Theme suggestions may not be in theme include files, Search and Views UI suggestions are not (always) found

Thanks for this! For the docs piece *.theme as well :)

For review

Patch doesn't apply anymore. Assigning it to me for a reroll.

Plain reroll.

Re-rolling patch for 8.3.x

+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -618,6 +618,9 @@ function hook_preprocess_HOOK(&$variables) {
+ * Implementations of this hook must be placed in *.module files.
+ * Or make sure that hook implementation available at any given time.
+ *

@@ -699,6 +702,9 @@ function hook_theme_suggestions_alter(array &$suggestions, array $variables, $ho
+ * Implementations of this hook must be placed in *.module files.
+ * Or make sure that hook implementation available at any given time.
+ *

These hooks are also available in themes, see

    // Invoke hook_theme_suggestions_alter() and
    // hook_theme_suggestions_HOOK_alter().
    $hooks = [
      'theme_suggestions',
      'theme_suggestions_' . $base_theme_hook,
    ];
    $this->moduleHandler->alter($hooks, $suggestions, $variables, $base_theme_hook);
    $this->alter($hooks, $suggestions, $variables, $base_theme_hook);

given that I'm wondering whether someone could misread this documentation. It says it has to be placed in .module files, maybe change it to "must be placed in *.module or *.theme files".

Rerolled patch 13 against current 8.3 branch w/ comment change to add "*.module or *.theme files"

Coding Standard Resolved "Short array syntax"

Ok for me.

Thanks for working on this and for providing better documentation so we don't make this mistake again in the future.

+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -613,6 +613,9 @@ function hook_preprocess_HOOK(&$variables) {
+ * Implementations of this hook must be placed in *.module or *.theme files.
+ * Or make sure that hook implementation available at any given time.

@@ -694,6 +697,9 @@ function hook_theme_suggestions_alter(array &$suggestions, array $variables, $ho
+ * Implementations of this hook must be placed in *.module or *.theme files.

There's a small grammar error here -- "or" should not start a new sentence because it's a conjunction. We can fix this by simply joining the sentences and rewording it a little:
Implementations of this hook must be placed in *.module or *.theme files, or othewise ensure that the hook implementation is always available.

Attaching another patch with xjm suggestion and re-rolling for 8.4.x

+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -613,6 +613,9 @@ function hook_preprocess_HOOK(&$variables) {
+ * Implementations of this hook must be placed in *.module or *.theme files, or
+ * make sure that hook implementation available at any given time.

This still needs some work on the grammar. "...or must otherwise make sure that the hook implementation is available at any given time."

Thanks!

#23 works are fixed

there were 2 comments with that (#26) correction

This should be fine now. Thanks.

Looks good now, thanks!

Committed and pushed to 8.5.x.

I briefly considered not backporting it to 8.4.x -- while the file location of procedural code is not considered API (https://www.drupal.org/core/d8-bc-policy#code-locations) we have accidentally disrupted things in the past by moving code between files (so we avoid changes like that in patch releases. However, since these are hook implementations, the .module file that provides them will always be available when the module is enabled -- and if the module is not enabled, the extending code would have to both be relying on a procedural code location and calling a hook implementation directly. That's two very unlikely things, neither of which are supported. So fixing the bug is definitely worth not protecting such a scenario.

So, long story short, I backported this to 8.4.x as well.

Thanks everyone!

Edit: Forgot about the issue crediting crosspost, so reverting and recommitting with the correct issue credit.

There we go, added credit for substantive reviews. :)