Move header files from include to src.

We have a semi-working C++ "interface" that's not really an interface. Some files are
impossible to be used without hacking inside XGBoost.

This PR hides the following files to reduce confusion and to reduce the need for broken
forward declarations:

- learner.h
- gbm.h
- cache.h
- predictor.h

Aside from automated headers sorting, I did not make much change, to make sure git
recognize this PR is doing a simple rename.

Author: trivialfis

plugin/sycl/predictor/predictor.cc

@@ -1,5 +1,5 @@
 /*!
- * Copyright by Contributors 2017-2025
+ * Copyright by Contributors 2017-2026
  */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wtautological-constant-compare"
@@ -17,7 +17,7 @@
 #include "dmlc/registry.h"
 
 #include "xgboost/tree_model.h"
-#include "xgboost/predictor.h"
+#include "../../../src/predictor/predictor.h"
 #include "xgboost/tree_updater.h"
 #include "../../../src/common/timer.h"
 

src/c_api/c_api.cc

@@ -1,5 +1,5 @@
 /**
- * Copyright 2014-2025, XGBoost Contributors
+ * Copyright 2014-2026, XGBoost Contributors
  */
 #include "xgboost/c_api.h"
 
@@ -29,6 +29,7 @@
 #include "../data/proxy_dmatrix.h"       // for DMatrixProxy
 #include "../data/simple_dmatrix.h"      // for SimpleDMatrix
 #include "../encoder/types.h"            // for Overloaded
+#include "../learner.h"                  // for Learner, PredictionType
 #include "c_api_error.h"                 // for xgboost_CHECK_C_ARG_PTR, API_END, API_BEGIN
 #include "c_api_utils.h"                 // for RequiredArg, OptionalArg, GetMissing, CastDM...
 #include "dmlc/base.h"                   // for BeginPtr
@@ -42,9 +43,7 @@
 #include "xgboost/global_config.h"       // for GlobalConfiguration, GlobalConfigThreadLocal...
 #include "xgboost/host_device_vector.h"  // for HostDeviceVector
 #include "xgboost/json.h"                // for Json, get, Integer, IsA, Boolean, String
-#include "xgboost/learner.h"             // for Learner, PredictionType
 #include "xgboost/logging.h"             // for LOG_FATAL, LogMessageFatal, CHECK, LogCheck_EQ
-#include "xgboost/predictor.h"           // for PredictionCacheEntry
 #include "xgboost/span.h"                // for Span
 #include "xgboost/string_view.h"         // for StringView, operator<<
 #include "xgboost/version_config.h"      // for XGBOOST_VER_MAJOR, XGBOOST_VER_MINOR, XGBOOS...

src/c_api/c_api.cu

@@ -1,5 +1,5 @@
 /**
- * Copyright 2019-2025, XGBoost Contributors
+ * Copyright 2019-2026, XGBoost Contributors
  */
 #include <thrust/transform.h>  // for transform
 
@@ -13,7 +13,7 @@
 #include "xgboost/c_api.h"
 #include "xgboost/data.h"
 #include "xgboost/json.h"
-#include "xgboost/learner.h"
+#include "../learner.h"  // for Learner
 #if defined(XGBOOST_USE_NCCL)
 #include <nccl.h>
 #endif  // defined(XGBOOST_USE_NCCL)

src/c_api/c_api_utils.h

@@ -1,5 +1,5 @@
 /**
- * Copyright 2021-2024, XGBoost Contributors
+ * Copyright 2021-2026, XGBoost Contributors
  */
 #ifndef XGBOOST_C_API_C_API_UTILS_H_
 #define XGBOOST_C_API_C_API_UTILS_H_
@@ -19,7 +19,7 @@
 #include "xgboost/data.h"         // DMatrix
 #include "xgboost/feature_map.h"  // for FeatureMap
 #include "xgboost/json.h"
-#include "xgboost/learner.h"
+#include "../learner.h"  // for Learner
 #include "xgboost/linalg.h"  // ArrayInterfaceHandler, MakeTensorView, ArrayInterfaceStr
 #include "xgboost/logging.h"
 #include "xgboost/string_view.h"  // StringView

src/common/api_entry.h

@@ -1,17 +1,18 @@
 /**
- * Copyright 2016-2023 by XGBoost contributors
+ * Copyright 2016-2026, XGBoost contributors
  */
 #ifndef XGBOOST_COMMON_API_ENTRY_H_
 #define XGBOOST_COMMON_API_ENTRY_H_
-#include <string>               // std::string
-#include <vector>               // std::vector
 
