add min rust version

was proposed in https://github.com/mthom/scryer-prolog/issues/1335#issuecomment-1062039516 and got positive feedback

diff --git a/Cargo.toml b/Cargo.toml
index 193d6ce6dff81635de2715d45f5c1ff3ca69b260..b9d8466df5134b02b760a7446ae57470683a1f39 100644 (file)
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -10,6 +10,7 @@ license = "BSD-3-Clause"
 keywords = ["prolog", "prolog-interpreter", "prolog-system"]
 categories = ["command-line-utilities"]
 build = "build.rs"
+rust-version = "1.57"
 
 [workspace]
 members = ["crates/num-rug-adapter",
@@ -72,4 +73,4 @@ predicates-core = "1.0.2"
 serial_test = "0.5.1"
 
 [profile.release]
-debug = true
\ No newline at end of file
+debug = true