CMake built-ins detection successful when compile_commands.json compiler

path is relative

Under certain conditions*, the compiler param in the command field of
the compile_commands.json is a relative path rather than absolute. When
this happens, the built-ins detection was not successful and the
following exception was thrown:

!ENTRY org.eclipse.cdt.core 4 0 2025-02-23 20:32:10.752
!MESSAGE Error: Cannot run program "gcc": Launching failed
!STACK 0
java.io.IOException: Cannot run program "gcc": Launching failed
	at org.eclipse.cdt.utils.spawner.Spawner.exec(Spawner.java:450)
	at org.eclipse.cdt.utils.spawner.Spawner.<init>(Spawner.java:147)
	at org.eclipse.cdt.utils.spawner.Spawner.<init>(Spawner.java:134)
	at org.eclipse.cdt.utils.spawner.ProcessFactory$Builder.start(ProcessFactory.java:273)
	at org.eclipse.cdt.utils.spawner.ProcessFactory.exec(ProcessFactory.java:366)
	at org.eclipse.cdt.core.CommandLauncher.execute(CommandLauncher.java:189)
	at org.eclipse.cdt.jsoncdb.core.internal.builtins.CompilerBuiltinsDetector.detectBuiltins(CompilerBuiltinsDetector.java:111)
	at org.eclipse.cdt.jsoncdb.core.CompileCommandsJsonParser.detectBuiltins(CompileCommandsJsonParser.java:290)
	at org.eclipse.cdt.jsoncdb.core.CompileCommandsJsonParser.processJsonFile(CompileCommandsJsonParser.java:193)
	at org.eclipse.cdt.jsoncdb.core.CompileCommandsJsonParser.parse(CompileCommandsJsonParser.java:455)
	at org.eclipse.cdt.cmake.core.CMakeBuildConfiguration.processCompileCommandsFile(CMakeBuildConfiguration.java:361)
	at org.eclipse.cdt.cmake.core.CMakeBuildConfiguration.build(CMakeBuildConfiguration.java:241)

This meant that source file includes were not indexed and could not be
opened using Open Declaration (F3) and info markers of the following
type appeared in the Problems view:

  gcc -E -P -dM -Wp,-v
"...extCmakegcc\\build\\cmake.run.win32.x86_64.Local\\detect_compiler_builtins.c"
  Cannot run program "gcc": Launching failed

  Error: Program "gcc" not found in PATH
  PATH=[...]
    extCmakegcc		Unknown	Compiler Builtins Detector Problem
  gcc -E -P -dM -Wp,-v
"...extCmakegcc\\build\\cmake.run.win32.x86_64.Local\\detect_compiler_builtins.c"
  Cannot run program "gcc": Launching failed

This patch fixes the environment issue for Core Build projects, by
calling ICBuildConfiguration.setBuildEnvironment(Map<String, String>) as
part of the built-ins detection setup, thereby supporting absolute and
relative compiler paths.

Addresses Issue: CDT CMake Improvements eclipse-cdt#1000, IDE-82683-REQ-011 Source
code navigation and Built-ins detection

*: CMake produces relative compiler path

When the CMAKE_<LANG>_COMPILER variable (eg CMAKE_C_COMPILER) is set in
the CMakeLists.txt *after* the project() or language commands, it causes
a relative path to be used. For example, in the CMakeLists.txt below,
gcc is set after project() command:

  cmake_minimum_required(VERSION 3.10)
  project (track2)
  set(CMAKE_C_COMPILER gcc)
  add_executable(${PROJECT_NAME} track2.c)

The above script creates a relative compiler path in
compile_commands.json:
  "command": "gcc   -O3 -DNDEBUG -o ...

Normally the CMAKE_C_COMPILER variable should be set before the
project() comannd.

  "command": "C:\\msys64\\mingw64\\bin\\c++.exe -IC...

Author: betamaxbandit

jsoncdb/org.eclipse.cdt.jsoncdb.core/src/org/eclipse/cdt/jsoncdb/core/internal/builtins/CompilerBuiltinsDetector.java

@@ -21,6 +21,8 @@
 import org.eclipse.cdt.core.CCorePlugin;
 import org.eclipse.cdt.core.ConsoleOutputStream;
 import org.eclipse.cdt.core.ICommandLauncher;
+import org.eclipse.cdt.core.build.ICBuildConfiguration;
+import org.eclipse.cdt.core.build.ICBuildConfigurationManager;
 import org.eclipse.cdt.core.resources.IConsole;
 import org.eclipse.cdt.jsoncdb.core.IParserPreferences;
 import org.eclipse.cdt.jsoncdb.core.IParserPreferencesAccess;
@@ -190,13 +192,28 @@ private String[] getEnvp(IProject project) {
 		// e.g. 'C:\msys64\mingw64\bin' must be part of the PATH environment variable. That's why we need PATH here:
 		// Fixes CDT #407
 		try {
-			final String path = "PATH"; //$NON-NLS-1$
-			var variables = CCorePlugin.getDefault().getBuildEnvironmentManager()
-					.getVariables(project.getActiveBuildConfig(), true);
-			for (var variable : variables) {
-				if (path.equalsIgnoreCase(variable.getName())) {
-					map.put(path, variable.getValue());
-					break;
+			final ICBuildConfiguration cBuildConfiguration;
+			var active = project.getActiveBuildConfig();
+			if (active != null) {
+				var manager = CCorePlugin.getService(ICBuildConfigurationManager.class);
+				cBuildConfiguration = manager.getBuildConfiguration(active);
+			} else {
+				cBuildConfiguration = null;
+			}
+
+			if (cBuildConfiguration != null) {
+				// If this project is Core Build, use its specialisation of getBuildEnvironmentManager
+				// to get the environment.
+				cBuildConfiguration.setBuildEnvironment(map);
+			} else {
+				final String path = "PATH"; //$NON-NLS-1$
+				var variables = CCorePlugin.getDefault().getBuildEnvironmentManager()
+						.getVariables(project.getActiveBuildConfig(), true);
+				for (var variable : variables) {
+					if (path.equalsIgnoreCase(variable.getName())) {
+						map.put(path, variable.getValue());
+						break;
+					}
 				}
 			}
 		} catch (CoreException e) {