refactor!: typed discriminators for InferenceError

Six of fifteen variants carried a `&'static str` discriminator, about
thirty magic strings between them, and the only thing a caller could do
with one was print it. Four new enums replace them:

    Parameter        13 variants, replacing 9 strings in InvalidParameter
    Shape             4 variants, replacing 10 in MismatchedShape
    OutcomeKind       2 variants, replacing WrongOutcomeKind's three fields
    CompetitorField   2 variants, replacing ConflictingCompetitorConfig's

`InvalidProbability` folds into `InvalidParameter` as
`Parameter::PDraw`. It was a bespoke variant for one scalar while every
other scalar shared `InvalidParameter`, and it omitted the parameter
name — so the same parameter had two mechanisms.

`JointUnavailable { reason: &'static str }` splits into `EmptyHistory`,
`JointRequiresScoredEvents` and `NotPositiveDefinite`. The three are
conditions a caller branches on differently — add events, use
`predict_win_probabilities`, or reconsider the priors — and telling them
apart used to mean string-matching English. One test already proved the
distinction was load-bearing: the blanket conversion mapped the
empty-history case onto the ranked one and `an_empty_history_has_no_joint`
caught it immediately.

`NonFiniteResult` splits into `NonFiniteStep { context, step }` and
`NonFiniteSkill { mu, sigma }`. One `step: (f64, f64)` field was
carrying a sweep step from `converge` and a skill's own moments from a
prediction — two situations in one variant, and a field name that could
only be right for one of them.

`InvalidParameter { name: "beta with point-mass skills" }` becomes
`NoPerformanceVariance`. It was never a parameter out of range: both
values are individually valid and it is their combination that leaves
nothing varying.

Three `Display` impls did not meet the standard the others set, and the
typed data is what makes fixing them possible:

    before  drift variance is invalid: NaN
    after   drift variance must be finite and non-negative (got NaN)

    before  kinds: expected length 3, got 2
    after   the outcome describes a different number of teams than the
            event has: expected 3, got 2

    before  Game::ranked: expected Outcome::Ranked, got Outcome::Scored
    after   expected Outcome::Ranked, got Outcome::Scored; call
            Game::scored for a scored outcome

`Parameter::range()` states each parameter's actual bounds, which no
`&'static str` name could have. `error::message_tests` renders every one
and asserts each is a sentence rather than a label, and that the three
above now carry a range or a next step.

The four internal `MismatchedShape` kinds — `results`, `times`, `kinds`,
and the weights array — collapse to `Shape::Internal`, whose `Display`
says plainly that reaching it is a bug in this crate. They are checks on
`add_events_with_prior`'s own parallel arrays and are unreachable
through the public API; they stay checked rather than becoming
`debug_assert!`s, because release is where this crate's defects hide.

Closes #74.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

tests/competitor_config.rs

@@ -184,7 +184,10 @@ fn a_batch_declaring_two_different_priors_is_rejected() {
     assert!(
         matches!(
             err,
-            InferenceError::ConflictingCompetitorConfig { field: "prior", .. }
+            InferenceError::ConflictingCompetitorConfig {
+                field: trueskill_tt::CompetitorField::Prior,
+                ..
+            }
         ),
         "got {err:?}"
     );

tests/degenerate_inputs.rs

@@ -157,7 +157,7 @@ fn event_builder_rejects_a_weights_length_mismatch() {
         matches!(
             err,
             InferenceError::MismatchedShape {
-                kind: "weights",
+                shape: trueskill_tt::Shape::Weights,
                 expected: 1,
                 got: 2,
                 ..
@@ -228,7 +228,7 @@ fn scored_event_rejects_non_positive_sigma() {
     assert!(matches!(
         err,
         InferenceError::InvalidParameter {
-            name: "score_sigma",
+            parameter: trueskill_tt::Parameter::ScoreSigma,
             ..
         }
     ));

tests/drift_scale.rs

@@ -279,7 +279,7 @@ fn reject(scale: f64) -> InferenceError {
 fn negative_scale_is_rejected() {
     assert!(matches!(
         reject(-1.0),
-        InferenceError::InvalidParameter { name: "drift_scale", value, .. }
+        InferenceError::InvalidParameter { parameter: trueskill_tt::Parameter::DriftScale, value, .. }
             if value == -1.0
     ));
 }
@@ -291,7 +291,7 @@ fn non_finite_scale_is_rejected() {
             matches!(
                 reject(scale),
                 InferenceError::InvalidParameter {
-                    name: "drift_scale",
+                    parameter: trueskill_tt::Parameter::DriftScale,
                     ..
                 }
             ),
@@ -488,7 +488,7 @@ fn a_batch_that_contradicts_itself_is_rejected() {
         matches!(
             err,
             InferenceError::ConflictingCompetitorConfig {
-                field: "drift_scale",
+                field: trueskill_tt::CompetitorField::DriftScale,
                 ..
             }
         ),

