Add examples of System.Private.CoreLib iterative workflow

docs/workflow/building/libraries/README.md

@@ -176,6 +176,25 @@ dotnet build System.Net.NetworkInformation.csproj /p:TargetOS=Linux
 dotnet build -c Release System.Net.NetworkInformation.csproj
 ```
 
+### Iterating on System.Private.CoreLib changes
+When changing `System.Private.CoreLib` after a full build, in order to test against those changes, you will need an updated `System.Private.CoreLib` in the testhost. In order to achieve that, you can build the `libs.pretest` subset which does testhost setup including copying over `System.Private.CoreLib`.
+
+After doing a build of the runtime:
+
+```
+build.cmd clr -rc Release
+```
+
+You can iterate on `System.Private.CoreLib` by running:
+
+```
+build.cmd clr.corelib+clr.nativecorelib+libs.pretest -rc Release
+```
+
+When this `System.Private.CoreLib` will be built in Release mode, then it will be crossgen'd and we will update the testhost to the latest version of corelib. 
+
+You can use the same workflow for mono runtime by using `mono.corelib+libs.pretest` subsets.
+
 ### Building for Mono
 By default the libraries will attempt to build using the CoreCLR version of `System.Private.CoreLib.dll`. In order to build against the Mono version you need to use the `/p:RuntimeFlavor=Mono` argument.
 

eng/build.ps1

@@ -93,9 +93,12 @@ function Get-Help() {
   Write-Host "* Build Mono runtime for Windows x64 on Release configuration."
   Write-Host ".\build.cmd mono -c release"
   Write-Host ""
-  Write-Host "It's important to mention that to build Mono for the first time,"
-  Write-Host "you need to build the CLR and Libs subsets beforehand."
-  Write-Host "This is done automatically if a full build is performed at first."
+  Write-Host "* Build Release coreclr corelib, crossgen corelib and update Debug libraries testhost to run test on an updated corelib."
+  Write-Host ".\build.cmd clr.corelib+clr.nativecorelib+libs.pretest -rc release"
+  Write-Host ""
+  Write-Host "* Build Debug mono corelib and update Release libraries testhost to run test on an updated corelib."
+  Write-Host ".\build.cmd mono.corelib+libs.pretest -rc debug -c release"
+  Write-Host ""
   Write-Host ""
   Write-Host "For more information, check out https://github.com/dotnet/runtime/blob/master/docs/workflow/README.md"
 }

eng/build.sh

@@ -107,9 +107,12 @@ usage()
   echo "* Build Mono runtime for Linux x64 on Release configuration."
   echo "./build.sh mono -c release"
   echo ""
-  echo "It's important to mention that to build Mono for the first time,"
-  echo "you need to build the CLR and Libs subsets beforehand."
-  echo "This is done automatically if a full build is performed at first."
+  echo "* Build Release coreclr corelib, crossgen corelib and update Debug libraries testhost to run test on an updated corelib."
+  echo "./build.sh clr.corelib+clr.nativecorelib+libs.pretest -rc release"
+  echo ""
+  echo "* Build Debug mono corelib and update Release libraries testhost to run test on an updated corelib."
+  echo "./build.sh mono.corelib+libs.pretest -rc debug -c release"
+  echo ""
   echo ""
   echo "For more general information, check out https://github.com/dotnet/runtime/blob/master/docs/workflow/README.md"
 }