[diff] IJPL-190026 Don't call LanguageUtil.getLanguageForPsi on EDT

`DiffLangSpecificProvider.findApplicable` is called from EDT, producing a slow operation error in `LanguageUtil.getLanguageForPsi -> ... -> findModuleForFile`.
Now `DiffUserDataKeys.LANGUAGE` is set on BGT and `DiffLangSpecificProvider.findApplicable` relies on it.

GitOrigin-RevId: dfe87819de3f3782ffcfee1174b0ba3d7dc69ea0

platform/diff-impl/api-dump.txt

@@ -545,14 +545,14 @@ com.intellij.diff.lang.DiffIgnoredRangeProvider
 com.intellij.diff.lang.DiffLangSpecificProvider
 - sf:Companion:com.intellij.diff.lang.DiffLangSpecificProvider$Companion
 - sf:EP_NAME:com.intellij.openapi.extensions.ExtensionPointName
-- s:findApplicable(com.intellij.openapi.project.Project,com.intellij.diff.contents.DiffContent,com.intellij.diff.contents.DiffContent):com.intellij.diff.lang.DiffLangSpecificProvider
+- s:findApplicable(com.intellij.diff.contents.DiffContent,com.intellij.diff.contents.DiffContent):com.intellij.diff.lang.DiffLangSpecificProvider
 - a:getDescription():java.lang.String
 - getLineFragments(com.intellij.openapi.project.Project,java.lang.CharSequence,java.lang.CharSequence,com.intellij.diff.tools.util.base.IgnorePolicy,com.intellij.diff.tools.util.base.HighlightPolicy,com.intellij.openapi.progress.ProgressIndicator):java.util.List
 - getPatchedLineFragments(com.intellij.openapi.project.Project,java.util.List,java.lang.CharSequence,java.lang.CharSequence,com.intellij.diff.tools.util.base.IgnorePolicy,com.intellij.diff.tools.util.base.HighlightPolicy,com.intellij.openapi.progress.ProgressIndicator):java.util.List
 - a:getShouldPrecalculateLineFragments():Z
 - a:isApplicable(com.intellij.lang.Language):Z
 f:com.intellij.diff.lang.DiffLangSpecificProvider$Companion
-- f:findApplicable(com.intellij.openapi.project.Project,com.intellij.diff.contents.DiffContent,com.intellij.diff.contents.DiffContent):com.intellij.diff.lang.DiffLangSpecificProvider
+- f:findApplicable(com.intellij.diff.contents.DiffContent,com.intellij.diff.contents.DiffContent):com.intellij.diff.lang.DiffLangSpecificProvider
 a:com.intellij.diff.lang.LangDiffIgnoredRangeProvider
 - com.intellij.diff.lang.DiffIgnoredRangeProvider
 - <init>():V
@@ -560,15 +560,14 @@ a:com.intellij.diff.lang.LangDiffIgnoredRangeProvider
 - pa:accepts(com.intellij.openapi.project.Project,com.intellij.lang.Language):Z
 - pa:computeIgnoredRanges(com.intellij.openapi.project.Project,java.lang.CharSequence,com.intellij.lang.Language):java.util.List
 - getIgnoredRanges(com.intellij.openapi.project.Project,java.lang.CharSequence,com.intellij.diff.contents.DiffContent):java.util.List
-- s:getLanguage(com.intellij.openapi.project.Project,com.intellij.diff.contents.DiffContent):com.intellij.lang.Language
 com.intellij.diff.merge.LangSpecificMergeConflictResolver
 - sf:Companion:com.intellij.diff.merge.LangSpecificMergeConflictResolver$Companion
 - sf:EP_NAME:com.intellij.openapi.extensions.ExtensionPointName
-- s:findApplicable(com.intellij.openapi.project.Project,java.util.List):com.intellij.diff.merge.LangSpecificMergeConflictResolver
+- s:findApplicable(java.util.List):com.intellij.diff.merge.LangSpecificMergeConflictResolver
 - a:isApplicable(com.intellij.lang.Language):Z
 - a:tryResolveMergeConflicts(com.intellij.diff.merge.LangSpecificMergeContext,kotlin.coroutines.Continuation):java.lang.Object
 f:com.intellij.diff.merge.LangSpecificMergeConflictResolver$Companion
-- f:findApplicable(com.intellij.openapi.project.Project,java.util.List):com.intellij.diff.merge.LangSpecificMergeConflictResolver
+- f:findApplicable(java.util.List):com.intellij.diff.merge.LangSpecificMergeConflictResolver
 f:com.intellij.diff.merge.LangSpecificMergeContext
 - <init>(com.intellij.openapi.project.Project,java.util.List,java.util.List,java.util.List):V
 - f:file(com.intellij.diff.util.ThreeSide):com.intellij.psi.PsiFile

