1.44.0 #

Released on: 4 June, 2020
Branched from master on: 17 April, 2020

Language #

Syntax-only changes

Expansion-driven outline module parsing

#[cfg(FALSE)]
mod foo {
    mod bar {
        mod baz; // `foo/bar/baz.rs` doesn't exist, but no error!
    }
}

These are still rejected semantically, so you will likely receive an error but these changes can be seen and parsed by macros and conditional compilation.

Compiler #

Rustc now respects the -C codegen-units flag in incremental mode. Additionally when in incremental mode rustc defaults to 256 codegen units.
Refactored catch_unwind to have zero-cost, unless unwinding is enabled and a panic is thrown.
Added tier 3* support for the aarch64-unknown-none and aarch64-unknown-none-softfloat targets.
Added tier 3 support for arm64-apple-tvos and x86_64-apple-tvos targets.

Libraries #

Special cased vec![] to map directly to Vec::new(). This allows vec![] to be able to be used in const contexts.
convert::Infallible now implements Hash.
OsString now implements DerefMut and IndexMut returning a &mut OsStr.
Unicode 13 is now supported.
String now implements From<&mut str>.
IoSlice now implements Copy.
Vec<T> now implements From<[T; N]>. Where N is at most 32.
proc_macro::LexError now implements fmt::Display and Error.
from_le_bytes, to_le_bytes, from_be_bytes, to_be_bytes, from_ne_bytes, and to_ne_bytes methods are now const for all integer types.

Stabilized APIs #

Cargo #

Added the cargo tree command which will print a tree graph of your dependencies. E.g.

  mdbook v0.3.2 (/Users/src/rust/mdbook)
├── ammonia v3.0.0
│   ├── html5ever v0.24.0
│   │   ├── log v0.4.8
│   │   │   └── cfg-if v0.1.9
│   │   ├── mac v0.1.1
│   │   └── markup5ever v0.9.0
│   │       ├── log v0.4.8 (*)
│   │       ├── phf v0.7.24
│   │       │   └── phf_shared v0.7.24
│   │       │       ├── siphasher v0.2.3
│   │       │       └── unicase v1.4.2
│   │       │           [build-dependencies]
│   │       │           └── version_check v0.1.5
...

You can also display dependencies on multiple versions of the same crate with cargo tree -d (short for cargo tree --duplicates).

Misc #

Rustdoc now allows you to specify --crate-version to have rustdoc include the version in the sidebar.

Compatibility Notes #

Rustc now correctly generates static libraries on Windows GNU targets with the .a extension, rather than the previous .lib.
Removed the -C no_integrated_as flag from rustc.
The file_name property in JSON output of macro errors now points the actual source file rather than the previous format of <NAME macros>. Note: this may not point to a file that actually exists on the user’s system.
The minimum required external LLVM version has been bumped to LLVM 8.
mem::{zeroed, uninitialised} will now panic when used with types that do not allow zero initialization such as NonZeroU8. This was previously a warning.
In 1.45.0 (the next release) converting a f64 to u32 using the as operator has been defined as a saturating operation. This was previously undefined behaviour, but you can use the {f64, f32}::to_int_unchecked methods to continue using the current behaviour, which may be desirable in rare performance sensitive situations.

Internal Only #

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.