Raise required compiler to rust 1.56

Author: dtolnay

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [1.31.0, stable, beta]
+        rust: [1.56.0, stable, beta]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v3

Cargo.toml

@@ -10,7 +10,7 @@ edition = "2018"
 keywords = ["macros", "syn"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/dtolnay/proc-macro2"
-rust-version = "1.31"
+rust-version = "1.56"
 
 [package.metadata.docs.rs]
 rustc-args = ["--cfg", "procmacro2_semver_exempt"]