Add PIP_BUILD_CONSTRAINT support for pip 26.2+ compatibility

[Copilot]

pip 26.2+ will enforce that PIP_CONSTRAINT no longer affects build constraints. The deprecation warning recommends using PIP_BUILD_CONSTRAINT for build-time constraints.

Changes

• build_env.py: Updated _create_constraints_file() to read PIP_BUILD_CONSTRAINT environment variable first, falling back to PIP_CONSTRAINT for backward compatibility
• config.py:
  • Added pip_build_constraint to BUILD_KEY_TO_VAR mapping to recognize the environment variable
  • Added default computed value for pip_build_constraint that defaults to $(PIP_CONSTRAINT) for environments where it's not explicitly set
• builder.py: Set both PIP_CONSTRAINT and PIP_BUILD_CONSTRAINT to the constraints file path when building packages
• test_build_env.py: Added comprehensive tests to verify PIP_BUILD_CONSTRAINT functionality:
  • Test that PIP_BUILD_CONSTRAINT is used when explicitly set
  • Test that default values from xbuildenv are used when no override is provided
  • Test that PIP_BUILD_CONSTRAINT takes precedence over PIP_CONSTRAINT when both are set
• CHANGELOG.md: Documented the new feature for pip 26.2+ compatibility

# builder.py - sets both environment variables
constraints_file = str(self._create_constraints_file())
build_env["PIP_CONSTRAINT"] = constraints_file
build_env["PIP_BUILD_CONSTRAINT"] = constraints_file

# build_env.py - prioritizes PIP_BUILD_CONSTRAINT with fallback
def _create_constraints_file() -> str:
    # PIP_BUILD_CONSTRAINT takes precedence; fall back to PIP_CONSTRAINT for backward compatibility
    try:
        constraints = get_build_flag("PIP_BUILD_CONSTRAINT")
    except ValueError:
        try:
            constraints = get_build_flag("PIP_CONSTRAINT")
        except ValueError:
            return ""
    ...

Behavior

• If PIP_BUILD_CONSTRAINT is set in the environment, it will be used for build constraints
• If not set, it defaults to PIP_CONSTRAINT value for backward compatibility
• Both variables are set in the build environment to ensure pip uses them correctly
• No binary xbuildenv file changes required

Testing

• ✅ All constraint-related tests pass (18 tests)
• ✅ All new tests for PIP_BUILD_CONSTRAINT functionality pass (3 tests)
• ✅ No linting issues
• ✅ Code review passed
• ✅ Security checks passed

This ensures compatibility with pip 26.2+ which will enforce the separation of build constraints from install constraints while maintaining full backward compatibility.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>PIP_CONSTRAINT deprecated</issue_title>
<issue_description> DEPRECATION: Setting PIP_CONSTRAINT will not affect build constraints in the future, pip 26.2 will enforce this behaviour change. A possible replacement is to specify build constraints using --build-constraint or PIP_BUILD_CONSTRAINT. To disable this warning without any build constraints set --use-feature=build-constraint or PIP_USE_FEATURE="build-constraint".

</issue_description>

<agent_instructions>We are setting PIP_CONSTRAINT. Update to also set PIP_BUILD_CONSTRAINT to handle it correctly.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes PIP_CONSTRAINT deprecated #291

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[ryanking13]

@copilot Do not change the binary xbuildenv file

[Copilot]

@copilot Do not change the binary xbuildenv file

Reverted the binary xbuildenv file and removed pip_build_constraint from the config mapping. PIP_BUILD_CONSTRAINT is now set directly in the build environment without needing config system changes. See commit bf63a45.

[ryanking13]

@copilot

Update _create_constraints_file to read the env variable PIP_BUILD_CONSTRAINT. And also update the config.py to handle PIP_BUILD_CONSTRAINT env variable.

[Copilot]

@copilot

Update _create_constraints_file to read the env variable PIP_BUILD_CONSTRAINT. And also update the config.py to handle PIP_BUILD_CONSTRAINT env variable.

Updated _create_constraints_file in build_env.py to read PIP_BUILD_CONSTRAINT first, falling back to PIP_CONSTRAINT for backward compatibility. Added pip_build_constraint to config.py BUILD_KEY_TO_VAR mapping and DEFAULT_CONFIG_COMPUTED with a default value of $(PIP_CONSTRAINT). See commits 8c4088d and b301349.

[ryanking13]

@copilot update test and changelog

[Copilot]

@copilot update test and changelog

Added three comprehensive tests for PIP_BUILD_CONSTRAINT functionality in test_build_env.py and updated CHANGELOG.md to document the new feature. See commits 1afa9ee and 02f4e82.