[GSoC24] Implement `rewrite_result` for AST nodes

[ding-young]

This issue tracks the necessary tasks to implement rewrite_result for all the nodes that haverewrite implementation.

Background

Current rewrite method returns Option, which means that it gives None without any context when formatting fails. To improve tracking the context of formatting failures, we plan to add a new method rewrite_result in Rewrite trait and gradually replace rewrite with rewrite_result.

Target AST nodes

Below is the list of structs that currently implement the rewrite method. Most of these structs are rustc AST node, but it may be rustfmt representation of AST nodes (ex. Chain). Each task requires both implementing rewrite_result for that node and then modifying rewrite body with calling rewrite_result. Still, rewrite signature will remain the same and the return value of rewrite_result will be squashed into Option until we are ready to propagate Result upwards.

rewrite_*** on the right are the functions that need to be modified to implement rewrite_result for corresponding ast node.
attr.rs

• ast::NestedMetaItem - rewrite_literal refactor rewrite_literal & combine_strs_with_missing_comments #6250
• ast::MetaItem - rewrite_path, rewrite_literal, rewrite_with_parens refactor rewrite_array, pair, tuple, call #6248
• ast::Attribute - rewrite_doc_comment refactor rewrite_literal & combine_strs_with_missing_comments #6250
• [ast::Attribute] - rewrite_(initial)doc_comment refactor rewrite_literal & combine_strs_with_missing_comments #6250

chains.rs

• ChainItem - rewrite_paren (local change), rewrite_method_call, rewrite_comment refactor rewrite_literal & combine_strs_with_missing_comments #6250
• Chain - methods of ChainFormatter trait update rewrite_chain to return RewriteResult #6270

expr.rs

• ast::Expr - tons of rewrite calls impl rewrite_result for ast::Expr #6311
• ast::Block - rewrite_block return RewriteResult for rewrite_block and rewrite_closure #6235
• ControlFlow - rewrite_block_with_visitor, format_expr impl rewrite_result for ControlFlow, Stmt, update rewrite_index #6291

imports.rs

• UseSegment - rewrite_nested_use_tree update rewrite functions in imports.rs #6261
• UseTree - UseSegment update rewrite functions in imports.rs #6261

items.rs

• ast::Local Impl rewrite_result for ast nodes in items.rs #6212
• TraitAliasBound Impl rewrite_result for ForeignItem, TraitAliasBounds, WherePredicate #6309
• ast::FieldDef Impl rewrite_result for ast nodes in items.rs #6212
• OpaqueType
• ast::FnRetTy Impl rewrite_result for ast nodes in items.rs #6212
• ast::Param Impl rewrite_result for ast nodes in items.rs #6212
• ast::ForeignItem Impl rewrite_result for ForeignItem, TraitAliasBounds, WherePredicate #6309

macros.rs

• ast::Item update macro rewrite functions to return RewriteResult #6271
• MacroArg update macro rewrite functions to return RewriteResult #6271
  rewrite_macro

matches.rs

• ArmWrapper - impl rewrite_result for ArmWrapper #6239

overflow.rs

• OverflowableItem - rewrite_result for
  Expr, GenericParam, MacroArg, NestedMetaItem, SegmentParam
  FieldDef, TuplePatField, Ty, Pat impl rewrite_result for ControlFlow, Stmt, update rewrite_index #6291

patterns.rs

• RangeOperand refactor rewrite_array, pair, tuple, call #6248
• (ast) Pat - write_list, rewrite_path, rewrite_unary_prefix impl rewrite_result for Pat, TuplePatField #6262
• (ast) PatField refactor rewrite_literal & combine_strs_with_missing_comments #6250
• TuplePatField impl rewrite_result for Pat, TuplePatField #6262

stmt.rs

• Stmt impl rewrite_result for ControlFlow, Stmt, update rewrite_index #6291

types.rs

• SegmentParam Impl rewrite_result for ast nodes in types.rs #6220
• PreciseCapturingArg return RewriteResult for rewrite_path & rewrite_struct_*** #6236
• ast::AssocItemConstraint return RewriteResult for rewrite_path & rewrite_struct_*** #6236
• ast::AssocConstraintKind Impl rewrite_result for ast nodes in types.rs #6220
• ast::WherePredicate Impl rewrite_result for ForeignItem, TraitAliasBounds, WherePredicate #6309
• ast::GenericArg Impl rewrite_result for ast nodes in types.rs #6220
• ast::AnonConst - format_expr impl rewrite_result for ast::Expr #6311
• ast::Lifetime impl rewrite_result for ControlFlow, Stmt, update rewrite_index #6291
• ast::GenericBound Impl rewrite_result for ast nodes in types.rs #6220
• ast::GenericBounds Impl rewrite_result for ast nodes in types.rs #6220
• ast::GenericParam Impl rewrite_result for ast nodes in types.rs #6220
• ast::PolyTraitRef return RewriteResult for rewrite_path & rewrite_struct_*** #6236
• ast::TraitRef return RewriteResult for rewrite_path & rewrite_struct_*** #6236
• ast:Ty Impl rewrite_result for ast nodes in types.rs #6220

Future Tasks

Future tasks have two directions: one is propagating errors upward, and the other is generating a Result from downward that can potentially cause failure. Corresponding gh issues will be created after the tasks for this issue are done.

Reference

This issue is part of the Google Summer of Code project. For detailed description, see GSoC project page.

[ding-young]

@rustbot claim

[ding-young]

List of rewrite_***

We need to complete refactoring following functions to implement rewrite_result for Expr

• rewrite_array
• rewrite_literal
• rewrite_call
• rewrite_all_pairs
• rewrite_pair
• rewrite_unary_op
• rewrite_struct_lit
• rewrite_tuple
• rewrite_let
• rewrite_block
• rewrite_block_with_visitor
• rewrite_match
• rewrite_path
• rewrite_assignment
• rewrite_unary_prefix
• rewrite_unary_suffix
• rewrite_closure
• rewrite_chain
• rewrite_macro
• rewrite_index
• rewrite_expr_addrof

Notable functions that are widely used

• combine_strs_with_missing_comments
• itemize_list, write_list, Iterator for ListItems
• rewrite_comment