Changed

• Switched from rustc-ap-* crates to rustc_private for consumption model of rustc internals
• annotate-snippets updated to v0.8 PR #4762
• Greatly improved the performance of cargo fmt in large workspaces utilizing the --all flag by updating to a newer version of cargo_metadata that leverages updated cargo output from v1.51+ PR #4997
• Improved formatting of long slice patterns #4530
  • Note you must have version = Two in your configuration to take advantage of the new formatting
• Stabilized match_block_trailing_comma configuration option #3380 - https://rust-lang.github.io/rustfmt/?version=v1.4.38&search=#match_block_trailing_comma
• Stabilized disable_all_formatting configuration option #5026 - https://rust-lang.github.io/rustfmt/?version=v1.4.38&search=#disable_all_formatting
• Various improvements to the configuration documentation website https://rust-lang.github.io/rustfmt/?version=v1.4.38
• Addressed various clippy and rustc warnings

Fixed

• Resolved issue where specious whitespace would be inserted when a block style comment was terminated within string literal processing #4312
• Nested out-of-line mods are again parsed and formatted #4874
• Accepts 2021 for edition value from rustfmt command line PR #4847
• Unstable command line options are no longer displayed in --help text on stable PR #4798
• Stopped panicking on patterns in match arms which start with non-ascii characters #4868
• Stopped stripping defaults on const params #4816
• Fixed issue with dropped content with GAT aliases with self bounds in impls #4911
• Stopped removing generic args on associated type constraints #4943
• Stopped dropping visibility on certain trait and impl items #4960
• Fixed dropping of qualified paths in struct patterns #4908 and #5005
• Fixed bug in line width calculation that was causing specious formatting of certain patterns #4031
  • Note that this bug fix may cause observable formatting changes in cases where code had been formatted with prior versions of rustfmt that contained the bug
• Fixed bug where rustfmt would drop parameter attributes if they were too long in certain cases #4579
• Resolved idempotency issue with extern body elements #4963
• rustfmt will now handle doc-style comments on function parameters, since they could appear with certain macro usage patterns even though it's generally invalid syntax #4936
• Fixed bug in match_block_trailing_comma where commas were not added to the blocks of bodies whose arm had a guard that did not fit on the same line as the pattern #4998
• Fixed bug in cases where derive attributes started with a block style comment #4984
• Fixed issue where the struct rest could be lost when struct_field_align_threshold was enabled #4926
• Handles cases where certain control flow type expressions have comments between patterns/keywords and the pattern ident contains the keyword #5009
• Handles tuple structs that have explicit visibilities and start with a block style comment #5011
• Handles leading line-style comments in certain types of macro calls #4615

Added

• Granular width heuristic options made available for user control PR #4782. This includes the following:
  • array_width
  • attr_fn_like_width
  • chain_width
  • fn_call_width
  • single_line_if_else_max_width
  • struct_lit_width
  • struct_variant_width

Note this hit the rustup distributions prior to the v1.4.38 release as part of an out-of-cycle updates, but is listed in this version because the feature was not in the other v1.4.37 releases. See also the use_small_heuristics section on the configuration site for more information https://rust-lang.github.io/rustfmt/?version=v1.4.38&search=#use_small_heuristics

• New One variant added to imports_granularity configuration option which can be used to reformat all imports into a single use statement #4669
• rustfmt will now skip files that are annotated with @generated at the top of the file #3958
• New configuration option hex_literal_case that allows user to control the casing utilized for hex literals PR #4903

See the section on the configuration site for more information https://rust-lang.github.io/rustfmt/?version=v1.4.38&search=#hex_literal_case

• cargo fmt now directly supports the --check flag, which means it's now possible to run cargo fmt --check instead of the more verbose cargo fmt -- --check #3888