Add a warning if license-file points to a file that does not exist.

Author: ehuss

src/cargo/ops/cargo_package.rs

@@ -200,6 +200,26 @@ fn check_metadata(pkg: &Package, config: &Config) -> CargoResult<()> {
             things = things
         ))?
     }
+
+    if let Some(license_path) = &md.license_file {
+        let license_path = Path::new(license_path);
+        let abs_license_path = pkg.root().join(license_path);
+        if !abs_license_path.exists() {
+            let rel_msg = if license_path.is_absolute() {
+                "".to_string()
+            } else {
+                format!(" (relative to `{}`)", pkg.root().display())
+            };
+            config.shell().warn(&format!(
+                "license-file `{}` does not appear to exist{}.\n\
+                Please update the license-file setting in the manifest at `{}`\n\
+                This may become a hard error in the future.",
+                license_path.display(),
+                rel_msg,
+                pkg.manifest_path().display()
+            ))?;
+        }
+    }
     Ok(())
 }
 

tests/testsuite/package.rs

@@ -1445,3 +1445,33 @@ fn exclude_dot_files_and_directories_by_default() {
          ",
     );
 }
+
+#[cargo_test]
+fn invalid_license_file_path() {
+    // Test warning when license-file points to a non-existent file.
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "1.0.0"
+            license-file = "does-not-exist"
+            description = "foo"
+            homepage = "foo"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("package --no-verify")
+        .with_stderr(
+            "\
+[WARNING] license-file `does-not-exist` does not appear to exist (relative to `[..]/foo`).
+Please update the license-file setting in the manifest at `[..]/foo/Cargo.toml`
+This may become a hard error in the future.
+[PACKAGING] foo v1.0.0 ([..]/foo)
+",
+        )
+        .run();
+}