Julia v1.13 Release Notes

New language features

New Base.@acquire macro for a non-closure version of Base.acquire(f, s::Base.Semaphore), like @lock. (#56845)
New nth function to access the n-th element of a generic iterable. (#56580)
New @__FUNCTION__ macro to refer to the innermost enclosing function. (#58940)
The character U+1F8B2 🢲 (RIGHTWARDS ARROW WITH LOWER HOOK), newly added by Unicode 16, is now a valid operator with arrow precedence, accessible as \hookunderrightarrow at the REPL. ([JuliaLang/JuliaSyntax.jl#525], #57143)
Support for Unicode 17 (#59534).

mod(x::AbstractFloat, -Inf) now returns x (as long as x is finite), this aligns with C standard and

is considered a bug fix (#47102)

The hash algorithm and its values have changed. Most hash specializations will remain correct and require no action. Types that reimplement the core hashing logic independently, such as some third-party string packages do, may require a migration to the new algorithm. (#57509)
Indexless getindex and setindex! (i.e. A[]) on ReinterpretArray now correctly throw a BoundsError when there is more than one element. (#58814)

The option --sysimage-native-code=no has been deprecated.
The JULIA_CPU_TARGET environment variable now supports a sysimage keyword to match (or extend) the CPU target used to build the current system image (#58970).
The --code-coverage=all option now automatically throws away sysimage caches so that code coverage can be accurately measured on methods within the sysimage. It is thrown away after startup (and after startup.jl), before any user code is executed (#59234)
New --trace-eval command-line option to show expressions being evaluated during top-level evaluation. Supports --trace-eval=loc or just --trace-eval (show location only), --trace-eval=full (show full expressions), and --trace-eval=no (disable tracing). Also adds Base.TRACE_EVAL global control that takes priority over the command-line option and can be set to :no, :loc, :full, or nothing (to use command-line setting). (#57137)

A new AbstractSpinLock is defined with SpinLock <: AbstractSpinLock (#55944).
A new PaddedSpinLock <: AbstractSpinLock is defined. It has extra padding to avoid false sharing (#55944).
New types are defined to handle the pattern of code that must run once per process, called a OncePerProcess{T} type, which allows defining a function that should be run exactly once the first time it is called, and then always return the same result value of type T every subsequent time afterwards. There are also OncePerThread{T} and OncePerTask{T} types for similar usage with threads or tasks. ([#TBD])

ispositive(::Real) and isnegative(::Real) are provided for performance and convenience (#53677).
The Test module now supports the JULIA_TEST_VERBOSE environment variable. When set to true, it enables verbose testset entry/exit messages with timing information and sets the default verbose=true for DefaultTestSet to show detailed hierarchical test summaries (#59295).
Exporting function fieldindex to get the index of a struct's field (#58119).
Base.donotdelete is now public. It prevents deadcode elimination of its arguments (#55774).
Sys.sysimage_target() returns the CPU target string used to build the current system image (#58970).
Iterators.findeach is a lazy version of findall (#54124)

fieldoffset now also accepts the field name as a symbol as fieldtype already did (#58100).
sort(keys(::Dict)) and sort(values(::Dict)) now automatically collect, they previously threw (#56978).
Base.AbstractOneTo is added as a supertype of one-based axes, with Base.OneTo as its subtype (#56902).
takestring!(::IOBuffer) removes the content from the buffer, returning the content as a String.
chopprefix and chopsuffix can now also accept an AbstractChar as the prefix/suffix to remove.
The macroexpand (with default true) and the new macroexpand! (with default false) functions now support a legacyscope boolean keyword argument to control whether to run the legacy scope resolution pass over the result. The legacy scope resolution code has known design bugs and will be disabled by default in a future version. Users should migrate now by calling legacyscope=false or using macroexpand!. This may often require fixes to the code calling macroexpand with Meta.unescape and Meta.reescape or by updating tests to expect hygienic-scope or escape markers might appear in the result.
Base.ScopedValues.LazyScopedValue{T} is introduced for scoped values that compute their default using a OncePerProcess{T} callback, allowing for lazy initialization of the default value. AbstractScopedValue is now the abstract base type for both ScopedValue and LazyScopedValue. (#59372)

randperm! and randcycle! now support non-Array AbstractArray inputs, assuming they are mutable and their indices are one-based (#58596).
shuffle now may take an argument of NTuple value (#56906).

The display of AbstractChars in the main REPL mode now includes LaTeX input information like what is shown in help mode (#58181).
Display of repeated frames and cycles in stack traces has been improved by bracketing them in the trace and treating them consistently (#55841).

Test failures when using the @test macro now show evaluated arguments for all function calls (#57825, #57839).
Transparent test sets (@testset let) now show context when tests error (#58727).
@test_throws now supports a three-argument form @test_throws ExceptionType pattern expr to test both exception type and message pattern in one call (#59117).
The testset stack was changed to use ScopedValue rather than task local storage (#53462).

Introspection utilities such as @code_typed, @which and @edit now accept type annotations as substitutes for values, recognizing forms such as f(1, ::Float64, 3) or even sum(::Vector{T}; init = ::T) where {T<:Real}. Type-annotated variables as in f(val::Int; kw::Float64) are not evaluated if the type annotation provides the necessary information, making this syntax compatible with signatures found in stacktraces (#57909, #58222).
Code introspection macros such as @code_lowered and @code_typed now have a much better support for broadcasting expressions, including broadcasting assignments of the form x .+= f(y) (#58349).