platform/diff-impl/src/com/intellij/diff/impl/CacheDiffRequestProcessor.java

@@ -4,6 +4,7 @@ package com.intellij.diff.impl;
 import com.intellij.CommonBundle;
 import com.intellij.diff.chains.DiffRequestProducer;
 import com.intellij.diff.chains.DiffRequestProducerException;
+import com.intellij.diff.lang.DiffLanguage;
 import com.intellij.diff.requests.*;
 import com.intellij.diff.tools.util.SoftHardCacheMap;
 import com.intellij.diff.util.DiffTaskQueue;
@@ -110,6 +111,9 @@ public abstract class CacheDiffRequestProcessor<T> extends DiffRequestProcessor
     myQueue.executeAndTryWait(
       indicator -> {
         final DiffRequest request = doLoadRequest(requestProvider, indicator);
+        if (request instanceof ContentDiffRequest contentDiffRequest) {
+          contentDiffRequest.getContents().forEach(content -> DiffLanguage.computeAndCacheLanguage(content, getProject()));
+        }
         return () -> finishRequestLoading(request, force, scrollToChangePolicy, requestProvider);
       },
       () -> applyRequest(new LoadingDiffRequest(getRequestName(requestProvider)), force, scrollToChangePolicy),

platform/diff-impl/src/com/intellij/diff/lang/DiffLangSpecificProvider.kt

@@ -61,11 +61,9 @@ interface DiffLangSpecificProvider {
     val EP_NAME: ExtensionPointName<DiffLangSpecificProvider> = ExtensionPointName.create<DiffLangSpecificProvider>("com.intellij.diff.lang.DiffLangSpecificAdjuster")
 
     @JvmStatic
-    fun findApplicable(project: Project?, leftContent: DiffContent, rightContent: DiffContent): DiffLangSpecificProvider? {
-      if (project == null) return null
-
-      val leftLanguage = LangDiffIgnoredRangeProvider.getLanguage(project, leftContent)
-      val rightLanguage = LangDiffIgnoredRangeProvider.getLanguage(project, rightContent)
+    fun findApplicable(leftContent: DiffContent, rightContent: DiffContent): DiffLangSpecificProvider? {
+      val leftLanguage = DiffLanguage.getLanguage(leftContent)
+      val rightLanguage = DiffLanguage.getLanguage(rightContent)
 
       if (leftLanguage == null || rightLanguage == null || leftLanguage != rightLanguage) return null
 

