try reading rust-version from Cargo.toml

Cargo.toml can contain a field `rust-version`, that acts like a MSRV of
clippy.toml file: https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field
This will try to read that field and use it, if the clippy.toml config
has no `msrv` entry

Author: hellow554

clippy_lints/src/lib.rs

@@ -50,6 +50,7 @@ extern crate rustc_typeck;
 #[macro_use]
 extern crate clippy_utils;
 
+use cargo_metadata::MetadataCommand;
 use clippy_utils::parse_msrv;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_lint::LintId;
@@ -455,7 +456,7 @@ pub fn read_conf(sess: &Session) -> Conf {
         },
     };
 
-    let TryConf { conf, errors } = utils::conf::read(&file_name);
+    let TryConf { mut conf, errors } = utils::conf::read(&file_name);
     // all conf errors are non-fatal, we just use the default conf in case of error
     for error in errors {
         sess.struct_err(&format!(
@@ -466,6 +467,23 @@ pub fn read_conf(sess: &Session) -> Conf {
         .emit();
     }
 
+    if conf.msrv.is_none() {
+        // let's try to get msrv from `Cargo.toml`s field `rust-version`
+        match MetadataCommand::new().no_deps().exec() {
+            Ok(metadata) => {
+                conf.msrv = metadata
+                    .packages
+                    .get(0)
+                    .and_then(|x| x.rust_version.as_ref())
+                    .and_then(|r| r.comparators.get(0))
+                    .map(|v| format!("{}.{}.{}", v.major, v.minor.unwrap_or(0), v.patch.unwrap_or(0)));
+            },
+            Err(e) => {
+                sess.struct_err(&format!("could not read cargo metadata: {}", e)).emit();
+            },
+        }
+    }
+
     conf
 }