1 files changed, 96 insertions, 0 deletions
diff --git a/rust/macros/paste.rs b/rust/macros/paste.rs
new file mode 100644
index 000000000000..385a78434224
--- /dev/null
+++ b/rust/macros/paste.rs
@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use proc_macro::{Delimiter, Group, Ident, Spacing, Span, TokenTree};
+
+fn concat(tokens: &[TokenTree], group_span: Span) -> TokenTree {
+    let mut tokens = tokens.iter();
+    let mut segments = Vec::new();
+    let mut span = None;
+    loop {
+        match tokens.next() {
+            None => break,
+            Some(TokenTree::Literal(lit)) => segments.push((lit.to_string(), lit.span())),
+            Some(TokenTree::Ident(ident)) => {
+                let mut value = ident.to_string();
+                if value.starts_with("r#") {
+                    value.replace_range(0..2, "");
+                }
+                segments.push((value, ident.span()));
+            }
+            Some(TokenTree::Punct(p)) if p.as_char() == ':' => {
+                let Some(TokenTree::Ident(ident)) = tokens.next() else {
+                    panic!("expected identifier as modifier");
+                };
+
+                let (mut value, sp) = segments.pop().expect("expected identifier before modifier");
+                match ident.to_string().as_str() {
+                    // Set the overall span of concatenated token as current span
+                    "span" => {
+                        assert!(
+                            span.is_none(),
+                            "span modifier should only appear at most once"
+                        );
+                        span = Some(sp);
+                    }
+                    "lower" => value = value.to_lowercase(),
+                    "upper" => value = value.to_uppercase(),
+                    v => panic!("unknown modifier `{v}`"),
+                };
+                segments.push((value, sp));
+            }
+            _ => panic!("unexpected token in paste segments"),
+        };
+    }
+
+    let pasted: String = segments.into_iter().map(|x| x.0).collect();
+    TokenTree::Ident(Ident::new(&pasted, span.unwrap_or(group_span)))
+}
+
+pub(crate) fn expand(tokens: &mut Vec<TokenTree>) {
+    for token in tokens.iter_mut() {
+        if let TokenTree::Group(group) = token {
+            let delimiter = group.delimiter();
+            let span = group.span();
+            let mut stream: Vec<_> = group.stream().into_iter().collect();
+            // Find groups that looks like `[< A B C D >]`
+            if delimiter == Delimiter::Bracket
+                && stream.len() >= 3
+                && matches!(&stream[0], TokenTree::Punct(p) if p.as_char() == '<')
+                && matches!(&stream[stream.len() - 1], TokenTree::Punct(p) if p.as_char() == '>')
+            {
+                // Replace the group with concatenated token
+                *token = concat(&stream[1..stream.len() - 1], span);
+            } else {
+                // Recursively expand tokens inside the group
+                expand(&mut stream);
+                let mut group = Group::new(delimiter, stream.into_iter().collect());
+                group.set_span(span);
+                *token = TokenTree::Group(group);
+            }
+        }
+    }
+
+    // Path segments cannot contain invisible delimiter group, so remove them if any.
+    for i in (0..tokens.len().saturating_sub(3)).rev() {
+        // Looking for a double colon
+        if matches!(
+            (&tokens[i + 1], &tokens[i + 2]),
+            (TokenTree::Punct(a), TokenTree::Punct(b))
+                if a.as_char() == ':' && a.spacing() == Spacing::Joint && b.as_char() == ':'
+        ) {
+            match &tokens[i + 3] {
+                TokenTree::Group(group) if group.delimiter() == Delimiter::None => {
+                    tokens.splice(i + 3..i + 4, group.stream());
+                }
+                _ => (),
+            }
+
+            match &tokens[i] {
+                TokenTree::Group(group) if group.delimiter() == Delimiter::None => {
+                    tokens.splice(i..i + 1, group.stream());
+                }
+                _ => (),
+            }
+        }
+    }
+}