Add partial_dependency method to dependencies

This adds a new method, partial_dependency to all dependencies. These
sub dependencies are copies of the original dependency, but with one or
more of the attributes replaced with an empty list. This allows creating
a sub dependency that has only cflags or drops link_arguments, for
example.

2 files changed, 51 insertions, 0 deletions

diff --git a/docs/markdown/Reference-manual.md b/docs/markdown/Reference-manual.md
index 42abe75..74e7ff2 100644
--- a/docs/markdown/Reference-manual.md
+++ b/docs/markdown/Reference-manual.md
@@ -1827,6 +1827,32 @@ an external dependency with the following methods:
 
  - `version()` is the version number as a string, for example `1.2.8`
 
+ - `partial_dependency(compile_args : false, link_args : false, links : false,
+   includes : false, source : false)` (*added 0.46.0) returns a new dependency
+   object with the same name, version, found status, type name, and methods as
+   the object that called it. This new object will only inherit other
+   attributes from its parent as controlled by keyword arguments.
+
+   If the parent has any dependencies, those will be applied to the new
+   partial dependency with the same rules. So , given:
+
+   ```meson
+   dep1 = declare_dependency(compiler_args : '-Werror=foo', link_with : 'libfoo')
+   dep2 = declare_dependency(compiler_args : '-Werror=bar', dependencies : dep1)
+   dep3 = dep2.partial_dependency(compile_args : true)
+   ```
+
+   dep3 will add `['-Werror=foo', '-Werror=bar']` to the compiler args of
+   any target it is added to, but libfoo will not be added to the link_args.
+
+   The following arguments will add the following attributes:
+
+   - compile_args: any arguments passed to the compiler
+   - link_args: any arguments passed to the linker
+   - links: anything passed via link_with or link_whole
+   - includes: any include_directories
+   - sources: any compiled or static sources the dependency has
+
 ### `disabler` object
 
 A disabler object is an object that behaves in much the same way as
diff --git a/docs/markdown/snippets/partial-dependencies.md b/docs/markdown/snippets/partial-dependencies.md
new file mode 100644
index 0000000..8d2136a
--- /dev/null
+++ b/docs/markdown/snippets/partial-dependencies.md
@@ -0,0 +1,25 @@
+## Added new partial_dependency method to dependencies and libraries
+
+It is now possible to use only part of a dependency in a target. This allows,
+for example, to only use headers with convenience libraries to avoid linking
+to the same library multiple times.
+
+```meson
+
+dep = dependency('xcb')
+
+helper = static_library(
+  'helper',
+  ['helper1.c', 'helper2.c'],
+  dependencies : dep.partial_dependency(includes : true),
+]
+
+final = shared_library(
+  'final',
+  ['final.c'],
+  dependencyes : dep,
+)
+```
+
+A partial dependency will have the same name version as the full dependency it
+is derived from, as well as any values requested.