platform/diff-impl/src/com/intellij/diff/lang/DiffLanguage.kt

@@ -0,0 +1,58 @@
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package com.intellij.diff.lang
+
+import com.intellij.diff.contents.DiffContent
+import com.intellij.diff.contents.DocumentContent
+import com.intellij.diff.lang.DiffLanguage.getLanguageOrCompute
+import com.intellij.diff.util.DiffUserDataKeys
+import com.intellij.lang.Language
+import com.intellij.lang.LanguageUtil
+import com.intellij.openapi.application.runReadAction
+import com.intellij.openapi.project.Project
+import com.intellij.util.concurrency.annotations.RequiresBackgroundThread
+import org.jetbrains.annotations.CalledInAny
+
+internal object DiffLanguage {
+  /**
+   * Note that unlike [getLanguageOrCompute] this method ignores [com.intellij.psi.LanguageSubstitutors] as their
+   * calculation might require a background thread.
+   *
+   * @return language saved as user data in [content] or language guessed by [content] file type
+   */
+  @CalledInAny
+  fun getLanguage(content: DiffContent): Language? =
+    getLanguageByDataKey(content) ?: getLanguageByFileType(content)
+
+  @JvmStatic
+  @RequiresBackgroundThread
+  fun getLanguageOrCompute(project: Project, content: DiffContent): Language? {
+    val language = getLanguageByDataKey(content)
+    if (language != null) return language
+
+    return computeLanguage(content, project)
+  }
+
+  @JvmStatic
+  @RequiresBackgroundThread
+  fun computeAndCacheLanguage(content: DiffContent, project: Project?) {
+    val language = computeLanguage(content, project)
+    if (language != null) {
+      content.putUserData(DiffUserDataKeys.LANGUAGE, language)
+    }
+  }
+
+  private fun computeLanguage(content: DiffContent, project: Project?): Language? {
+    if (project == null) return getLanguageByFileType(content)
+
+    val fileType = content.getContentType()
+    val languageForPsi = (content as? DocumentContent)?.highlightFile?.let { highlightingFile ->
+      runReadAction { LanguageUtil.getLanguageForPsi(project, highlightingFile, fileType) }
+    }
+
+    return languageForPsi ?: getLanguageByFileType(content)
+  }
+
+  private fun getLanguageByDataKey(content: DiffContent): Language? = content.getUserData(DiffUserDataKeys.LANGUAGE)
+
+  private fun getLanguageByFileType(content: DiffContent): Language? = content.getContentType()?.let(LanguageUtil::getFileTypeLanguage)
+}

platform/diff-impl/src/com/intellij/diff/lang/LangDiffIgnoredRangeProvider.java

@@ -2,15 +2,9 @@
 package com.intellij.diff.lang;
 
 import com.intellij.diff.contents.DiffContent;
-import com.intellij.diff.contents.DocumentContent;
-import com.intellij.diff.util.DiffUserDataKeys;
 import com.intellij.lang.Language;
-import com.intellij.lang.LanguageUtil;
-import com.intellij.openapi.application.ReadAction;
-import com.intellij.openapi.fileTypes.FileType;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.util.TextRange;
-import com.intellij.openapi.vfs.VirtualFile;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
@@ -24,7 +18,7 @@ public abstract class LangDiffIgnoredRangeProvider implements DiffIgnoredRangePr
   @Override
   public final boolean accepts(@Nullable Project project, @NotNull DiffContent content) {
     if (project == null) return false;
-    Language language = getLanguage(project, content);
+    Language language = DiffLanguage.getLanguageOrCompute(project, content);
     if (language == null) return false;
     return accepts(project, language);
   }
@@ -32,21 +26,9 @@ public abstract class LangDiffIgnoredRangeProvider implements DiffIgnoredRangePr
   @Override
   public @NotNull List<TextRange> getIgnoredRanges(@Nullable Project project, @NotNull CharSequence text, @NotNull DiffContent content) {
     assert project != null;
-    Language language = getLanguage(project, content);
+    Language language = DiffLanguage.getLanguageOrCompute(project, content);
     assert language != null;
 
     return computeIgnoredRanges(project, text, language);
   }
-
-  public static @Nullable Language getLanguage(@NotNull Project project, @NotNull DiffContent content) {
-    Language language = content.getUserData(DiffUserDataKeys.LANGUAGE);
-    if (language != null) return language;
-
-    FileType fileType = content.getContentType();
-    VirtualFile file = content instanceof DocumentContent ? ((DocumentContent)content).getHighlightFile() : null;
-    if (file != null) {
-      return ReadAction.compute(() -> LanguageUtil.getLanguageForPsi(project, file, fileType));
-    }
-    return fileType == null ? null : LanguageUtil.getFileTypeLanguage(fileType);
-  }
 }

platform/diff-impl/src/com/intellij/diff/merge/LangSpecificMergeConflictResolver.kt

