Julia v1.4 Release Notes

New language features

Structs with all isbits and isbitsunion fields are now stored inline in arrays (#32448).
import now allows quoted symbols, e.g. import Base.:+ (#33158).
a[begin] can now be used to address the first element of an integer-indexed collection a. The index is computed by firstindex(a) (#33946).

The syntax (;), which used to parse as an empty block expression, is deprecated. In the future it will indicate an empty named tuple (#30115).

Values can now be interpolated into @async and @spawn via $, which copies the value directly into the constructed underlying closure (#33119).

Windows build installer has switched to Inno Setup. Installer command line parameters have thus changed. For example, to extract the installer to a specific directory, the command line parameter is now /DIR=x:\dirname. Use julia-installer.exe /? to list all new command line parameters.

The new only(x) function returns the one-and-only element of a collection x, and throws an ArgumentError if x contains zero or multiple elements (#33129).
takewhile and dropwhile have been added to the Iterators submodule (#33437).
accumulate has been added to the Iterators submodule (#34033).
There is a now an evalpoly function meant to take the role of the @evalpoly macro. The function is just as efficient as the macro while giving added flexibility, so it should be preferred over @evalpoly. evalpoly takes a list of coefficients as a tuple, so where one might write @evalpoly(x, p1, p2, p3) one would instead write evalpoly(x, (p1, p2, p3)) (#32753).
pkgdir(ModuleName) now provides a simpler way to return the package root directory of a module (or submodule) than the typically used dirname(dirname(pathof(ModuleName))) (#33128).

Function composition now supports multiple functions: ∘(f, g, h) = f ∘ g ∘ h and splatting ∘(fs...) for composing an iterable collection of functions (#33568).
Functions gcd, lcm, and gcdx now support Rational arguments (#33910).
The splitpath function now accepts any AbstractString whereas previously it only accepted paths of type String (#33012).
filter can now act on a Tuple (#32968).
The tempname function now takes an optional parent::AbstractString argument to give it a directory in which to attempt to produce a temporary path name (#33090).
The tempname function now takes a cleanup::Bool keyword argument defaulting to true, which causes the process to try to ensure that any file or directory at the path returned by tempname is deleted upon process exit (#33090).
The readdir function now takes a join::Bool keyword argument defaulting to false, which when set causes readdir to join its directory argument with each listed name (#33113).
div now accepts a rounding mode as the third argument, consistent with the corresponding argument to rem. Support for rounding division, by passing one of the RoundNearest modes to this function, was added. For future compatibility, library authors should now extend this function, rather than extending the two-argument fld/cld/div directly (#33040).
methods now accepts a module (or a list thereof) to filter methods defined in it (#33403).

Calling show or repr on an undef/UndefInitializer() array initializer now shows valid Julia code (#33211).
Calling show or repr on a 0-dimensional AbstractArray now shows valid code for creating an equivalent 0-dimensional array, instead of only showing the contained value (#33206).
readdir output is now guaranteed to be sorted. The sort keyword allows opting out of sorting to get names in OS-native order (#33542).
The methods of mktemp and mktempdir that take a function to pass temporary paths to no longer throw errors if the path is already deleted when the function returns (#33091).
Verbose display of Char (text/plain output) now shows the codepoint value in standard-conforming "U+XXXX" format (#33291).
Iterators.partition now uses views (or smartly re-computed ranges) for partitions of all AbstractArrays (#33533).
Sets are now displayed less compactly in the REPL, as a column of elements, like vectors and dictionaries (#33300).
delete! on WeakKeyDicts now returns the WeakKeyDict itself instead of the underlying Dict used for implementation

qr and qr! functions support blocksize keyword argument (#33053).
dot now admits a 3-argument method dot(x, A, y) to compute generalized dot products dot(x, A*y), but without computing and storing the intermediate result A*y (#32739).
ldlt and non-pivoted lu now throw a new ZeroPivotException type (#33372).
cond(A, p) with p=1 or p=Inf now computes the exact condition number instead of an estimate (#33547).
UniformScaling objects may now be exponentiated such that (a*I)^x = a^x * I.

AbstractRNGs now behave like scalars when used in broadcasting (#33213).
The performance of rand(::Tuple) is improved in some cases (#32208). As a consequence, the stream of generated values produced for a given seed has changed.

The attributes of the implicit IOContext used by the REPL to display objects can be modified by the user (experimental feature) (#29249).

The return value of zero(x::AbstractSparseArray) has no stored zeros anymore (#31835). Previously, it would have stored zeros wherever x had them. This makes the operation constant time instead of O(<number of stored values>).
Products involving sparse arrays now allow more general sparse eltypes, such as StaticArrays (#33205)