Add check for empty cfg all condition

Author: GuillaumeGomez

Diff for: clippy_lints/src/attrs.rs

@@ -796,6 +796,14 @@ fn check_nested_cfg(cx: &EarlyContext<'_>, items: &[NestedMetaItem]) {
                             }
                         },
                     );
+                } else if list.is_empty() && meta.has_name(sym::all) {
+                    span_lint_and_then(
+                        cx,
+                        NON_MINIMAL_CFG,
+                        meta.span,
+                        "unneeded sub `cfg` when there is no condition",
+                        |_| {},
+                    );
                 }
             }
         }

Diff for: tests/ui/non_minimal_cfg.fixed

@@ -11,4 +11,7 @@ fn wasi() {}
 #[cfg(all(unix, not(windows)))]
 fn the_end() {}
 
+#[cfg(any())]
+fn any() {}
+
 fn main() {}

Diff for: tests/ui/non_minimal_cfg.rs

@@ -11,4 +11,7 @@ fn wasi() {}
 #[cfg(all(any(unix), all(not(windows))))]
 fn the_end() {}
 
+#[cfg(any())]
+fn any() {}
+
 fn main() {}

Diff for: tests/ui/non_minimal_cfg2.rs

@@ -0,0 +1,6 @@
+#![allow(unused)]
+
+#[cfg(all())]
+fn all() {}
+
+fn main() {}

Diff for: tests/ui/non_minimal_cfg2.stderr

@@ -0,0 +1,10 @@
+error: unneeded sub `cfg` when there is no condition
+  --> $DIR/non_minimal_cfg2.rs:3:7
+   |
+LL | #[cfg(all())]
+   |       ^^^^^
+   |
+   = note: `-D clippy::non-minimal-cfg` implied by `-D warnings`
+
+error: aborting due to previous error
+