Add rust-version field to Cargo.toml

https://doc.rust-lang.org/nightly/cargo/reference/manifest.html#the-rust-version-field

Author: taiki-e

.github/workflows/ci.yml

@@ -75,7 +75,7 @@ jobs:
         range:
           # This is the minimum supported Rust version of this crate.
           # When updating this, the reminder to update the minimum supported
-          # Rust version in README.md and .clippy.toml.
+          # Rust version in README.md, Cargo.toml, and .clippy.toml.
           - 1.37..
     runs-on: ubuntu-latest
     steps:

Cargo.toml

@@ -3,6 +3,7 @@ name = "pin-project"
 version = "1.0.8"
 authors = ["Taiki Endo <te316e89@gmail.com>"]
 edition = "2018"
+rust-version = "1.37"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/taiki-e/pin-project"
 keywords = ["pin", "macros", "attribute"]