Version 7.9.0

Breaking changes in dynamic branching

Embrace the vctrs paradigm and its type stability for dynamic branching (#1105, #1106).
Accept target as a symbol by default in read_trace(). Required for the trace to make sense in #1107.

Repair reference to custom HPC resources in the "future" backend (#1083, @jennysjaarda).
Properly copy data when importing targets from one cache into another (#1120, @brendanf).
Prevent dynamic vector sizes from conflicting with file sizes in metadata.

Add a new log_build_times argument to make() and drake_config(). Allows users to disable the recording of build times. Produces a speedup of up to 20% on Macs (#1078).
Implement cache locking to prohibit concurrent calls to make(), outdated(make_imports = TRUE), recoverable(make_imports = TRUE), vis_drake_graph(make_imports = TRUE), clean(), etc. on the same cache.
Add a new format trigger to invalidate targets when the specialized data format changes (#1104, @kendonB).
Add new functions cache_planned() and cache_unplanned() to help selectively clean workflows with dynamic targets (#1110, @kendonB).
Add S3 classes and pretty print methods for drake_config() objects and analyze_code() objects.
Add a new "qs" format (#1121, @kendonB).

Avoid setting seeds for imports (#1086, @adamkski).
Avoid working directly with POSIXct times (#1086, @adamkski)
Avoid excessive calls to %||% (%|||% is faster). (#1089, @billdenney)
Remove %||NA due to slowness (#1089, @billdenney).
Use hash tables to speed up is_dynamic() and is_subtarget() (#1089, @billdenney).
Use getVDigest() instead of digest() (#1089, #1092, eddelbuettel/digest#139 (comment), @eddelbuettel, @billdenney).
Pre-compute backtick and .deparseOpts() to speed up deparse() (#1086, https://stackoverflow.com/users/516548/g-grothendieck, @adamkski).
Pre-compute which targets exist in advance (#1095).
Avoid gratuitous cache interactions and data frame operations in build_times() (#1098).
Use mget_hash() in progress() (#1098).
Get target progress info only once in drake_graph_info() (#1098).
Speed up the retrieval of old metadata in outdated() (#1098).
In make(), avoid checking for nonexistent metadata for missing targets.
Reduce logging in drake_config().

Write a complete project structure in use_drake() (#1097, @lorenzwalthert, @tjmahr).
Add a minor logger note to say how many dynamic sub-targets are registered at a time (#1102, @kendonB).
Handle dependencies that are dynamic targets but not declared as such for the current target (#1107).
Internally, the "layout" data structure is now called the "workflow specification", or "spec" for short. The spec is drake's interpretation of the plan. In the plan, all the dependency relationships among targets and files are implicit. In the spec, they are all explicit. We get from the plan to the spec using static code analysis, e.g. analyze_code().