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externals/vcpkg/docs/users/selecting-library-features.md

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+# Selecting Library Features
+
+**The latest version of this documentation is available on [GitHub](https://github.com/Microsoft/vcpkg/tree/master/docs/users/selecting-library-features.md).**
+
+## Installing a library
+
+We will look at [llvm](https://llvm.org/) as an example. You could install it using:
+
+```powershell
+> vcpkg install llvm
+```
+or via a manifest with
+```json
+{
+  "dependencies": ["llvm"]
+}
+```
+
+With llvm now installed, we can execute:
+
+```powershell
+> installed\x86-windows\bin\llc.exe --version
+```
+
+we see:
+
+```powershell
+  Registered Targets:
+    x86    - 32-bit X86: Pentium-Pro and above
+    x86-64 - 64-bit X86: EM64T and AMD64
+```
+
+## Installing additional features
+
+But [llvm supports many more targets](https://llvm.org/docs/GettingStarted.html#local-llvm-configuration), from ARM to SPARC to SystemZ.
+However, clearly our current installation doesn't include ARM as a target;
+thus, we need to learn how vcpkg allows us to install other LLVM targets.
+The llvm port allows this via the "target-*" features.
+
+If we do:
+
+```powershell
+> vcpkg search llvm
+```
+
+We can see:
+
+```
+llvm                 10.0.0#6         The LLVM Compiler Infrastructure
+llvm[clang]                           Build C Language Family Front-end.
+llvm[clang-tools-extra]               Build Clang tools.
+...
+llvm[target-all]                      Build with all backends.
+llvm[target-amdgpu]                   Build with AMDGPU backend.
+llvm[target-arm]                      Build with ARM backend.
+...
+```
+
+We can install any of these targets by using the install-feature syntax:
+
+```powershell
+> vcpkg install llvm[target-arm] # Installs LLVM with the ARM target
+```
+```json
+{
+  "dependencies": [{ "name": "llvm", "features": ["target-arm"] }]
+}
+```
+
+## Opting out of default features
+
+The llvm port includes a few default features that you as a user may not want: for example,
+the `clang` feature is default, which means that `vcpkg install llvm` will also build and install clang.
+If you are writing a compiler that uses LLVM as a backend,
+you're likely not interested in installing clang as well,
+and we can do that by disabling default features with the special `core` "feature":
+```powershell
+> vcpkg install llvm[core,target-arm] # removing the default-feature with "core" also removes all of the default targets you get
+```
+or in manifest files:
+```json
+{
+  "dependencies": [{
+    "name": "llvm",
+    "default-features": false,
+    "features": ["target-arm"]
+  }]
+}
+```
+
+# Further reading
+- The [Feature Packages](../specifications/feature-packages.md) specification was the initial design for features.