spacing::Joint)),
+                TokenTree::from(Punct::new('>', Spacing::Alone))
+            );
+            c.0.set_span(Span::def_site());
+            c.1.set_span(Span::def_site());
+            [c.0, c.1].into_iter().collect::<TokenStream>()
+        }
+    };
+    (+=) => {
+        {
+            let mut c = (
+                TokenTree::from(Punct::new('+', Spacing::Joint)),
+                TokenTree::from(Punct::new('=', Spacing::Alone))
+            );
+            c.0.set_span(Span::def_site());
+            c.1.set_span(Span::def_site());
+            [c.0, c.1].into_iter().collect::<TokenStream>()
+        }
+    };
+    (!=) => {
+        {
+            let mut c = (
+                TokenTree::from(Punct::new('!', Spacing::Joint)),
+                TokenTree::from(Punct::new('=', Spacing::Alone))
+            );
+            c.0.set_span(Span::def_site());
+            c.1.set_span(Span::def_site());
+            [c.0, c.1].into_iter().collect::<TokenStream>()
+        }
+    };
     ($t:tt) => { TokenTree::from(minimal_quote_tt!($t)) };
 }
 
@@ -71,17 +284,99 @@ pub fn quote(stream: TokenStream) -> TokenStream {
     let mut after_dollar = false;
 
     let mut tokens = crate::TokenStream::new();
-    for tree in stream {
+    let mut iter = stream.into_iter().peekable();
+    while let Some(tree) = iter.next() {
         if after_dollar {
             after_dollar = false;
             match tree {
+                TokenTree::Group(tt) => {
+                    // Handles repetition by expanding `$( CONTENTS ) SEP_OPT *` to `{ REP_EXPANDED }`.
+                    let contents = tt.stream();
+
+                    // The `*` token is also consumed here.
+                    let sep_opt: Option<Punct> = match (iter.next(), iter.peek()) {
+                        (Some(TokenTree::Punct(sep)), Some(TokenTree::Punct(star)))
+                            if sep.spacing() == Spacing::Joint && star.as_char() == '*' =>
+                        {
+                            iter.next();
+                            Some(sep)
+                        }
+                        (Some(TokenTree::Punct(star)), _) if star.as_char() == '*' => None,
+                        _ => panic!("`$(...)` must be followed by `*` in `quote!`"),
+                    };
+
+                    let mut rep_expanded = TokenStream::new();
+
+                    // Append setup code for a `while`, where recursively quoted `CONTENTS`
+                    // and `SEP_OPT` are repeatedly processed, to `REP_EXPANDED`.
+                    let meta_vars = collect_meta_vars(contents.clone());
+                    minimal_quote!(
+                        use crate::ext::*;
+                        (@ if sep_opt.is_some() {
+                            minimal_quote!(let mut _i = 0usize;)
+                        } else {
+                            minimal_quote!(();)
+                        })
+                        let has_iter = crate::ThereIsNoIteratorInRepetition;
+                    )
+                    .to_tokens(&mut rep_expanded);
+                    for meta_var in &meta_vars {
+                        minimal_quote!(
+                            #[allow(unused_mut)]
+                            let (mut (@ meta_var), i) = (@ meta_var).quote_into_iter();
+                            let has_iter = has_iter | i;
+                        )
+                        .to_tokens(&mut rep_expanded);
+                    }
+                    minimal_quote!(let _: crate::HasIterator = has_iter;)
+                        .to_tokens(&mut rep_expanded);
+
+                    // Append the `while` to `REP_EXPANDED`.
+                    let mut while_body = TokenStream::new();
+                    for meta_var in &meta_vars {
+                        minimal_quote!(
+                            let (@ meta_var) = match (@ meta_var).next() {
+                                Some(_x) => crate::RepInterp(_x),
+                                None => break,
+                            };
+                        )
+                        .to_tokens(&mut while_body);
+                    }
+                    minimal_quote!(
+                        (@ if let Some(sep) = sep_opt {
+                            minimal_quote!(
+                                if _i > 0 {
+                                    (@ minimal_quote!(crate::ToTokens::to_tokens(&crate::TokenTree::Punct(crate::Punct::new(
+                                        (@ TokenTree::from(Literal::character(sep.as_char()))),
+                                        (@ minimal_quote!(crate::Spacing::Alone)),
+                                    )), &mut ts);))
+                                }
+                                _i += 1;
+                            )
+                        } else {
+                            minimal_quote!(();)
+                        })
+                        (@ quote(contents.clone())).to_tokens(&mut ts);
+                    )
+                        .to_tokens(&mut while_body);
+                    rep_expanded.extend(vec![
+                        TokenTree::Ident(Ident::new("while", Span::call_site())),
+                        TokenTree::Ident(Ident::new("true", Span::call_site())),
+                        TokenTree::Group(Group::new(Delimiter::Brace, while_body)),
+                    ]);
+
+                    minimal_quote!((@ TokenTree::Group(Group::new(Delimiter::Brace, rep_expanded)))).to_tokens(&mut tokens);
+                    continue;
+                }
                 TokenTree::Ident(_) => {
                     minimal_quote!(crate::ToTokens::to_tokens(&(@ tree), &mut ts);)
                         .to_tokens(&mut tokens);
                     continue;
                 }
                 TokenTree::Punct(ref tt) if tt.as_char() == '$' => {}
-                _ => panic!("`$` must be followed by an ident or `$` in `quote!`"),
+                _ => panic!(
+                    "`$` must be followed by an ident or `$` or a repetition group in `quote!`"
+                ),
             }
         } else if let TokenTree::Punct(ref tt) = tree {
             if tt.as_char() == '$' {
@@ -155,6 +450,33 @@ pub fn quote(stream: TokenStream) -> TokenStream {
     }
 }
 
+/// Helper function to support macro repetitions like `$( CONTENTS ) SEP_OPT *` in `quote!`.
+/// Recursively collects all `Ident`s (meta-variables) that follow a `$`
+/// from the given `CONTENTS` stream, preserving their order of appearance.
+fn collect_meta_vars(content_stream: TokenStream) -> Vec<Ident> {
+    fn helper(stream: TokenStream, out: &mut Vec<Ident>) {
+        let mut iter = stream.into_iter().peekable();
+        while let Some(tree) = iter.next() {
+            match &tree {
+                TokenTree::Punct(tt) if tt.as_char() == '$' => {
+                    if let Some(TokenTree::Ident(id)) = iter.peek() {
+                        out.push(id.clone());
+                        iter.next();
+                    }
+                }
+                TokenTree::Group(tt) => {
+                    helper(tt.stream(), out);
+                }
+                _ => {}
+            }
+        }
+    }
+
+    let mut vars = Vec::new();
+    helper(content_stream, &mut vars);
+    vars
+}
+
 /// Quote a `Span` into a `TokenStream`.
 /// This is needed to implement a custom quoter.
 #[unstable(feature = "proc_macro_quote", issue = "54722")]