Changelog

with_seed() now caches the check for R version, so is now faster (#170)
with_makevars() and local_makevars() now eagerly evaluate the path argument (#169)

local_options() now lets you set an option to NULL as intended (#156)
local_tempfile() argument envir is deprecated, in favor of .local_envir. All withr functions except local_tempfile() used .local_envir to specify environments, so this makes this function consistent with the rest. (#157)
with_environment() now passing pos and warn.conflicts to attach(), as intended (#161).
with_seed() now also sets the RNG via new arguments .rng_kind, .rng_normal_kind and .rng_sample_kind (#162, @AshesITR).
with_timezone() now works after recent changes to Sys.timezone() in R-devel (#165)

withr is now licensed as MIT (#154).
Tests for with_cairo_pdf() and with_cairo_ps() have been removed, as they fail if Cairo is not available, such as with M1 macOS systems (#158)
local_seed() is now exported (#153)

Deprecations

local_tempfile() argument new is deprecated, in favor of returning the path to the new tempfile. calls like local_tempfile("xyz") should be replaced with xyx <- local_tempfile() in your code (#141).

New local_seed() function and local_preserve_seed() functions to correspond to with_seed() and with_preserve_seed() (#139).
New local_tempdir() function added to create a temp directory (#140)
local_*() functions now take dots (...), which can simplify calls in some cases, e.g. you can now use local_options(foo = "bar") rather than local_options(c(foo = "bar")).

defer() now throws an error if an error occurs in the deferred expression (#148)
with_file() and local_file() can now work if the file is actually a directory (#144).

defer() can set deferred events on .GlobalEnv to facilitate the interactive development of code inside a function or test. Helpers deferred_run() (and deferred_clear()) provide a way to explicity run and clear (or just clear) deferred events (#76, @jennybc).
with_connection() now works when existing objects or connections exist with the same names (#120)
with_makevars() now uses tools::makevars_user() to determine the default user makevars file (#77, @siddharthab).
with_options() no longer uses do.call(), so optiosn are not evaluated on exit (#73, @mtmorgan).
with_package() no longer has the help argument (#94, @wendtke).
with_package() now does not try to detach the package if it is already attached before calling with_package() (#107)
with_preserve_seed() now restores .Random.seed if it is not set originally (#124).
Add with_rng_version() and local_rng_version() functions to change the version of the RNG (#90, @gaborcsardi).
with_svg() documentation now is consistent across R versions (#129)
Add with_timezone() and local_timezone() functions to change the time zone (#92, @gaborcsardi).
with_tempfile() and local_tempfile() now delete recursively directories on exit (#84, @meta00).

set_makevars() is now exported (#68, @gaborcsardi).
with_temp_libpaths() gains an action argument, to specify how the temporary library path will be added (#66, @krlmlr).

with_connection() function to automatically close R file connections.
with_db_connection() function to automatically disconnect from DBI database connections.
with_gctorture2 command to run code with gctorture2, useful for testing (#47).
with_package(), with_namespace() and with_environment() (and equivalent locals) functions added, to run code with a modified object search path (#38,

#48).
Add with_tempfile() and local_tempfile() functions to create temporary files which are cleanup up afterwards. (#32)
Remove the code argument from local_ functions (#50).

Each with_ function now has a local_ variant, which reset at the end of their local scope, generally at the end of the function body.
New functions with_seed() and with_preserve_seed() for running code with a given random seed (#45, @krlmlr).

with_makevars() gains an assignment argument to allow specifying additional assignment types.