fix(compare): fall back to a line diff when semantic rows fail to build

bindings/node/index.d.ts

@@ -134,7 +134,7 @@ export interface Fallback {
    */
   kind: 'no_grammar' | 'grammar_unavailable' | 'semantic' | 'request_budget'
   /** Present when `kind` is `semantic`. */
-  reason?: 'too_large' | 'parse_errors' | 'error_coverage' | 'unparsed' | 'budget' | 'timeout'
+  reason?: 'too_large' | 'parse_errors' | 'error_coverage' | 'unparsed' | 'budget' | 'timeout' | 'invalid'
   /** For people; wording may change. Branch on `kind` and `reason`. */
   message: string
 }

crates/diffy-compare/FORMAT.md

@@ -58,7 +58,7 @@ Render order: for each hunk, `gap_before`, header, rows; then `gap_after`.
 | Field | Type | |
 | --- | --- | --- |
 | `kind` | `"no_grammar"` \| `"grammar_unavailable"` \| `"semantic"` \| `"request_budget"` | `no_grammar`: no language for the path. `grammar_unavailable`: the language's grammar failed to load. `semantic`: the semantic diff gave up, see `reason`. `request_budget`: the request's deadline passed before this file was diffed. |
-| `reason` | `"too_large"` \| `"parse_errors"` \| `"error_coverage"` \| `"unparsed"` \| `"budget"` \| `"timeout"`? | Present when `kind` is `semantic`, in order: file too large; too many parse errors; parse errors cover too much; parser gave up; work budget ran out; time budget ran out. |
+| `reason` | `"too_large"` \| `"parse_errors"` \| `"error_coverage"` \| `"unparsed"` \| `"budget"` \| `"timeout"` \| `"invalid"`? | Present when `kind` is `semantic`, in order: file too large; too many parse errors; parse errors cover too much; parser gave up; work budget ran out; time budget ran out; its result was unusable (a bug). |
 | `message` | string | For people, e.g. `"exceeded the semantic time budget"`. Wording may change; branch on `kind` and `reason`. |
 
 ## Hunk
@@ -213,7 +213,7 @@ type File = {
 
 type Fallback = {
   kind: "no_grammar" | "grammar_unavailable" | "semantic" | "request_budget";
-  reason?: "too_large" | "parse_errors" | "error_coverage" | "unparsed" | "budget" | "timeout";
+  reason?: "too_large" | "parse_errors" | "error_coverage" | "unparsed" | "budget" | "timeout" | "invalid";
   message: string;
 };
 

crates/diffy-compare/src/file.rs

@@ -132,10 +132,22 @@ pub(crate) fn diff_one(
                     })
                     .collect(),
             };
-            Ok(FileDiff {
-                document: document_from_rows(meta, old, new, chunks, true, semantics)?,
-                fallback: None,
-            })
+            match document_from_rows(meta.clone(), old, new, chunks, true, semantics) {
+                Ok(document) => Ok(FileDiff {
+                    document,
+                    fallback: None,
+                }),
+                // A bug in the semantic rows; the file still gets a diff.
+                Err(error) => {
+                    tracing::warn!(
+                        target: "diffy::semantic",
+                        path = input.display_path(),
+                        %error,
+                        "semantic rows did not build; using a line diff"
+                    );
+                    line(meta, Fallback::Invalid(error.to_string()))
+                }
+            }
         }
         Err(fallback) => line(meta, Fallback::Semantic(fallback)),
     }

crates/diffy-compare/src/lib.rs

@@ -73,6 +73,8 @@ pub enum Fallback {
     GrammarUnavailable(String),
     /// The semantic diff gave up.
     Semantic(diffy_semantic::Fallback),
+    /// The semantic diff's rows did not build a document (a bug).
+    Invalid(String),
     /// The request's deadline passed before this file was diffed.
     RequestBudget,
 }
@@ -83,6 +85,7 @@ impl std::fmt::Display for Fallback {
             Self::NoGrammar => f.write_str("no grammar for this file"),
             Self::GrammarUnavailable(tag) => write!(f, "{tag} grammar unavailable"),
             Self::Semantic(fallback) => fallback.fmt(f),
+            Self::Invalid(error) => write!(f, "semantic result unusable: {error}"),
             Self::RequestBudget => f.write_str("request time budget exhausted"),
         }
     }

crates/diffy-compare/src/wire.rs

@@ -153,6 +153,7 @@ pub enum SemanticReason {
     Unparsed,
     Budget,
     Timeout,
+    Invalid,
 }
 
 impl From<&crate::Fallback> for Fallback {
@@ -162,6 +163,7 @@ impl From<&crate::Fallback> for Fallback {
             crate::Fallback::NoGrammar => (FallbackKind::NoGrammar, None),
             crate::Fallback::GrammarUnavailable(_) => (FallbackKind::GrammarUnavailable, None),
             crate::Fallback::RequestBudget => (FallbackKind::RequestBudget, None),
+            crate::Fallback::Invalid(_) => (FallbackKind::Semantic, Some(SemanticReason::Invalid)),
             crate::Fallback::Semantic(semantic) => {
                 let reason = match semantic {
                     Semantic::TooLarge(_) => SemanticReason::TooLarge,

crates/diffy-compare/src/wire/tests.rs

@@ -682,6 +682,14 @@ fn fallbacks_serialize_as_stable_kinds() {
                 "message": "5 bytes exceeds the semantic byte limit",
             }),
         ),
+        (
+            crate::Fallback::Invalid("bad rows".into()),
+            serde_json::json!({
+                "kind": "semantic",
+                "reason": "invalid",
+                "message": "semantic result unusable: bad rows",
+            }),
+        ),
         (
             crate::Fallback::GrammarUnavailable("zig".into()),
             serde_json::json!({