Merge #7894

7894: generate_function assist: convert arg names to lower snake case r=Veykril a=JoshMcguigan

This PR fixes one of the points listed by @TimoFreiberg in #3639. Specifically that all generated argument names should be converted to lower snake case. 

```rust
struct BazBaz;

fn foo() {
    bar$0(BazBaz);
    // ^ when triggering the assist here, you get the output below
}

// BEFORE
fn bar(BazBaz: BazBaz) ${0:-> ()} {
    todo!()
}

// AFTER
fn bar(baz_baz: BazBaz) ${0:-> ()} {
    todo!()
}
```

Co-authored-by: Josh Mcguigan <joshmcg88@gmail.com>

Author: bors[bot]

crates/ide_assists/src/handlers/generate_function.rs

@@ -1,6 +1,7 @@
 use hir::HirDisplay;
 use ide_db::{base_db::FileId, helpers::SnippetCap};
 use rustc_hash::{FxHashMap, FxHashSet};
+use stdx::to_lower_snake_case;
 use syntax::{
     ast::{
         self,
@@ -257,14 +258,15 @@ fn deduplicate_arg_names(arg_names: &mut Vec<String>) {
 fn fn_arg_name(fn_arg: &ast::Expr) -> Option<String> {
     match fn_arg {
         ast::Expr::CastExpr(cast_expr) => fn_arg_name(&cast_expr.expr()?),
-        _ => Some(
-            fn_arg
+        _ => {
+            let s = fn_arg
                 .syntax()
                 .descendants()
                 .filter(|d| ast::NameRef::can_cast(d.kind()))
                 .last()?
-                .to_string(),
-        ),
+                .to_string();
+            Some(to_lower_snake_case(&s))
+        }
     }
 }
 
@@ -447,6 +449,52 @@ mod baz {
         )
     }
 
+    #[test]
+    fn add_function_with_upper_camel_case_arg() {
+        check_assist(
+            generate_function,
+            r"
+struct BazBaz;
+fn foo() {
+    bar$0(BazBaz);
+}
+",
+            r"
+struct BazBaz;
+fn foo() {
+    bar(BazBaz);
+}
+
+fn bar(baz_baz: BazBaz) ${0:-> ()} {
+    todo!()
+}
+",
+        );
+    }
+
+    #[test]
+    fn add_function_with_upper_camel_case_arg_as_cast() {
+        check_assist(
+            generate_function,
+            r"
+struct BazBaz;
+fn foo() {
+    bar$0(&BazBaz as *const BazBaz);
+}
+",
+            r"
+struct BazBaz;
+fn foo() {
+    bar(&BazBaz as *const BazBaz);
+}
+
+fn bar(baz_baz: *const BazBaz) ${0:-> ()} {
+    todo!()
+}
+",
+        );
+    }
+
     #[test]
     fn add_function_with_function_call_arg() {
         check_assist(