-#include "xgboost/base.h"       // GradientPair,bst_ulong
-#include "xgboost/predictor.h"  // PredictionCacheEntry
+#include <string>  // for string
+#include <vector>  // for vector
+
+#include "../predictor/predictor.h"  // PredictionCacheEntry
+#include "xgboost/base.h"            // GradientPair,bst_ulong
 
 namespace xgboost {
 /**
- * \brief entry to to easily hold returning information
+ * @brief entry to to easily hold returning information
  */
 struct XGBAPIThreadLocalEntry {
   /*! \brief result holder for returning string */

include/xgboost/cache.h src/common/cache.hinclude/xgboost/cache.h renamed to src/common/cache.h

@@ -1,9 +1,7 @@
 /**
- * Copyright 2023 by XGBoost contributors
+ * Copyright 2023-2026, XGBoost contributors
  */
-#ifndef XGBOOST_CACHE_H_
-#define XGBOOST_CACHE_H_
-
+#pragma once
 #include <xgboost/logging.h>  // for CHECK_EQ, CHECK
 
 #include <cstddef>            // for size_t
@@ -201,4 +199,3 @@ class DMatrixCache {
   }
 };
 }  // namespace xgboost
-#endif  // XGBOOST_CACHE_H_

src/data/data.cc

@@ -1,5 +1,5 @@
 /**
- * Copyright 2015-2025, XGBoost Contributors
+ * Copyright 2015-2026, XGBoost Contributors
  * \file data.cc
  */
 #include "xgboost/data.h"
@@ -52,7 +52,7 @@
 #include "xgboost/base.h"                     // for bst_group_t, bst_idx_t, bst_float, bst_ulong
 #include "xgboost/context.h"                  // for Context
 #include "xgboost/host_device_vector.h"       // for HostDeviceVector
-#include "xgboost/learner.h"                  // for HostDeviceVector
+#include "../learner.h"                       // for Learner
 #include "xgboost/linalg.h"                   // for Tensor, Stack, TensorView, Vector, ArrayInte...
 #include "xgboost/logging.h"                  // for Error, LogCheck_EQ, CHECK, CHECK_EQ, LOG
 #include "xgboost/span.h"                     // for Span, operator!=, SpanIterator

src/gbm/gblinear.cc

@@ -1,5 +1,5 @@
 /**
- * Copyright 2014-2025, XGBoost Contributors
+ * Copyright 2014-2026, XGBoost Contributors
  * \file gblinear.cc
  * \brief Implementation of Linear booster, with L1/L2 regularization: Elastic Net
  *        the update rule is parallel coordinate descent (shotgun)
@@ -17,13 +17,13 @@
 #include "../common/threading_utils.h"
 #include "../common/timer.h"
 #include "gblinear_model.h"
-#include "xgboost/gbm.h"
+#include "gbm.h"  // for GradientContainer
 #include "xgboost/json.h"
-#include "xgboost/learner.h"
+#include "../learner.h"  // for LearnerModelParam
 #include "xgboost/linalg.h"
 #include "xgboost/linear_updater.h"
 #include "xgboost/logging.h"
-#include "xgboost/predictor.h"
+#include "../predictor/predictor.h"  // for Predictor
 
 namespace xgboost::gbm {
 DMLC_REGISTRY_FILE_TAG(gblinear);

src/gbm/gblinear_model.h

@@ -1,10 +1,10 @@
 /**
- * Copyright 2018-2025, XGBoost Contributors
+ * Copyright 2018-2026, XGBoost Contributors
  */
 #pragma once
 #include <dmlc/io.h>
 #include <dmlc/parameter.h>
-#include <xgboost/learner.h>
+#include "../learner.h"  // for LearnerModelParam
 
 #include <vector>
 #include <string>

src/gbm/gbm.cc

@@ -1,16 +1,16 @@
 /**
- * Copyright 2015-2025, XGBoost Contributors
+ * Copyright 2015-2026, XGBoost Contributors
  * \file gbm.cc
  * \brief Registry of gradient boosters.
  */
-#include "xgboost/gbm.h"
+#include "gbm.h"
 
 #include <dmlc/registry.h>
 
 #include <string>
 
 #include "xgboost/context.h"
-#include "xgboost/learner.h"
+#include "../learner.h"  // for LearnerModelParam
 
 namespace dmlc {
 DMLC_REGISTRY_ENABLE(::xgboost::GradientBoosterReg);