Add ide-assist: convert_to_format_string

Example
---
```rust
fn foo() {
    let n = 2;
    let s = "n: {n$0}";
}
```
->
```rust
fn foo() {
    let n = 2;
    let s = format!("n: {n}");
}
```

Author: A4-Tacks

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

@@ -0,0 +1,363 @@
+use std::iter::once;
+
+use ide_db::syntax_helpers::format_string::is_format_string;
+use syntax::{
+    AstNode, AstToken, NodeOrToken, T,
+    ast::{self, make::tokens, syntax_factory::SyntaxFactory},
+};
+
+use crate::{
+    AssistId,
+    assist_context::{AssistContext, Assists},
+};
+
+// Assist: convert_to_format_string
+//
+// Convert string literal to `format!()`.
+//
+// ```
+// fn foo() {
+//     let n = 2;
+//     let s = "n: {n$0}";
+// }
+// ```
+// ->
+// ```
+// fn foo() {
+//     let n = 2;
+//     let s = format!("n: {n}");
+// }
+// ```
+pub(crate) fn convert_to_format_string(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    let str_token = ctx.find_token_at_offset::<ast::String>()?;
+    let parent = str_token.syntax().parent()?;
+    let text = str_token.syntax().text();
+
+    if ctx
+        .sema
+        .descend_into_macros(str_token.syntax().clone())
+        .into_iter()
+        .filter_map(ast::String::cast)
+        .any(|string| is_format_string(&string))
+    {
+        return None;
+    }
+
+    let offset_in_text = ctx.offset().checked_sub(str_token.syntax().text_range().start())?;
+    let (left, variable, right) = split_curly(text, offset_in_text.into())?;
+    let scope = ctx.sema.scope(&parent)?;
+
+    if !variable.is_empty() && !exist_variable(variable, scope) {
+        return None;
+    }
+
+    acc.add(
+        AssistId::refactor_rewrite("convert_to_format_string"),
+        "Convert to `format!()`",
+        str_token.syntax().text_range(),
+        |builder| {
+            let left = escape_format_string(left);
+            let right = escape_format_string(right);
+
+            let mut edit = builder.make_editor(&parent);
+            let make = SyntaxFactory::with_mappings();
+
+            let new_str = format!("{left}{{{variable}}}{right}");
+            let new_str = tokens::literal(&new_str);
+            let args = once(NodeOrToken::Token(new_str)).chain(
+                variable
+                    .is_empty()
+                    .then(|| {
+                        [
+                            NodeOrToken::Token(make.token(T![,])),
+                            NodeOrToken::Token(make.whitespace(" ")),
+                        ]
+                    })
+                    .into_iter()
+                    .flatten(),
+            );
+            let tt = make.token_tree(T!['('], args);
+            let expr_macro = make.expr_macro(make.ident_path("format"), tt);
+            edit.replace(str_token.syntax(), expr_macro.syntax());
+
+            if variable.is_empty()
+                && let Some(cap) = ctx.config.snippet_cap
+                && let Some(macro_call) = expr_macro.macro_call()
+                && let Some(token_tree) = macro_call.token_tree()
+                && let Some(NodeOrToken::Token(last)) = token_tree.token_trees_and_tokens().last()
+            {
+                let annotation = builder.make_tabstop_before(cap);
+                edit.add_annotation(last, annotation);
+            }
+
+            edit.add_mappings(make.finish_with_mappings());
+            builder.add_file_edits(ctx.vfs_file_id(), edit);
+        },
+    )
+}
+
+fn escape_format_string(s: &str) -> String {
+    let mut replaced = s.replace('{', "{{");
+    stdx::replace(&mut replaced, '}', "}}");
+    replaced
+}
+
+fn exist_variable(variable: &str, scope: hir::SemanticsScope<'_>) -> bool {
+    let mut exist = false;
+    scope.process_all_names(&mut |name, def| {
+        if !matches!(def, hir::ScopeDef::Local(_)) || exist {
+            return;
+        }
+        exist = name.as_str() == variable;
+    });
+    exist
+}
+
+fn split_curly(text: &str, offset: usize) -> Option<(&str, &str, &str)> {
+    let offset = improve_side_offset(text, offset).unwrap_or(offset);
+    let (left, right) = text.split_at_checked(offset)?;
+    let l_curly = left.rfind('{')?;
+    let r_curly = right.find('}')? + left.len();
+
+    Some((&text[..l_curly], &text[l_curly + 1..r_curly], &text[r_curly + 1..]))
+}
+
+fn improve_side_offset(text: &str, offset_in_text: usize) -> Option<usize> {
+    text.get(offset_in_text..)
+        .and_then(|s| (s.chars().next()? == '{').then_some(offset_in_text + 1))
+        .or_else(|| {
+            (text.get(..offset_in_text)?.chars().next_back()? == '}').then_some(offset_in_text - 1)
+        })
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn empty_format() {
