[host dependencies] add support in the scripts (#16627)

This contains all the docs and scripts changes from #16479,
without any of the ports changes, for easier CR

5 files changed, 93 insertions, 3 deletions

diff --git a/docs/index.md b/docs/index.md
index 492890bce..af8584fb9 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -16,6 +16,7 @@ Vcpkg helps you manage C and C++ libraries on Windows, Linux and MacOS. This too
 - [Configuration and Environment](users/config-environment.md)
 - [Usage with Android](users/android.md)
 - [Using a manifest file to declare your dependencies](users/manifests.md)
+- [Host Dependencies](users/host-dependencies.md)
 
 ### Maintainer help
 
diff --git a/docs/maintainers/vcpkg_common_definitions.md b/docs/maintainers/vcpkg_common_definitions.md
index 5fcb11ab2..a97723c1b 100644
--- a/docs/maintainers/vcpkg_common_definitions.md
+++ b/docs/maintainers/vcpkg_common_definitions.md
@@ -19,6 +19,10 @@ VCPKG_TARGET_IMPORT_LIBRARY_SUFFIX       import library suffix for target (same
 VCPKG_FIND_LIBRARY_PREFIXES              target dependent prefixes used for find_library calls in portfiles
 VCPKG_FIND_LIBRARY_SUFFIXES              target dependent suffixes used for find_library calls in portfiles
 VCPKG_SYSTEM_LIBRARIES                   list of libraries are provide by the toolchain and are not managed by vcpkg
+TARGET_TRIPLET                           the name of the current triplet to build for
+CURRENT_INSTALLED_DIR                    the absolute path to the installed files for the current triplet
+HOST_TRIPLET                             the name of the triplet corresponding to the host
+CURRENT_HOST_INSTALLED_DIR               the absolute path to the installed files for the host triplet
 ```
 
 CMAKE_STATIC_LIBRARY_(PREFIX|SUFFIX), CMAKE_SHARED_LIBRARY_(PREFIX|SUFFIX) and CMAKE_IMPORT_LIBRARY_(PREFIX|SUFFIX) are defined for the target
diff --git a/docs/users/config-environment.md b/docs/users/config-environment.md
index 309a78654..e164bc0e7 100644
--- a/docs/users/config-environment.md
+++ b/docs/users/config-environment.md
@@ -38,11 +38,15 @@ Example: `D:\2017`
 

 This environment variable can be set to a triplet name which will be used for unqualified triplet references in command lines.

 

+#### VCPKG_DEFAULT_HOST_TRIPLET

+

+This environment variable can be set to a triplet name which will be used for unqualified host port references in command lines and all host port references in dependency lists. See [the Tools documentation](tools.md) for more information.

+

 #### VCPKG_OVERLAY_PORTS

 

 This environment variable allows users to override ports with alternate versions according to the

-[ports overlay](../specifications/ports-overlay.md) specification. List paths to overlays using 

-the platform dependent PATH seperator (Windows `;` | others `:`) 

+[ports overlay](../specifications/ports-overlay.md) specification. List paths to overlays using

+the platform dependent PATH seperator (Windows `;` | others `:`)

 

 Example (Windows): `C:\custom-ports\boost;C:\custom-ports\sqlite3`

 

@@ -50,7 +54,7 @@ Example (Windows): `C:\custom-ports\boost;C:\custom-ports\sqlite3`
 

 This environment variable allows users to add directories to search for triplets.

 [Example: overlay triplets](../examples/overlay-triplets-linux-dynamic.md).

-List paths to overlays using the platform dependent PATH seperator (Windows `;`, others `:`) 

+List paths to overlays using the platform dependent PATH seperator (Windows `;`, others `:`)

 

 #### VCPKG_FORCE_SYSTEM_BINARIES

 

diff --git a/docs/users/host-dependencies.md b/docs/users/host-dependencies.md
new file mode 100644
index 000000000..061e21804
--- /dev/null
+++ b/docs/users/host-dependencies.md
@@ -0,0 +1,59 @@
+# Host Dependencies
+
+Tools used at build time by other ports to generate code or implement a custom build system can be packaged inside vcpkg.
+
+## Consuming
+
+When consuming a port as a tool, you must set the dependency's `"host"` field to true. For example:
+```json
+{
+    "name": "contoso-http-library",
+    "version-string": "1.0.0",
