Completion = $0 after keyval cfg predicate

Example
---
```rust
//- /main.rs cfg:test,dbg=false,opt_level=2
#[cfg($0)]
```

**Before this PR**

```rust
#[cfg(opt_level)]
```

**After this PR**

```rust
#[cfg(opt_level = $0)]
```

Author: A4-Tacks

crates/ide-completion/src/completions/attribute/cfg.rs

@@ -2,7 +2,7 @@
 
 use ide_db::SymbolKind;
 use itertools::Itertools;
-use syntax::{AstToken, Direction, NodeOrToken, SyntaxKind, algo, ast::Ident};
+use syntax::{AstToken, Direction, NodeOrToken, SmolStr, SyntaxKind, algo, ast::Ident};
 
 use crate::{CompletionItem, completions::Completions, context::CompletionContext};
 
@@ -56,10 +56,15 @@ pub(crate) fn complete_cfg(acc: &mut Completions, ctx: &CompletionContext<'_>) {
         None => ctx
             .krate
             .potential_cfg(ctx.db)
-            .get_cfg_keys()
-            .unique()
-            .map(|s| (s.as_str(), ""))
-            .chain(CFG_CONDITION.iter().copied())
+            .into_iter()
+            .map(|x| match x {
+                hir::CfgAtom::Flag(key) => (key.as_str(), "".into()),
+                hir::CfgAtom::KeyValue { key, .. } => {
+                    (key.as_str(), SmolStr::from_iter([key.as_str(), " = $0"]))
+                }
+            })
+            .chain(CFG_CONDITION.iter().map(|&(k, snip)| (k, SmolStr::new_static(&snip))))
+            .unique_by(|&(s, _)| s)
             .for_each(|(s, snippet)| {
                 let mut item = CompletionItem::new(
                     SymbolKind::BuiltinAttr,

crates/ide-completion/src/tests/attribute.rs

@@ -982,6 +982,34 @@ mod cfg {
         );
     }
 
+    #[test]
+    fn complete_key_attr() {
+        check_edit(
+            "test",
+            r#"
+//- /main.rs cfg:test,dbg=false,opt_level=2
+#[cfg($0)]
+"#,
+            r#"
+#[cfg(test)]
+"#,
+        );
+    }
+
+    #[test]
+    fn complete_key_value_attr() {
+        check_edit(
+            "opt_level",
+            r#"
+//- /main.rs cfg:test,dbg=false,opt_level=2
+#[cfg($0)]
+"#,
+            r#"
+#[cfg(opt_level = $0)]
+"#,
+        );
+    }
+
     #[test]
     fn cfg_target_endian() {
         check(