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "{$0}";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let s = format!("{}", $0);
+            }
+            "#,
+        );
+
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "left{$0}right";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let s = format!("left{}right", $0);
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn curly_offsets() {
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "left{}$0right";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let s = format!("left{}right", $0);
+            }
+            "#,
+        );
+
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "left$0{}right";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let s = format!("left{}right", $0);
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn biased_curlys() {
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "left{}$0{}right";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let s = format!("left{{}}{}right", $0);
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn not_format_other_curlys() {
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "{left{$0}right}";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let s = format!("{{left{}right}}", $0);
+            }
+            "#,
+        );
+
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "{{}left{{$0}}right{}}";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let s = format!("{{{{}}left{{{}}}right{{}}}}", $0);
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn format_variable() {
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let var = 2;
+                let s = "{left{var$0}right}";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let var = 2;
+                let s = format!("{{left{var}right}}");
+            }
+            "#,
+        );
+
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "{{}left{{$0}}right{}}";
+            }
+            "#,
+            r#"
+            fn foo() {
+                let s = format!("{{{{}}left{{{}}}right{{}}}}", $0);
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn applicable_in_macro() {
+        check_assist(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let var = 2;
+                let s = some_macro!("{left{var$0}right}");
+            }
+            "#,
+            r#"
+            fn foo() {
+                let var = 2;
+                let s = some_macro!(format!("{{left{var}right}}"));
+            }
+            "#,
+        );
+
+        check_assist(
+            convert_to_format_string,
+            r#"
+            //- minicore: fmt
+            fn foo() {
+                let var = 2;
+                let s = print!("{}", "{left{var$0}right}");
+            }
+            "#,
+            r#"
+            fn foo() {
+                let var = 2;
+                let s = print!("{}", format!("{{left{var}right}}"));
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_outside_curly() {
+        check_assist_not_applicable(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "left{}r$0ight";
+            }
+            "#,
+        );
+
+        check_assist_not_applicable(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "l$0eft{}right";
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_unknown_variable() {
+        check_assist_not_applicable(
+            convert_to_format_string,
+            r#"
+            fn foo() {
+                let s = "left{var$0}right";
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_is_format_string() {
+        check_assist_not_applicable(
+            convert_to_format_string,
+            r#"
+            //- minicore: fmt
+            fn foo() {
+                let s = print!("left{$0}right");
+            }
+            "#,
+        );
+    }
+}

crates/ide-assists/src/lib.rs

@@ -132,6 +132,7 @@ mod handlers {
     mod convert_named_struct_to_tuple_struct;
     mod convert_nested_function_to_closure;
     mod convert_range_for_to_while;
+    mod convert_to_format_string;
     mod convert_to_guarded_return;
     mod convert_tuple_return_type_to_struct;
     mod convert_tuple_struct_to_named_struct;
@@ -270,6 +271,7 @@ mod handlers {
             convert_named_struct_to_tuple_struct::convert_named_struct_to_tuple_struct,
             convert_nested_function_to_closure::convert_nested_function_to_closure,
             convert_range_for_to_while::convert_range_for_to_while,
+            convert_to_format_string::convert_to_format_string,
             convert_to_guarded_return::convert_to_guarded_return,
             convert_tuple_return_type_to_struct::convert_tuple_return_type_to_struct,
             convert_tuple_struct_to_named_struct::convert_tuple_struct_to_named_struct,