clippy::common-cargo-metadata: check for license-file

When license property is missing in Cargo.toml check for license-file
as it may be used instead of the former. The check implemented here is
very naive as it only verifies that the field is present and is not
empty. More scrutiny can be applied by verifying the file is actually
present.

Fixes #4517

clippy_lints/src/cargo_common_metadata.rs

@@ -1,5 +1,7 @@
 //! lint on missing cargo common metadata
 
+use std::path::PathBuf;
+
 use crate::utils::span_lint;
 use rustc::lint::{EarlyContext, EarlyLintPass, LintArray, LintPass};
 use rustc::{declare_lint_pass, declare_tool_lint};
@@ -47,6 +49,10 @@ fn is_empty_str(value: &Option<String>) -> bool {
     value.as_ref().map_or(true, String::is_empty)
 }
 
+fn is_empty_path(value: &Option<PathBbuf>) -> bool {
+    value.as_ref().and_then(|x| x.to_str()).map_or(true, str::is_empty)
+}
+
 fn is_empty_vec(value: &[String]) -> bool {
     // This works because empty iterators return true
     value.iter().all(std::string::String::is_empty)
@@ -72,8 +78,8 @@ impl EarlyLintPass for CargoCommonMetadata {
                 missing_warning(cx, &package, "package.description");
             }
 
-            if is_empty_str(&package.license) {
-                missing_warning(cx, &package, "package.license");
+            if is_empty_str(&package.license) || is_empty_path(&package.license_file) {
+                missing_warning(cx, &package, "either package.license or package.license_file");
             }
 
             if is_empty_str(&package.repository) {