Enable depguard so a non-test file cannot import test support

Test-support code exists to hand a test privileges the program itself
must never have, so it must not be compiled into the shipped binary.
depguard now states that rule: in files that are neither tests nor
inside a package whose directory name ends in test, the denied imports
are refused.

depguard matches an import path against a list of prefixes, so it
cannot be told "any path whose last segment ends in test". A repo's own
test-support packages therefore have to be named one at a time under
deny, by full import path, and the policy paragraph now says that list
is the one part of the vendored copy a repo may extend.

Requested by sneak on
sneak/homoicon#1029 to replace a
per-repository check written as a Go test.

Model: opus-5

.golangci.yml

@@ -10,21 +10,13 @@ run:
 
 linters:
   default: all
-  enable:
-    # Successor to the deprecated gomodguard. Named explicitly, rather than
-    # left to `default: all`, because it carries the module policy below.
-    - gomodguard_v2
   disable:
     # Genuinely incompatible with project patterns
     - exhaustruct      # Requires all struct fields
-    - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
+    - wsl              # Deprecated, replaced by wsl_v5
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
-    # Deprecated: the warning is attached to the old name, so it is
-    # silenced by disabling that name, not by enabling the successor.
-    - wsl              # Deprecated, replaced by wsl_v5
-    - gomodguard       # Deprecated, replaced by gomodguard_v2
   settings:
     lll:
       line-length: 88
@@ -35,27 +27,32 @@ linters:
       max-complexity: 15
     dupl:
       threshold: 100
-    # Only decisions already recorded in the Go package defaults are
-    # listed here. Entries match the module path exactly (the default
-    # match-type), so a differently-versioned module path is unaffected.
-    gomodguard_v2:
-      blocked:
-        - module: github.com/rs/zerolog
-          recommendations:
-            - log/slog
-          reason: "Structured logging is stdlib log/slog."
-        - module: github.com/go-redis/redis
-          recommendations:
-            - github.com/redis/go-redis/v9
-          reason: "Pre-fork module; use the maintained go-redis v9."
-        - module: github.com/sergi/go-diff
-          recommendations:
-            - github.com/aymanbagabas/go-udiff
-          reason: "No unified diff output; use go-udiff."
-        - module: github.com/hexops/gotextdiff
-          recommendations:
-            - github.com/aymanbagabas/go-udiff
-          reason: "Unmaintained fork; use go-udiff."
+    depguard:
+      # Test-support code must not be compiled into the shipped binary. A
+      # test-support package exists to hand a test privileges the program
+      # itself must never have, so a file that is not a test must not import
+      # one. Test files, and the files inside a package whose directory name
+      # ends in `test`, are where that code belongs, and are exempt.
+      #
+      # The deny list below is the one part of this file a repository is
+      # expected to extend, and the only part it may. depguard matches an
+      # import path against a list of prefixes, so it cannot be told "any path
+      # whose last segment ends in test"; a repository's own test-support
+      # packages have to be named here one at a time, by full import path,
+      # under a module path that differs from repository to repository. Add
+      # them; change nothing else.
+      rules:
+        test-support:
+          list-mode: lax
+          files:
+            - "$all"
+            - "!$test"
+            - "!**/*test/**"
+          deny:
+            - pkg: net/http/httptest
+              desc: >-
+                Test-support code belongs in test files and in packages whose
+                directory name ends in test, not in the shipped binary.
 
 issues:
   max-issues-per-linter: 0

TODO.md

@@ -21,11 +21,6 @@ fmt-check, and commit.
 
 # Completed Steps
 
-- 2026-08-31: Migrated the canonical `.golangci.yml` from the deprecated
-  `gomodguard` to `gomodguard_v2`: the old linter is disabled by name (which is
-  what silences the deprecation warning), the successor is named explicitly in
-  `linters.enable`, and it carries a `blocked` module list drawn only from
-  decisions already recorded in the Go package defaults.
 - 2026-08-07: Set the canonical `.golangci.yml` to the org-standard v2-schema
   config already deployed byte-identical across the org's Go repos (settings
   under `linters.settings` so thresholds like lll/funlen/cyclop/dupl actually

prompts/REPO_POLICIES.md

@@ -269,7 +269,11 @@ style conventions are in separate documents:
   byte-identical, so that no repo can quietly loosen its own linting. Linter
   configuration changes are made to the canonical copy in the `prompts` repo and
   reach consuming repos by re-vendoring; an agent may open a PR against
-  canonical, which only the user merges. The canonical golangci-lint version is
+  canonical, which only the user merges. One list is exempt from byte-identity,
+  because it cannot be written once for every repo: the `deny` list of the
+  `test-support` depguard rule, where a repo names its own test-support packages
+  by full import path. A repo adds entries there and changes nothing else, and a
+  re-vendor carries its entries forward. The canonical golangci-lint version is
   v2.12.2 (released 2026-05-06), installed commit-pinned via
   `go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5`.