feat: extract_struct_from_function_signature

made the edit actually have a effect by putting the edit_file first
made the parameter in original function lower snake case
made the checking if pre existing struct with new name use the new name
as opposed to the original function name
fixed autogenerated test

Author: mendelsshop

crates/ide-assists/src/handlers/extract_struct_from_function_signature.rs

@@ -1,7 +1,7 @@
 use hir::{Function, ModuleDef};
 use ide_db::{RootDatabase, assists::AssistId, path_transform::PathTransform};
 use itertools::Itertools;
-use stdx::to_camel_case;
+use stdx::{to_camel_case, to_lower_snake_case};
 use syntax::{
     AstNode, SyntaxElement, SyntaxKind, SyntaxNode, T,
     ast::{
@@ -18,16 +18,13 @@ use crate::{AssistContext, Assists};
 // Extracts a struct (part) of the signature of a function.
 //
 // ```
-// fn foo(bar: u32, baz: u32) { ... }
+// fn foo($0bar: u32, baz: u32) { ... }
 // ```
 // ->
 // ```
-// struct FooStruct {
-//      bar: u32,
-//      baz: u32,
-// }
+// struct FooStruct{ bar: u32, baz: u32 }
 //
-// fn foo(FooStruct) { ... }
+// fn foo(foo_struct: FooStruct) { ... }
 // ```
 
 pub(crate) fn extract_struct_from_function_signature(
@@ -42,9 +39,10 @@ pub(crate) fn extract_struct_from_function_signature(
     // go through the fn, the text_range is the whole function.
     let params_list = ctx.find_node_at_offset::<ast::ParamList>()?;
     let fn_name = fn_ast.name()?;
+    let name = make::name(&format!("{}Struct", to_camel_case(fn_name.text_non_mutable())));
 
     let fn_hir = ctx.sema.to_def(&fn_ast)?;
-    if existing_definition(ctx.db(), &fn_name, &fn_hir) {
+    if existing_definition(ctx.db(), &name, &fn_hir) {
         cov_mark::hit!(test_extract_function_signature_not_applicable_if_struct_exists);
         return None;
     }
@@ -69,16 +67,16 @@ pub(crate) fn extract_struct_from_function_signature(
             })
             .collect::<Option<Vec<_>>>()?,
     );
-    let name = make::name(&format!("{}Struct", to_camel_case(fn_name.text_non_mutable())));
     acc.add(
         AssistId::refactor_rewrite("extract_struct_from_function_signature"),
         "Extract struct from signature of a function",
         target,
         |builder| {
             // TODO: update calls to the function
-            let fn_ast = builder.make_mut(fn_ast);
             tracing::info!("extract_struct_from_function_signature: starting edit");
             builder.edit_file(ctx.vfs_file_id());
+            // this has to be after the edit_file (order matters)
+            let fn_ast = builder.make_mut(fn_ast);
             tracing::info!("extract_struct_from_function_signature: editing main file");
 
             let generic_params = fn_ast
@@ -154,7 +152,8 @@ fn update_function(
                 p.pat()
                     .is_some_and(|p| matches!(p, ast::Pat::IdentPat(p) if p.mut_token().is_some()))
             }),
-            name,
+            // TODO: maybe make a method that maps over a name's text
+            make::name(&to_lower_snake_case(name.text_non_mutable())),
         )),
         ty,
     );
@@ -327,3 +326,21 @@ fn existing_definition(db: &RootDatabase, variant_name: &ast::Name, variant: &Fu
         })
         .any(|(name, _)| name.as_str() == variant_name.text().trim_start_matches("r#"))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::tests::check_assist_not_applicable;
+
+    #[test]
+    fn test_extract_function_signature_not_applicable_if_struct_exists() {
+        cov_mark::check!(test_extract_function_signature_not_applicable_if_struct_exists);
+        check_assist_not_applicable(
+            extract_struct_from_function_signature,
+            r#"
+struct OneStruct;
+fn one($0x: u8, y: u32) {}
+"#,
+        );
+    }
+}

crates/ide-assists/src/tests/generated.rs

@@ -1187,6 +1187,21 @@ enum A { One(One) }
     )
 }
 
+#[test]
+fn doctest_extract_struct_from_function_signature() {
+    check_doc_test(
+        "extract_struct_from_function_signature",
+        r#####"
+fn foo($0bar: u32, baz: u32) { ... }
+"#####,
+        r#####"
+struct FooStruct{ bar: u32, baz: u32 }
+
+fn foo(foo_struct: FooStruct) { ... }
+"#####,
+    )
+}
+
 #[test]
 fn doctest_extract_type_alias() {
     check_doc_test(