cmd/go: refuse to run when the main module or workspace needs a newer Go

We already refuse to build code in modules are too new (CL 476279).
This is a more comprehensive check: refuse to do anything at all with
modules or workspaces that are too new.

Since the module or workspace is new, it may have semantics we don't
understand and misinterpret well before we get to the actual building of code.
For example when we switched from // +build to //go:build that changed
the decision about which files go into a package, which affects the way
the overall load phase runs and which errors it reports. Waiting until the
building of code would miss earlier changes like that one.

Leaving the test from CL 476279 alone, but it's not load-bearing anymore.

For #57001.

Change-Id: I8c39943db1d7ddbcb9b5cae68d80459fddd68151
Reviewed-on: https://go-review.googlesource.com/c/go/+/497435
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Auto-Submit: Russ Cox <[email protected]>

Author: rsc

src/cmd/go/internal/modload/init.go

@@ -698,6 +698,9 @@ func LoadModFile(ctx context.Context) *Requirements {
 		if err != nil {
 			base.Fatalf("reading go.work: %v", err)
 		}
+		if gover.Compare(workFileGoVersion, gover.Local()) > 0 {
+			base.Fatalf("go: %s requires go %v (running go %v)", base.ShortPath(workFilePath), workFileGoVersion, gover.Local())
+		}
 		for _, modRoot := range modRoots {
 			sumFile := strings.TrimSuffix(modFilePath(modRoot), ".mod") + ".sum"
 			modfetch.WorkspaceGoSumFiles = append(modfetch.WorkspaceGoSumFiles, sumFile)
@@ -760,6 +763,9 @@ func LoadModFile(ctx context.Context) *Requirements {
 				base.Fatalf("go: %v", err)
 			}
 		}
+		if f.Go != nil && gover.Compare(f.Go.Version, gover.Local()) > 0 {
+			base.Fatalf("go: %s requires go %v (running go %v)", base.ShortPath(gomod), f.Go.Version, gover.Local())
+		}
 
 		modFiles = append(modFiles, f)
 		mainModule := f.Module.Mod

src/cmd/go/internal/modload/vendor.go

@@ -43,7 +43,8 @@ func readVendorList(mainModule module.Version) {
 		vendorPkgModule = make(map[string]module.Version)
 		vendorVersion = make(map[string]string)
 		vendorMeta = make(map[module.Version]vendorMetadata)
-		data, err := os.ReadFile(filepath.Join(MainModules.ModRoot(mainModule), "vendor/modules.txt"))
+		vendorFile := filepath.Join(MainModules.ModRoot(mainModule), "vendor/modules.txt")
+		data, err := os.ReadFile(vendorFile)
 		if err != nil {
 			if !errors.Is(err, fs.ErrNotExist) {
 				base.Fatalf("go: %s", err)
@@ -110,6 +111,9 @@ func readVendorList(mainModule module.Version) {
 					if goVersion, ok := strings.CutPrefix(entry, "go "); ok {
 						meta.GoVersion = goVersion
 						rawGoVersion.Store(mod, meta.GoVersion)
+						if gover.Compare(goVersion, gover.Local()) > 0 {
+							base.Fatalf("go: %s in %s requires go %v (running go %v)", mod.Path, base.ShortPath(vendorFile), goVersion, gover.Local())
+						}
 					}
 					// All other tokens are reserved for future use.
 				}

src/cmd/go/testdata/script/mod_goline_too_new.txt

@@ -0,0 +1,47 @@
+# Go should refuse to build code that is too new according to go.mod.
+
+# go.mod too new
+env GOTOOLCHAIN=local
+! go build .
+stderr '^go: go.mod requires go 1.99999 \(running go 1\..+\)$'
+
+# go.mod referenced from go.work too new
+cp go.work.old go.work
+! go build .
+stderr '^go: go.mod requires go 1.99999 \(running go 1\..+\)$'
+
+# go.work too new
+cp go.work.new go.work
+cp go.mod.old go.mod
+! go build .
+stderr '^go: go.work requires go 1.99999 \(running go 1\..+\)$'
+
+# vendor too new
+rm go.work
+mv notvendor vendor
+! go build -mod=vendor .
+stderr '^go: golang.org/x/text in vendor'${/}'modules.txt requires go 1.99999 \(running go 1\..+\)$'
+
+-- go.mod --
+module example
+go 1.99999
+
+-- p.go --
+package p
+
+-- go.mod.old --
+module example
+go 1.10
+
+-- go.work.new --
+go 1.99999
+use .
+
+-- go.work.old --
+go 1.10
+use .
+
+-- notvendor/modules.txt --
+# golang.org/x/text v0.9.0
+## explicit; go 1.99999
+golang.org/x/text/internal/language