From 38d93c76aedde037557b2c31dfc074f99f4ceb76 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Mon, 18 Aug 2025 17:15:47 -0700
Subject: [PATCH] Replace ~const syntax with [const] conditionally const syntax

---
 src/generics.rs | 93 ++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 68 insertions(+), 25 deletions(-)

diff --git a/src/generics.rs b/src/generics.rs
index 3f225b8..6c9688b 100644
--- a/src/generics.rs
+++ b/src/generics.rs
@@ -105,8 +105,7 @@ impl Printer {
         match type_param_bound {
             #![cfg_attr(all(test, exhaustive), deny(non_exhaustive_omitted_patterns))]
             TypeParamBound::Trait(trait_bound) => {
-                let tilde_const = false;
-                self.trait_bound(trait_bound, tilde_const);
+                self.trait_bound(trait_bound, TraitBoundConst::None);
             }
             TypeParamBound::Lifetime(lifetime) => self.lifetime(lifetime),
             TypeParamBound::PreciseCapture(precise_capture) => {
@@ -117,17 +116,21 @@ impl Printer {
         }
     }
 
-    fn trait_bound(&mut self, trait_bound: &TraitBound, tilde_const: bool) {
+    fn trait_bound(&mut self, trait_bound: &TraitBound, constness: TraitBoundConst) {
         if trait_bound.paren_token.is_some() {
             self.word("(");
         }
-        if tilde_const {
-            self.word("~const ");
-        }
-        self.trait_bound_modifier(&trait_bound.modifier);
         if let Some(bound_lifetimes) = &trait_bound.lifetimes {
             self.bound_lifetimes(bound_lifetimes);
         }
+        match constness {
+            TraitBoundConst::None => {}
+            #[cfg(feature = "verbatim")]
+            TraitBoundConst::Conditional => self.word("[const] "),
+            #[cfg(feature = "verbatim")]
+            TraitBoundConst::Unconditional => self.word("const "),
+        }
+        self.trait_bound_modifier(&trait_bound.modifier);
         for segment in trait_bound.path.segments.iter().delimited() {
             if !segment.is_first || trait_bound.path.leading_colon.is_some() {
                 self.word("::");
@@ -154,34 +157,67 @@ impl Printer {
     #[cfg(feature = "verbatim")]
     fn type_param_bound_verbatim(&mut self, tokens: &TokenStream) {
         use syn::parse::{Parse, ParseStream, Result};
-        use syn::{parenthesized, token, Token};
+        use syn::{
+            bracketed, parenthesized, token, ParenthesizedGenericArguments, Path, PathArguments,
+            Token,
+        };
 
         enum TypeParamBoundVerbatim {
             Ellipsis,
-            TildeConst(TraitBound),
+            Const(TraitBound, TraitBoundConst),
         }
 
         impl Parse for TypeParamBoundVerbatim {
             fn parse(input: ParseStream) -> Result<Self> {
+                if input.peek(Token![...]) {
+                    input.parse::<Token![...]>()?;
+                    return Ok(TypeParamBoundVerbatim::Ellipsis);
+                }
+
                 let content;
-                let (paren_token, content) = if input.peek(token::Paren) {
-                    (Some(parenthesized!(content in input)), &content)
+                let content = if input.peek(token::Paren) {
+                    parenthesized!(content in input);
+                    &content
                 } else {
-                    (None, input)
+                    input
                 };
-                let lookahead = content.lookahead1();
-                if lookahead.peek(Token![~]) {
-                    content.parse::<Token![~]>()?;
+
+                let lifetimes: Option<BoundLifetimes> = content.parse()?;
+
+                let constness = if content.peek(token::Bracket) {
+                    let conditionally_const;
+                    bracketed!(conditionally_const in content);
+                    conditionally_const.parse::<Token![const]>()?;
+                    TraitBoundConst::Conditional
+                } else if content.peek(Token![const]) {
                     content.parse::<Token![const]>()?;
-                    let mut bound: TraitBound = content.parse()?;
-                    bound.paren_token = paren_token;
-                    Ok(TypeParamBoundVerbatim::TildeConst(bound))
-                } else if lookahead.peek(Token![...]) {
-                    content.parse::<Token![...]>()?;
-                    Ok(TypeParamBoundVerbatim::Ellipsis)
+                    TraitBoundConst::Unconditional
                 } else {
-                    Err(lookahead.error())
+                    TraitBoundConst::None
+                };
+
+                let modifier: TraitBoundModifier = content.parse()?;
+
+                let mut path: Path = content.parse()?;
+                if path.segments.last().unwrap().arguments.is_empty()
+                    && (content.peek(token::Paren)
+                        || content.peek(Token![::]) && content.peek3(token::Paren))
+                {
+                    content.parse::<Option<Token![::]>>()?;
+                    let args: ParenthesizedGenericArguments = content.parse()?;
+                    let parenthesized = PathArguments::Parenthesized(args);
+                    path.segments.last_mut().unwrap().arguments = parenthesized;
                 }
+
+                Ok(TypeParamBoundVerbatim::Const(
+                    TraitBound {
+                        paren_token: None,
+                        modifier,
+                        lifetimes,
+                        path,
+                    },
+                    constness,
+                ))
             }
         }
 
@@ -194,9 +230,8 @@ impl Printer {
             TypeParamBoundVerbatim::Ellipsis => {
                 self.word("...");
             }
-            TypeParamBoundVerbatim::TildeConst(trait_bound) => {
-                let tilde_const = true;
-                self.trait_bound(&trait_bound, tilde_const);
+            TypeParamBoundVerbatim::Const(trait_bound, constness) => {
+                self.trait_bound(&trait_bound, constness);
             }
         }
     }
@@ -381,3 +416,11 @@ impl Printer {
         }
     }
 }
+
+enum TraitBoundConst {
+    None,
+    #[cfg(feature = "verbatim")]
+    Conditional,
+    #[cfg(feature = "verbatim")]
+    Unconditional,
+}