fix(lint): resolve false positive in noAssignInExpressions for Svelte {@const} blocks (#10079)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: Damix48

.changeset/fix-svelte-const-false-positive.md

@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed false positive in `noAssignInExpressions` for Svelte `{@const}` blocks. Assignments in `{@const name = value}` are now correctly recognized as declarations rather than accidental assignments in expressions.

crates/biome_js_analyze/src/lint/suspicious/no_assign_in_expressions.rs

@@ -72,6 +72,12 @@ impl Rule for NoAssignInExpressions {
             return None;
         }
 
+        // Skip assignments in Svelte const blocks ({@const name = value})
+        // These are declaration statements, not accidental assignments in expressions
+        if file_source.is_svelte_const_block() {
+            return None;
+        }
+
         let assign = ctx.query();
         let mut ancestor = assign
             .syntax()

crates/biome_js_analyze/tests/specs/suspicious/noAssignInExpressions/valid-svelte-const.svelte

@@ -0,0 +1,9 @@
+<!-- should not generate diagnostics -->
+<script lang="ts">
+  const numbers = [1, 2, 3, 4];
+</script>
+
+{#each numbers as number, index (number)}
+  {@const n = number + 1}
+  <p>{n}</p>
+{/each}

crates/biome_js_analyze/tests/specs/suspicious/noAssignInExpressions/valid-svelte-const.svelte.snap

@@ -0,0 +1,18 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+assertion_line: 149
+expression: valid-svelte-const.svelte
+---
+# Input
+```svelte
+<!-- should not generate diagnostics -->
+<script lang="ts">
+  const numbers = [1, 2, 3, 4];
+</script>
+
+{#each numbers as number, index (number)}
+  {@const n = number + 1}
+  <p>{n}</p>
+{/each}
+
+```

crates/biome_js_parser/tests/spec_test.rs

@@ -89,6 +89,7 @@ pub fn run(test_case: &str, _snapshot_name: &str, test_directory: &str, outcome_
             is_source: false,
             is_function_signature: false,
             kind: biome_js_syntax::SvelteFileKind::Component,
+            is_const_block: false,
         });
     }
 

crates/biome_js_syntax/src/file_source.rs

@@ -165,6 +165,9 @@ pub enum EmbeddingKind {
 
         /// Whether this is the declaration of a function, usually declared in `#snippet`
         is_function_signature: bool,
+
+        /// Whether this is a `{@const name = value}` block.
+        is_const_block: bool,
     },
     #[default]
     None,
@@ -222,6 +225,15 @@ impl EmbeddingKind {
             }
         )
     }
+    pub const fn is_svelte_const_block(&self) -> bool {
+        matches!(
+            self,
+            Self::Svelte {
+                is_const_block: true,
+                ..
+            }
+        )
+    }
 }
 
 #[cfg_attr(feature = "schema", derive(schemars::JsonSchema))]
@@ -315,6 +327,7 @@ impl JsFileSource {
             is_source: true,
             is_function_signature: false,
             kind: SvelteFileKind::Component,
+            is_const_block: false,
         })
     }
 
@@ -421,6 +434,11 @@ impl JsFileSource {
         self.embedding_kind.is_vue_event_handler()
     }
 
+    /// Returns true if this is a Svelte `{@const}` block
+    pub const fn is_svelte_const_block(&self) -> bool {
+        self.embedding_kind.is_svelte_const_block()
+    }
+
     pub const fn as_embedding_kind(&self) -> &EmbeddingKind {
         &self.embedding_kind
     }
@@ -510,6 +528,7 @@ impl JsFileSource {
                 is_source: true,
                 is_function_signature: false,
                 kind: SvelteFileKind::SourceModule,
+                is_const_block: false,
             }));
         }
 

crates/biome_service/src/file_handlers/html/parse_embedded_nodes.rs

@@ -790,6 +790,7 @@ fn parse_matched_embed(
                             is_source: true,
                             is_function_signature: false,
                             kind: SvelteFileKind::Component,
+                            is_const_block: false,
                         });
                     } else if ctx.host_file_source.is_vue() {
                         js_source = js_source.with_embedding_kind(EmbeddingKind::Vue {
@@ -813,6 +814,10 @@ fn parse_matched_embed(
                             is_source: false,
                             is_function_signature,
                             kind: SvelteFileKind::Component,
+                            is_const_block: matches!(
+                                block_kind,
+                                EmbedBlockKind::Svelte(SvelteBlockKind::Const)
+                            ),
                         });
                     } else if ctx.host_file_source.is_vue() {
                         js_source = js_source.with_embedding_kind(EmbeddingKind::Vue {
@@ -846,6 +851,7 @@ fn parse_matched_embed(
                                 is_source: false,
                                 is_function_signature: false,
                                 kind: SvelteFileKind::Component,
+                                is_const_block: false,
                             });
                         }
                     }

crates/biome_service/src/file_handlers/svelte.rs

@@ -82,6 +82,7 @@ impl SvelteFileHandler {
                             is_source: true,
                             is_function_signature: false,
                             kind: SvelteFileKind::Component,
+                            is_const_block: false,
                         }),
                 )
             })