[WIP] Port TypeScript PR #59767: Rewrite relative import extensions with flag

internal/checker/checker.go

@@ -14497,6 +14497,36 @@ func (c *Checker) resolveExternalModule(location *ast.Node, moduleReference stri
 						tsExtension,
 					)
 				}
+			} else if c.compilerOptions.RewriteRelativeImportExtensions.IsTrue() &&
+				location.Flags&ast.NodeFlagsAmbient == 0 &&
+				!tspath.IsDeclarationFileName(moduleReference) &&
+				!ast.IsLiteralImportTypeNode(location) &&
+				!ast.IsPartOfTypeOnlyImportOrExportDeclaration(location) {
+				shouldRewrite := tspath.ShouldRewriteModuleSpecifier(moduleReference, c.compilerOptions.RewriteRelativeImportExtensions.IsTrue())
+				if !resolvedModule.ResolvedUsingTsExtension && shouldRewrite {
+					// Get relative path from current source file to resolved file
+					sourceFileDir := tspath.GetDirectoryPath(importingSourceFile.FileName())
+					targetPath := resolvedModule.ResolvedFileName
+					canonicalSourceDir := tspath.GetCanonicalFileName(sourceFileDir, !c.program.UseCaseSensitiveFileNames())
+					canonicalTargetPath := tspath.GetCanonicalFileName(targetPath, !c.program.UseCaseSensitiveFileNames())
+					relativePath := tspath.GetRelativePathFromDirectory(canonicalSourceDir, canonicalTargetPath, tspath.ComparePathsOptions{
+						CurrentDirectory:          c.program.GetCurrentDirectory(),
+						UseCaseSensitiveFileNames: c.program.UseCaseSensitiveFileNames(),
+					})
+					c.error(
+						errorNode,
+						diagnostics.This_relative_import_path_is_unsafe_to_rewrite_because_it_looks_like_a_file_name_but_actually_resolves_to_0,
+						relativePath,
+					)
+				} else if resolvedModule.ResolvedUsingTsExtension && !shouldRewrite {
+					// For now, we'll skip the sourceFileMayBeEmitted check to keep it simple
+					c.error(
+						errorNode,
+						diagnostics.This_import_uses_a_0_extension_to_resolve_to_an_input_TypeScript_file_but_will_not_be_rewritten_during_emit_because_it_is_not_a_relative_path,
+						tspath.GetAnyExtensionFromPath(moduleReference, nil, !c.program.UseCaseSensitiveFileNames()),
+					)
+				}
+				// TODO: Add project reference validation for the third error case
 			}
 		}
 

internal/transformers/utilities.go

@@ -336,7 +336,7 @@ func tryRenameExternalModule(factory *printer.NodeFactory, moduleName *ast.Liter
 }
 
 func rewriteModuleSpecifier(emitContext *printer.EmitContext, node *ast.Expression, compilerOptions *core.CompilerOptions) *ast.Expression {
-	if node == nil || !ast.IsStringLiteral(node) || !shouldRewriteModuleSpecifier(node.Text(), compilerOptions) {
+	if node == nil || !ast.IsStringLiteral(node) || !tspath.ShouldRewriteModuleSpecifier(node.Text(), compilerOptions.RewriteRelativeImportExtensions.IsTrue()) {
 		return node
 	}
 	updatedText := tspath.ChangeExtension(node.Text(), outputpaths.GetOutputExtension(node.Text(), compilerOptions.Jsx))
@@ -350,10 +350,6 @@ func rewriteModuleSpecifier(emitContext *printer.EmitContext, node *ast.Expressi
 	return node
 }
 
-func shouldRewriteModuleSpecifier(specifier string, compilerOptions *core.CompilerOptions) bool {
-	return compilerOptions.RewriteRelativeImportExtensions.IsTrue() && tspath.PathIsRelative(specifier) && !tspath.IsDeclarationFileName(specifier) && tspath.HasTSFileExtension(specifier)
-}
-
 func singleOrMany(nodes []*ast.Node, factory *printer.NodeFactory) *ast.Node {
 	if len(nodes) == 1 {
 		return nodes[0]

internal/tsoptions/declscompiler.go

@@ -810,8 +810,8 @@ var commonOptionsWithBuild = []*CommandLineOption{
 		AffectsSemanticDiagnostics: true,
 		AffectsBuildInfo:           true,
 		Category:                   diagnostics.Modules,
-		// description: diagnostics.Rewrite_ts_tsx_mts_and_cts_file_extensions_in_relative_import_paths_to_their_JavaScript_equivalent_in_output_files,
-		DefaultValueDescription: false,
+		Description:                diagnostics.Rewrite_ts_tsx_mts_and_cts_file_extensions_in_relative_import_paths_to_their_JavaScript_equivalent_in_output_files,
+		DefaultValueDescription:    false,
 	},
 	{
 		Name:                    "resolvePackageJsonExports",

internal/tspath/path.go

@@ -902,3 +902,7 @@ func ForEachAncestorDirectoryPath[T any](directory Path, callback func(directory
 func HasExtension(fileName string) bool {
 	return strings.Contains(GetBaseFileName(fileName), ".")
 }
+
+func ShouldRewriteModuleSpecifier(specifier string, rewriteRelativeImportExtensions bool) bool {
+	return rewriteRelativeImportExtensions && PathIsRelative(specifier) && !IsDeclarationFileName(specifier)
+}