tests/event_builder_members.rs

@@ -136,7 +136,7 @@ fn weights_still_guards_a_members_team() {
         matches!(
             err,
             InferenceError::MismatchedShape {
-                kind: "weights",
+                shape: trueskill_tt::Shape::Weights,
                 expected: 2,
                 got: 1,
                 ..
@@ -164,7 +164,7 @@ fn an_invalid_drift_scale_surfaces_from_commit() {
             matches!(
                 err,
                 InferenceError::InvalidParameter {
-                    name: "drift_scale",
+                    parameter: trueskill_tt::Parameter::DriftScale,
                     ..
                 }
             ),

tests/game.rs

@@ -57,7 +57,7 @@ fn game_ranked_rejects_bad_p_draw() {
         },
     )
     .unwrap_err();
-    assert!(matches!(err, InferenceError::InvalidProbability { .. }));
+    assert!(matches!(err, InferenceError::InvalidParameter { .. }));
 }
 
 #[test]
@@ -227,7 +227,13 @@ mod malformed_games {
             )
             .unwrap_err();
             assert!(
-                matches!(err, InferenceError::InvalidParameter { name: "score", .. }),
+                matches!(
+                    err,
+                    InferenceError::InvalidParameter {
+                        parameter: trueskill_tt::Parameter::Score,
+                        ..
+                    }
+                ),
                 "{bad}: {err:?}"
             );
         }

tests/ingestion_shape.rs

@@ -112,7 +112,13 @@ fn a_non_finite_score_is_rejected_at_ingestion() {
             }])
             .unwrap_err();
         assert!(
-            matches!(err, InferenceError::InvalidParameter { name: "score", .. }),
+            matches!(
+                err,
+                InferenceError::InvalidParameter {
+                    parameter: trueskill_tt::Parameter::Score,
+                    ..
+                }
+            ),
             "{bad}: {err:?}"
         );
         assert!(h.current_skill(&"a").is_none(), "{bad} was recorded anyway");
@@ -136,7 +142,13 @@ fn a_non_finite_weight_is_rejected_at_ingestion() {
             .commit()
             .unwrap_err();
         assert!(
-            matches!(err, InferenceError::InvalidParameter { name: "weight", .. }),
+            matches!(
+                err,
+                InferenceError::InvalidParameter {
+                    parameter: trueskill_tt::Parameter::Weight,
+                    ..
+                }
+            ),
             "{bad}: {err:?}"
         );
         assert!(h.current_skill(&"a").is_none(), "{bad} reached the history");

tests/joint_handle.rs

@@ -231,16 +231,20 @@ fn a_ranked_history_has_no_exact_joint() {
     let _ = h.converge().unwrap();
     assert!(matches!(
         h.joint().unwrap_err(),
-        InferenceError::JointUnavailable { .. }
+        InferenceError::JointRequiresScoredEvents
     ));
 }
 
+/// Distinguishable from the ranked case, which is the point of splitting
+/// `JointUnavailable { reason: &str }` into three variants (#74): "add events"
+/// and "use predict_win_probabilities" are different instructions, and telling
+/// them apart used to mean matching on English prose.
 #[test]
 fn an_empty_history_has_no_joint() {
     let h = history(UnknownKeys::Reject);
     assert!(matches!(
         h.joint().unwrap_err(),
-        InferenceError::JointUnavailable { .. }
+        InferenceError::EmptyHistory
     ));
 }
 

tests/non_finite_results.rs

@@ -56,7 +56,7 @@ fn overflow_during_inference_is_reported_not_hidden() {
 
     for (name, sigma, beta, score_sigma, scores) in cases {
         match scored_fit(sigma, beta, score_sigma, scores) {
-            Err(InferenceError::NonFiniteResult { context, step, .. }) => {
+            Err(InferenceError::NonFiniteStep { context, step, .. }) => {
                 assert_eq!(context, "History::converge", "{name}");
                 assert!(
                     !step.0.is_finite() || !step.1.is_finite(),
@@ -86,7 +86,7 @@ fn a_broken_fit_is_never_reported_as_converged() {
 
     let err = h.converge().unwrap_err();
     assert!(
-        matches!(err, InferenceError::NonFiniteResult { .. }),
+        matches!(err, InferenceError::NonFiniteStep { .. }),
         "a breakdown must not be reported as convergence: {err:?}"
     );
 
@@ -104,7 +104,7 @@ fn a_broken_fit_is_never_reported_as_converged() {
     .unwrap();
     assert!(matches!(
         h2.converge_partial().unwrap_err(),
-        InferenceError::NonFiniteResult { .. }
+        InferenceError::NonFiniteStep { .. }
     ));
 }
 