+    "description": "Contoso's http runtime library",
+    "dependencies": [
+        "contoso-core-library",
+        {
+            "name": "contoso-code-generator",
+            "host": true
+        },
+        {
+            "name": "contoso-build-system",
+            "host": true
+        }
+    ]
+}
+```
+In this case, the `contoso-code-generator` and `contoso-build-system` (including any transitive dependencies) will be built and installed for the host triplet before `contoso-http-library` is built.
+
+>Note: Consumers must use `vcpkg.json` instead of `CONTROL` as their metadata format. You can easily convert an existing `CONTROL` file using `vcpkg format-manifest /path/to/CONTROL`.
+
+Then, within the portfile of the consumer (`contoso-http-library` in the example), the CMake variable `CURRENT_HOST_INSTALLED_DIR` will be defined to `installed/<host-triplet>` and should be used to locate any required assets. In the example, `contoso-code-generator` might have installed `tools/contoso-code-generator/ccg.exe` which the consumer would add to its local path via
+```cmake
+# ports/contoso-http-library/portfile.cmake
+vcpkg_add_to_path(${CURRENT_HOST_INSTALLED_DIR}/tools/contoso-code-generator)
+```
+
+## Specifying the Host Triplet
+
+The default host triplets are chosen based on the host architecture and operating system, for example `x64-windows`, `x64-linux`, or `x64-osx`. They can be overridden via:
+
+1. In CMake-based manifest mode, calling `set(VCPKG_HOST_TRIPLET "<triplet>" CACHE STRING "")` before the first `project()` directive
+2. In MSBuild-based manifest mode, setting the `VcpkgHostTriplet` property
+3. On the command line, via the flag `--host-triplet=...`
+4. The `VCPKG_DEFAULT_HOST_TRIPLET` environment variable
+
+## Producing
+
+Producing a tool has no special requirements; tools should be authored as a standard port, following all the normal policies and practices. Notably, they should build against `TARGET_TRIPLET`, not `HOST_TRIPLET` within the context of their portfile.
+
+Sometimes, it can be useful to determine whether the current context is a cross-compiling one or not. This should be done by comparing the strings `TARGET_TRIPLET` and `HOST_TRIPLET`. For example:
+
+```cmake
+string(COMPARE EQUAL "${TARGET_TRIPLET}" "${HOST_TRIPLET}" I_AM_NOT_CROSSCOMPILING)
+
+if(TARGET_TRIPLET STREQUAL HOST_TRIPLET)
+    # This is a native build
+else()
+    # This is a cross build
+endif()
+```
diff --git a/docs/users/manifests.md b/docs/users/manifests.md
index e9736e364..2ee64ec3e 100644
--- a/docs/users/manifests.md
+++ b/docs/users/manifests.md
@@ -323,6 +323,20 @@ automatically bootstrapped if missing and invoked to install your dependencies i
 All vcpkg-affecting variables must be defined before the first `project()` directive, such as via the command line or
 `set()` statements.
 
+#### `VCPKG_TARGET_TRIPLET`
+
+This variable controls which triplet dependencies will be installed for.
+
+If unset, vcpkg will automatically detect an appropriate default triplet given the current compiler settings.
+
+#### `VCPKG_HOST_TRIPLET`
+
+This variable controls which triplet host dependencies will be installed for.
+
+If unset, vcpkg will automatically detect an appropriate native triplet (x64-windows, x64-osx, x64-linux).
+
+See also [Host Dependencies](host-dependencies.md).
+
 #### `VCPKG_MANIFEST_MODE`
 
 This variable controls whether vcpkg operates in manifest mode or in classic mode. To disable manifest mode even with a
@@ -443,6 +457,14 @@ This can be set to "false" to explicitly disable vcpkg integration for the proje
 
 This can be set to a custom triplet to use for integration (such as x64-windows-static)
 
+#### `VcpkgHostTriplet` (Host Triplet)
+
+This can be set to a custom triplet to use for resolving host dependencies.
+
+If unset, this will default to the "native" triplet (x64-windows, x64-osx, x64-linux).
+
+See also [Host Dependencies](host-dependencies.md).
+
 #### `VcpkgAdditionalInstallOptions` (Additional Options)
 
 When using a manifest, this option specifies additional command line flags to pass to the underlying vcpkg tool