Bypass negative literal workaround on 1.56+

Cargo.toml

@@ -36,7 +36,7 @@ proc-macro = ["proc-macro2/proc-macro", "quote/proc-macro"]
 test = ["syn-test-suite/all-features"]
 
 [dependencies]
-proc-macro2 = { version = "1.0.27", default-features = false }
+proc-macro2 = { version = "1.0.32", default-features = false }
 quote = { version = "1.0", optional = true, default-features = false }
 unicode-xid = "0.2"
 

build.rs

@@ -19,6 +19,10 @@ fn main() {
         println!("cargo:rustc-cfg=syn_no_const_vec_new");
     }
 
+    if compiler.minor < 56 {
+        println!("cargo:rustc-cfg=syn_no_negative_literal_parse");
+    }
+
     if !compiler.nightly {
         println!("cargo:rustc-cfg=syn_disable_nightly_tests");
     }

src/lit.rs

@@ -1539,31 +1539,37 @@ mod value {
         }
     }
 
+    #[allow(clippy::unnecessary_wraps)]
     pub fn to_literal(repr: &str, digits: &str, suffix: &str) -> Option<Literal> {
-        if repr.starts_with('-') {
-            let f64_parse_finite = || digits.parse().ok().filter(|x: &f64| x.is_finite());
-            let f32_parse_finite = || digits.parse().ok().filter(|x: &f32| x.is_finite());
-            if suffix == "f64" {
-                f64_parse_finite().map(Literal::f64_suffixed)
-            } else if suffix == "f32" {
-                f32_parse_finite().map(Literal::f32_suffixed)
-            } else if suffix == "i64" {
-                digits.parse().ok().map(Literal::i64_suffixed)
-            } else if suffix == "i32" {
-                digits.parse().ok().map(Literal::i32_suffixed)
-            } else if suffix == "i16" {
-                digits.parse().ok().map(Literal::i16_suffixed)
-            } else if suffix == "i8" {
-                digits.parse().ok().map(Literal::i8_suffixed)
-            } else if !suffix.is_empty() {
-                None
-            } else if digits.contains('.') {
-                f64_parse_finite().map(Literal::f64_unsuffixed)
-            } else {
-                digits.parse().ok().map(Literal::i64_unsuffixed)
+        #[cfg(syn_no_negative_literal_parse)]
+        {
+            // Rustc older than https://github.com/rust-lang/rust/pull/87262.
+            if repr.starts_with('-') {
+                let f64_parse_finite = || digits.parse().ok().filter(|x: &f64| x.is_finite());
+                let f32_parse_finite = || digits.parse().ok().filter(|x: &f32| x.is_finite());
+                return if suffix == "f64" {
+                    f64_parse_finite().map(Literal::f64_suffixed)
+                } else if suffix == "f32" {
+                    f32_parse_finite().map(Literal::f32_suffixed)
+                } else if suffix == "i64" {
+                    digits.parse().ok().map(Literal::i64_suffixed)
+                } else if suffix == "i32" {
+                    digits.parse().ok().map(Literal::i32_suffixed)
+                } else if suffix == "i16" {
+                    digits.parse().ok().map(Literal::i16_suffixed)
+                } else if suffix == "i8" {
+                    digits.parse().ok().map(Literal::i8_suffixed)
+                } else if !suffix.is_empty() {
+                    None
+                } else if digits.contains('.') {
+                    f64_parse_finite().map(Literal::f64_unsuffixed)
+                } else {
+                    digits.parse().ok().map(Literal::i64_unsuffixed)
+                };
             }
-        } else {
-            Some(repr.parse::<Literal>().unwrap())
         }
+        let _ = digits;
+        let _ = suffix;
+        Some(repr.parse::<Literal>().unwrap())
     }
 }

tests/test_lit.rs

@@ -215,12 +215,6 @@ fn negative() {
     assert_eq!("-1.5f64", LitFloat::new("-1.5f64", span).to_string());
 }
 
-#[test]
-fn negative_overflow() {
-    assert!(syn::parse_str::<LitFloat>("-1.0e99f64").is_ok());
-    assert!(syn::parse_str::<LitFloat>("-1.0e999f64").is_err());
-}
-
 #[test]
 fn suffix() {
     fn get_suffix(token: &str) -> String {