@@ -161,7 +161,7 @@ fn a_nan_competitor_is_not_masked_by_a_healthy_one() {
         .converge()
         .expect_err("a NaN fit must never be reported as converged");
     assert!(
-        matches!(err, InferenceError::NonFiniteResult { .. }),
+        matches!(err, InferenceError::NonFiniteStep { .. }),
         "{err:?}"
     );
 }

tests/prediction_guards.rs

@@ -49,7 +49,7 @@ fn nan_poisoned() -> H {
     );
     let err = h.converge().expect_err("this fixture must not converge");
     assert!(
-        matches!(err, InferenceError::NonFiniteResult { .. }),
+        matches!(err, InferenceError::NonFiniteStep { .. }),
         "{err:?}"
     );
     h
@@ -101,7 +101,7 @@ fn a_nan_poisoned_fit_is_refused_by_every_prediction_path() {
 
     all_predictions!(h, |name: &str, r: Result<(), InferenceError>| {
         match r {
-            Err(InferenceError::NonFiniteResult { .. }) => {}
+            Err(InferenceError::NonFiniteSkill { .. }) => {}
             other => panic!("{name} answered from a NaN fit: {other:?}"),
         }
     });
@@ -121,7 +121,7 @@ fn degenerate_performances_are_refused_rather_than_answered_wrongly() {
     // and every skill is a point mass.
     all_predictions!(h, |name: &str, r: Result<(), InferenceError>| {
         match r {
-            Err(InferenceError::InvalidParameter { .. }) => {}
+            Err(InferenceError::NoPerformanceVariance) => {}
             other => panic!("{name} predicted from a degenerate fit: {other:?}"),
         }
     });

tests/registration.rs

@@ -172,7 +172,13 @@ fn a_weight_on_a_registration_is_rejected() {
         .register(Member::new("layout").with_weight(0.5))
         .unwrap_err();
     assert!(
-        matches!(err, InferenceError::InvalidParameter { name: "weight", .. }),
+        matches!(
+            err,
+            InferenceError::InvalidParameter {
+                parameter: trueskill_tt::Parameter::Weight,
+                ..
+            }
+        ),
         "{err:?}"
     );
 }
@@ -188,7 +194,7 @@ fn an_invalid_drift_scale_on_a_registration_is_rejected() {
             matches!(
                 err,
                 InferenceError::InvalidParameter {
-                    name: "drift_scale",
+                    parameter: trueskill_tt::Parameter::DriftScale,
                     ..
                 }
             ),
@@ -280,7 +286,7 @@ mod conflicting_configuration {
             matches!(
                 err,
                 InferenceError::ConflictingCompetitorConfig {
-                    field: "drift_scale",
+                    field: trueskill_tt::CompetitorField::DriftScale,
                     ..
                 }
             ),
@@ -297,7 +303,7 @@ mod conflicting_configuration {
             matches!(
                 err,
                 InferenceError::ConflictingCompetitorConfig {
-                    field: "drift_scale",
+                    field: trueskill_tt::CompetitorField::DriftScale,
                     ..
                 }
             ),

tests/validation.rs

@@ -49,7 +49,13 @@ fn ranked_rejects_a_zero_damping_factor() {
     )
     .expect_err("alpha = 0 must be rejected");
     assert!(
-        matches!(err, InferenceError::InvalidParameter { name: "alpha", .. }),
+        matches!(
+            err,
+            InferenceError::InvalidParameter {
+                parameter: trueskill_tt::Parameter::Alpha,
+                ..
+            }
+        ),
         "got {err:?}"
     );
 }
@@ -65,7 +71,13 @@ fn ranked_rejects_an_out_of_range_damping_factor() {
         )
         .expect_err("alpha out of (0, 1] must be rejected");
         assert!(
-            matches!(err, InferenceError::InvalidParameter { name: "alpha", .. }),
+            matches!(
+                err,
+                InferenceError::InvalidParameter {
+                    parameter: trueskill_tt::Parameter::Alpha,
+                    ..
+                }
+            ),
             "alpha={alpha}: got {err:?}"
         );
     }
@@ -81,7 +93,13 @@ fn scored_rejects_a_bad_damping_factor() {
     )
     .expect_err("alpha = 0 must be rejected");
     assert!(
-        matches!(err, InferenceError::InvalidParameter { name: "alpha", .. }),
+        matches!(
+            err,
+            InferenceError::InvalidParameter {
+                parameter: trueskill_tt::Parameter::Alpha,
+                ..
+            }
+        ),
         "got {err:?}"
     );
 }
@@ -360,7 +378,7 @@ mod constructor_parameters {
                 matches!(
                     err,
                     InferenceError::InvalidParameter {
-                        name: "drift variance",
+                        parameter: trueskill_tt::Parameter::DriftVariance,
                         ..
                     }
                 ),