[release-branch.go1.20] cmd/go: refuse to build Go 1.22 code

With #60078 accepted, we expect Go 1.22 will have different
for loop semantics than Go 1.20 did. Once Go 1.22 is released,
Go 1.20 will be unsupported, but add a check anyway, just to
help catch some mistakes and usage of old Go toolchains
beyond their end-of-support.

Note that Go 1.20 can keep being used indefinitely with pre-Go 1.22 code.
This change only makes it refuse to build code that says it needs
Go 1.22 semantics, because Go 1.20 does not provide those.

For #60078.

Change-Id: I75118d6fbd0cc08a6bc309aca54c389a255ba7dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/518675
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: rsc

src/cmd/go/internal/work/exec.go

@@ -531,6 +531,19 @@ func (b *Builder) build(ctx context.Context, a *Action) (err error) {
 		return errors.New("binary-only packages are no longer supported")
 	}
 
+	// Go 1.22 is likely to change for loop semantics.
+	// If we try to build code written for Go 1.22,
+	// it may have aliasing bugs that it shouldn't have.
+	// See go.dev/issue/60078.
+	// When Go 1.22 is released, Go 1.20 will no longer
+	// be supported, so no one should ever run into this condition,
+	// but we are adding this check anyway,
+	// just to help catch some mistakes and usage of old
+	// Go toolchains beyond their end-of-support.
+	if p.Module != nil && !(allowedVersion(p.Module.GoVersion) || p.Module.GoVersion == "1.21") {
+		return errors.New("cannot compile Go " + p.Module.GoVersion + " code")
+	}
+
 	if err := b.Mkdir(a.Objdir); err != nil {
 		return err
 	}

src/cmd/go/testdata/script/build_go122.txt

@@ -0,0 +1,24 @@
+! go build
+stderr '^m: cannot compile Go 1.22 code$'
+
+cd dep
+! go build
+stderr '^m: cannot compile Go 1.22 code$'
+
+-- go.mod --
+module m
+go 1.22
+
+-- p.go --
+package p
+
+-- dep/go.mod --
+module dep
+go 1.19
+require m v1.0.0
+replace m v1.0.0 => ../
+
+-- dep/p.go --
+package p
+
+import "m"

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

@@ -8,23 +8,23 @@ go build sub.1
 go build subver.1
 ! stderr 'module requires'
 ! go build badsub.1
-stderr '^note: module requires Go 1.11111$'
+stderr '^note: module requires Go 1.21$'
 
 go build versioned.1
 go mod edit -require [email protected]
 ! go build versioned.1
-stderr '^note: module requires Go 1.99999$'
+stderr '^note: module requires Go 1.21$'
 
 [short] stop
 
 # The message should be printed even if the compiler emits no output.
 go build -o $WORK/nooutput.exe nooutput.go
 ! go build -toolexec=$WORK/nooutput.exe versioned.1
-stderr '^# versioned.1\nnote: module requires Go 1.99999$'
+stderr '^# versioned.1\nnote: module requires Go 1.21$'
 
 -- go.mod --
 module m
-go 1.999
+go 1.21
 require (
 	sub.1 v1.0.0
 	subver.1 v1.0.0
@@ -51,14 +51,14 @@ package x
 
 -- subver/go.mod --
 module m
-go 1.11111
+go 1.21
 
 -- subver/x.go --
 package x
 
 -- badsub/go.mod --
 module m
-go 1.11111
+go 1.21
 
 -- badsub/x.go --
 package x
@@ -73,7 +73,7 @@ package x
 
 -- versioned2/go.mod --
 module versioned
-go 1.99999
+go 1.21
 
 -- versioned2/x.go --
 package x