TPE: Use helper functions to build the return value

[luxas]

Description of changes

Broken out from #2162, prefactoring.
This PR makes it clear that the type information is not actually used to make any decisions (yet) during TPE (ref: #2092).
This should decrease a bit of the cognitive complexity when reading the source code too, hopefully.
In #2162, it will become convenient to add logic in these helper functions, e.g. preserving the Loc of the residual, or adding a Loc to the error type.

Issue #, if available

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)

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• Does not update the CHANGELOG because my change does not significantly impact released code.

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• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

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• Does not require updates because my change does not impact the Cedar language specification.

[luxas]

the VS Code diff is nicer in that it shows that principal.into() stayed intact here, but Github does not, even in hide whitespace mode

[github-actions]

Coverage Report

Head Commit: 41ecf77d7ef77626d983eea9b61268630caeea8d

Base Commit: 16bb13698aaffc4e6dd08458584516456d3dfebb

Download the full coverage report.

Coverage of Added or Modified Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 79.61%

Status: FAILED ❌

Details

File	Status	Covered	Coverage	Missed Lines
cedar-policy-core/src/tpe/evaluator.rs	🟡	121/152	79.61%	78, 123, 182, 243, 263, 276, 290, 295, 319, 335, 357, 365, 368, 371, 382, 385, 388, 391, 394, 410, 413, 418, 424, 430, 434, 455, 479, 504, 509, 541, 546

Coverage of All Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 87.60%

Status: PASSED ✅

Details

Package	Status	Covered	Coverage	Base Coverage
cedar-language-server	🟢	4722/5102	92.55%	--
cedar-policy	🟡	4227/5355	78.94%	--
cedar-policy-cli	🟡	1210/1614	74.97%	--
cedar-policy-core	🟢	23821/27090	87.93%	--
cedar-policy-formatter	🟢	914/1088	84.01%	--
cedar-policy-symcc	🟢	6752/7262	92.98%	--
cedar-wasm	🔴	0/28	0.00%	--

[john-h-kastner-aws]

You've obviously not written any new uncovered code here. But, if you're in the mood, I think some of the uncovered lines being reported should be easy enough to reach with a test case. (some others are type errors which we don't expect to be reachable)

[john-h-kastner-aws]

looks great

[luxas]

You've obviously not written any new uncovered code here. But, if you're in the mood, I think some of the uncovered lines being reported should be easy enough to reach with a test case. (some others are type errors which we don't expect to be reachable)

Yeah, I'll look at this in some of the coming PRs to see if I can make the coverage checker happier 😄