@@ -2,11 +2,10 @@
 package com.intellij.diff.merge
 
 import com.intellij.diff.contents.DocumentContent
-import com.intellij.diff.lang.LangDiffIgnoredRangeProvider
+import com.intellij.diff.lang.DiffLanguage
 import com.intellij.diff.util.ThreeSide
 import com.intellij.lang.Language
 import com.intellij.openapi.extensions.ExtensionPointName
-import com.intellij.openapi.project.Project
 
 /**
  * An interface for resolving merge conflicts in a way that requires semantic of some language.
@@ -31,10 +30,10 @@ interface LangSpecificMergeConflictResolver {
     val EP_NAME: ExtensionPointName<LangSpecificMergeConflictResolver> = ExtensionPointName.create("com.intellij.diff.merge.conflict.semantic.resolver")
 
     @JvmStatic
-    fun findApplicable(project: Project?, contentList: List<DocumentContent>): LangSpecificMergeConflictResolver? {
-      if (project == null || contentList.size != ThreeSide.entries.size) return null
+    fun findApplicable(contentList: List<DocumentContent>): LangSpecificMergeConflictResolver? {
+      if (contentList.size != ThreeSide.entries.size) return null
 
-      val languageList = contentList.mapNotNull { LangDiffIgnoredRangeProvider.getLanguage(project, it) }
+      val languageList = contentList.mapNotNull { DiffLanguage.getLanguage(it) }
       
       if (languageList.size != ThreeSide.entries.size || languageList.distinct().size != 1) return null
 

platform/diff-impl/src/com/intellij/diff/merge/LangSpecificMergeConflictResolverWrapper.kt

@@ -28,7 +28,7 @@ import org.jetbrains.annotations.ApiStatus
 @ApiStatus.Internal
 class LangSpecificMergeConflictResolverWrapper(private val project: Project?, contentList: List<DocumentContent>) {
   private val resolver: LangSpecificMergeConflictResolver? = if (`is`("semantic.merge.conflict.resolution", false)) {
-    LangSpecificMergeConflictResolver.findApplicable(project, contentList)
+    LangSpecificMergeConflictResolver.findApplicable(contentList)
   }
   else {
     null

platform/diff-impl/src/com/intellij/diff/tools/util/text/SmartTextDiffProvider.java

@@ -49,7 +49,7 @@ public class SmartTextDiffProvider extends TwosideTextDiffProviderBase implement
     DiffContent content1 = Side.LEFT.select(request.getContents());
     DiffContent content2 = Side.RIGHT.select(request.getContents());
     DiffIgnoredRangeProvider ignoredRangeProvider = getIgnoredRangeProvider(project, content1, content2);
-    DiffLangSpecificProvider diffProvider = DiffLangSpecificProvider.findApplicable(project, content1, content2);
+    DiffLangSpecificProvider diffProvider = DiffLangSpecificProvider.findApplicable(content1, content2);
     IgnorePolicy[] ignorePolicies = getIgnorePolicies();
     return new SmartTextDiffProvider(project, content1, content2, settings, rediff, disposable, ignoredRangeProvider, diffProvider,
                                      ignorePolicies);
@@ -63,7 +63,7 @@ public class SmartTextDiffProvider extends TwosideTextDiffProviderBase implement
     DiffContent content1 = Side.LEFT.select(request.getContents());
     DiffContent content2 = Side.RIGHT.select(request.getContents());
     DiffIgnoredRangeProvider ignoredRangeProvider = getIgnoredRangeProvider(project, content1, content2);
-    DiffLangSpecificProvider diffAdjuster = DiffLangSpecificProvider.findApplicable(project, content1, content2);
+    DiffLangSpecificProvider diffAdjuster = DiffLangSpecificProvider.findApplicable(content1, content2);
     return new SmartTextDiffProvider.NoIgnore(project, content1, content2, settings, rediff, disposable, ignoredRangeProvider, diffAdjuster);
   }