[dev.cmdgo] cmd/go: add the workspace mode

This change adds the outline of the implementation of the workspace mode.
The go command will now locate go.work files, and read them to determine
which modules are in the workspace. It will then put those modules in
the root of the workspace when building the build list. It supports
building, running, testing, and listing in workspaces. There are still
many TODOs for undone work and other changes to fix certain cases. Some
of these undone parts include: replaces and go.work.sum files, as well
as go mod {test,why,verify}, excludes in workspaces, updating work files
to include module names in comments and setting the GOWORK variable.

For #45713

Change-Id: I72716af7a300a2896087fc8a79c04e951d248278
Reviewed-on: https://go-review.googlesource.com/c/go/+/334934
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index 0ed2389cd5..c51dd398c2 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -121,6 +121,14 @@ and test commands:
 		directory, but it is not accessed. When -modfile is specified, an
 		alternate go.sum file is also used: its path is derived from the
 		-modfile flag by trimming the ".mod" extension and appending ".sum".
+  -workfile file
+    in module aware mode, use the given go.work file as a workspace file.
+		By default or when -workfile is "auto", the go command searches for a
+		file named go.work in the current directory and then containing directories
+		until one is found. If a valid go.work file is found, the modules
+		specified will collectively be used as the main modules. If -workfile
+		is "off", or a go.work file is not found in "auto" mode, workspace
+		mode is disabled.
 	-overlay file
 		read a JSON config file that provides an overlay for build operations.
 		The file is a JSON struct with a single field, named 'Replace', that
@@ -201,6 +209,7 @@ func init() {
 
 	AddBuildFlags(CmdBuild, DefaultBuildFlags)
 	AddBuildFlags(CmdInstall, DefaultBuildFlags)
+	base.AddWorkfileFlag(&CmdBuild.Flag)
 }
 
 // Note that flags consulted by other parts of the code
@@ -364,6 +373,7 @@ var pkgsFilter = func(pkgs []*load.Package) []*load.Package { return pkgs }
 var runtimeVersion = runtime.Version()
 
 func runBuild(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
 	BuildInit()
 	var b Builder
 	b.Init()