The mesh can compile Go, which is why nothing delivered the host

novox/hq 04-ISSUES/142, and ADR 0141's own progressive insight naming
this as the first of two things missing: "nothing can compile it". The
toolchain list was a closed set of typescript and python, whose warning —
every language is another implementation of the contracts modules share —
does not attach to Go. Go is how the host, the control plane and the
builder are written, and none of them is a module in that sense: the host
is what APPLIES modules.

The toolchain names mesh-tools-go as its base rather than pinning an
upstream release here (ADR 0142, 0044): named and not pinned means the
mesh answers with the copy it holds, and moving compiler is a build
instead of an edit to this file.

Two things beyond the list also assumed one language, and both would have
failed after the entry was added:

  sourcesFor turned every entrypoint into a `.ts` file. The extension is
  the toolchain's now — one language's file extension written into the
  code that serves every language is a wall the next one hits.

  The output directory was the compiler's to create. tsc --outDir makes
  one; go build -o writes into a directory and does not make it, failing
  with a message about a path rather than about a build. Made here for
  every toolchain, because which compilers are forgiving is not something
  a reader should have to know.

And a toolchain now says what it is pointed at: a file list from the
module's entrypoints, or the one package the artifact is built `from`.
Pointing `go build` at a file list builds a program out of exactly those
files and ignores the rest of the package — a missing symbol rather than a
legible refusal.

Static and -trimpath: what a machine holds is a file, not a container, so
a binary needing a libc it did not bring is a delivery that works until a
machine differs; and a version comes from where a component sits rather
than from its linker, so two builds of one commit are the same bytes.

internal/builder/builder.go

@@ -861,6 +861,15 @@ func compile(ctx context.Context, run Runner, tree string, chain Toolchain,
 	// each other and then be packed together, so each bundle compiles and packs alone.
 	out := Out(a.Name)
 
+	// **The output directory exists before the compiler is told about it.** `tsc --outDir` makes
+	// one; `go build -o` writes a file into a directory and does not create it, failing with a
+	// message about a path rather than about a build. Made here for every toolchain, because which
+	// compilers happen to be forgiving is not a thing a reader should have to know
+	// (novox/hq 04-ISSUES/142).
+	if err := os.MkdirAll(filepath.Join(tree, out), 0o755); err != nil {
+		return "", fmt.Errorf("making the output directory for %s: %w", a.Name, err)
+	}
+
 	invocation := []string{
 		"run", "--rm",
 		"--volume", tree + ":" + within,
@@ -873,8 +882,14 @@ func compile(ctx context.Context, run Runner, tree string, chain Toolchain,
 	}
 	// What to compile. Named by the module rather than discovered, so adding a file does not
 	// silently change what a build produces.
-	if len(a.Entrypoints) > 0 {
-		invocation = append(invocation, sourcesFor(a.Entrypoints, out)...)
+	switch {
+	case chain.Unit == UnitPackage:
+		// One directory, compiled whole: the thing the artifact is built `from`. Relative, because
+		// the compiler runs with the module's own root as its working directory and a package path
+		// that looked absolute would name one inside the toolchain image.
+		invocation = append(invocation, "./"+strings.Trim(a.From, "./"))
+	case len(a.Entrypoints) > 0:
+		invocation = append(invocation, sourcesFor(a.Entrypoints, out, chain.SourceExt)...)
 	}
 	if _, err := run(ctx, tree, "docker", invocation...); err != nil {
 		return "", err
@@ -887,14 +902,16 @@ func compile(ctx context.Context, run Runner, tree string, chain Toolchain,
 // A module names what a tool host should LOAD — compiled paths under the bundle's root — because
 // that is the thing anything else needs to know. What to compile is the same list with the
 // language's own extension, which is the toolchain's business rather than the module's.
-func sourcesFor(entrypoints []string, out string) []string {
+func sourcesFor(entrypoints []string, out, ext string) []string {
 	sources := make([]string, 0, len(entrypoints))
 	for _, e := range entrypoints {
 		// An entrypoint is named as it will be FOUND — a path inside the unpacked bundle — so the
 		// source is the same path with the output directory taken off the front and the language's
-		// own extension on the end.
+		// own extension on the end. **The extension is the toolchain's**, where it used to be the
+		// literal `.ts`: one language's file extension written into the code that serves every
+		// language is a wall the next one hits (novox/hq 04-ISSUES/142).
 		at := strings.TrimPrefix(strings.TrimPrefix(e, out), "/")
-		sources = append(sources, strings.TrimSuffix(at, filepath.Ext(at))+".ts")
+		sources = append(sources, strings.TrimSuffix(at, filepath.Ext(at))+ext)
 	}
 	return sources
 }