fix: improve fn_param adds comma location

Example
---
```rust
fn bar(file_id: usize) {}
fn baz(
    foo: ()
    file$0
) {}
```

**Before this PR**

```rust
fn bar(file_id: usize) {}
fn baz(
    foo: ()
    , file_id: usize
) {}
```

**After this PR**

```rust
fn bar(file_id: usize) {}
fn baz(
    foo: (),
    file_id: usize,
) {}
```

Author: A4-Tacks

crates/ide-completion/src/completions/fn_param.rs

@@ -1,7 +1,7 @@
 //! See [`complete_fn_param`].
 
 use hir::HirDisplay;
-use ide_db::FxHashMap;
+use ide_db::{FxHashMap, text_edit::TextEdit};
 use itertools::Either;
 use syntax::{
     AstNode, Direction, SmolStr, SyntaxKind, TextRange, TextSize, ToSmolStr, algo,
@@ -40,17 +40,17 @@ pub(crate) fn complete_fn_param(
         } else {
             format_smolstr!("{qualifier}{label}")
         };
-        let mk_item = |insert_text: &str, range: TextRange| {
+        let mk_item = |edit, range: TextRange| {
             let mut item =
                 CompletionItem::new(CompletionItemKind::Binding, range, label, ctx.edition);
-            if insert_text != label {
-                item.insert_text(insert_text);
-            }
+            item.text_edit(edit);
             item
         };
         let item = match &comma_wrapper {
-            Some((fmt, range)) => mk_item(&fmt(&insert), *range),
-            None => mk_item(&insert, ctx.source_range()),
+            Some((fmt, range)) => mk_item(fmt(&insert), *range),
+            None => {
+                mk_item(TextEdit::replace(ctx.source_range(), insert.into()), ctx.source_range())
+            }
         };
         // Completion lookup is omitted intentionally here.
         // See the full discussion: https://github.com/rust-lang/rust-analyzer/issues/12073
@@ -196,7 +196,9 @@ fn should_add_self_completions(
     }
 }
 
-fn comma_wrapper(ctx: &CompletionContext<'_, '_>) -> Option<(impl Fn(&str) -> SmolStr, TextRange)> {
+fn comma_wrapper(
+    ctx: &CompletionContext<'_, '_>,
+) -> Option<(impl Fn(&str) -> TextEdit, TextRange)> {
     let param =
         ctx.original_token.parent_ancestors().find(|node| node.kind() == SyntaxKind::PARAM)?;
 
@@ -210,16 +212,45 @@ fn comma_wrapper(ctx: &CompletionContext<'_, '_>) -> Option<(impl Fn(&str) -> Sm
         let t = algo::skip_whitespace_token(t, Direction::Prev)?;
         t.kind()
     };
+    let prev_param = param.prev_sibling().and_then(ast::Param::cast);
 
-    let has_trailing_comma =
-        matches!(next_token_kind, SyntaxKind::COMMA | SyntaxKind::R_PAREN | SyntaxKind::PIPE);
-    let trailing = if has_trailing_comma { "" } else { "," };
+    let needs_comma_before =
+        !matches!(prev_token_kind, SyntaxKind::COMMA | SyntaxKind::L_PAREN | SyntaxKind::PIPE);
+    let needs_comma_after = match next_token_kind {
+        SyntaxKind::COMMA => false,
+        SyntaxKind::R_PAREN | SyntaxKind::PIPE => param
+            .next_sibling_or_token()
+            .and_then(|it| it.into_token())
+            .is_some_and(|it| it.text().contains("\n")),
+        _ => true,
+    };
 
-    let has_leading_comma =
-        matches!(prev_token_kind, SyntaxKind::COMMA | SyntaxKind::L_PAREN | SyntaxKind::PIPE);
-    let leading = if has_leading_comma { "" } else { ", " };
+    let insert_comma_before = prev_param.filter(|_| needs_comma_before).map(|prev_param| {
+        let needs_space_before =
+            prev_param.syntax().next_sibling_or_token().is_none_or(|it| !it.kind().is_trivia());
+        (prev_param.syntax().text_range().end(), if needs_space_before { ", " } else { "," })
+    });
 
-    Some((move |label: &_| format_smolstr!("{leading}{label}{trailing}"), param.text_range()))
+    let needs_space_after = param.next_sibling_or_token().is_none_or(|it| !it.kind().is_trivia());
+    let trailing = match (needs_comma_after, needs_space_after) {
+        (true, true) => ", ",
+        (true, false) => ",",
+        (false, _) => "",
+    };
+    let range = param.text_range();
+
+    Some((
+        move |label: &_| {
+            let insert_text = format!("{label}{trailing}");
+            let mut edit = TextEdit::builder();
+            if let Some((offset, comma)) = insert_comma_before {
+                edit.insert(offset, comma.to_owned());
+            }
+            edit.replace(range, insert_text);
+            edit.finish()
+        },
+        range,
+    ))
 }
 
 fn is_simple_param(param: &ast::Param) -> bool {

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

@@ -32,6 +32,46 @@ fn baz(foo: (), file$0) {}
 fn foo(file_id: usize) {}
 fn bar(file_id: usize) {}
 fn baz(foo: (), file_id: usize) {}
+"#,
+    );
+
+    check_edit(
+        "file_id: usize",
+        r#"
+fn foo(file_id: usize) {}
+fn bar(file_id: usize) {}
+fn baz(
+    foo: ()
+    file$0
+) {}
+"#,
+        r#"
+fn foo(file_id: usize) {}
+fn bar(file_id: usize) {}
+fn baz(
+    foo: (),
+    file_id: usize,
+) {}
+"#,
+    );
+
+    check_edit(
+        "file_id: usize",
+        r#"
+fn foo(file_id: usize) {}
+fn bar(file_id: usize) {}
+fn baz(
+    foo: (),
+    file$0
+) {}
+"#,
+        r#"
+fn foo(file_id: usize) {}
+fn bar(file_id: usize) {}
+fn baz(
+    foo: (),
+    file_id: usize,
+) {}
 "#,
     );
 }
@@ -49,6 +89,26 @@ fn baz(file$0 id: u32) {}
 fn foo(file_id: usize) {}
 fn bar(file_id: usize) {}
 fn baz(file_id: usize, id: u32) {}
+"#,
+    );
+
+    check_edit(
+        "file_id: usize",
+        r#"
+fn foo(file_id: usize) {}
+fn bar(file_id: usize) {}
+fn baz(
+    file$0
+    id: u32
+) {}
+"#,
+        r#"
+fn foo(file_id: usize) {}
+fn bar(file_id: usize) {}
+fn baz(
+    file_id: usize,
+    id: u32
+) {}
 "#,
     );
 }