biomejs · Mokto · Jun 3, 2026 · Jun 3, 2026 · Jun 3, 2026 · Jun 3, 2026
diff --git a/.changeset/no-duplicate-else-if.md b/.changeset/no-duplicate-else-if.md
@@ -0,0 +1,13 @@
+---
+"@biomejs/biome": patch
+---
+
+Added the nursery rule [`noDuplicateElseIf`](https://biomejs.dev/linter/rules/no-duplicate-else-if/) for Svelte templates: disallow duplicate conditions in `{#if}` / `{:else if}` chains. A condition identical to a previous one can never execute:
+
+```svelte
+{#if a}
+  <div>a</div>
+{:else if a}
+  <div>unreachable</div>
+{/if}
+```
diff --git a/.changeset/no-svelte-duplicate-style-properties.md b/.changeset/no-svelte-duplicate-style-properties.md
@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Added the nursery rule [`noSvelteDuplicateStyleProperties`](https://biomejs.dev/linter/rules/no-svelte-duplicate-style-properties/) for Svelte templates: disallow duplicate `style:` directives on the same element. `<div style:color="red" style:color="blue"></div>` is invalid.
diff --git a/.changeset/no-svelte-duplicate-use-directives.md b/.changeset/no-svelte-duplicate-use-directives.md
@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Added the nursery rule [`noSvelteDuplicateUseDirectives`](https://biomejs.dev/linter/rules/no-svelte-duplicate-use-directives/) for Svelte templates: disallow duplicate `use:` directives on the same element. `<div use:tooltip use:tooltip></div>` is invalid.
diff --git a/crates/biome_cli/src/execute/migrate/eslint_any_rule_to_biome.rs b/crates/biome_cli/src/execute/migrate/eslint_any_rule_to_biome.rs
diff --git a/crates/biome_configuration/src/analyzer/linter/rules.rs b/crates/biome_configuration/src/analyzer/linter/rules.rs
diff --git a/crates/biome_configuration/src/generated/linter_options_check.rs b/crates/biome_configuration/src/generated/linter_options_check.rs
diff --git a/crates/biome_diagnostics_categories/src/categories.rs b/crates/biome_diagnostics_categories/src/categories.rs
diff --git a/crates/biome_html_analyze/src/lint/nursery/no_duplicate_else_if.rs b/crates/biome_html_analyze/src/lint/nursery/no_duplicate_else_if.rs
@@ -0,0 +1,131 @@
+use biome_analyze::{
+    Ast, Rule, RuleDiagnostic, RuleDomain, RuleSource, context::RuleContext, declare_lint_rule,
+};
+use biome_console::markup;
+use biome_html_syntax::SvelteIfBlock;
+use biome_rowan::{AstNode, AstNodeList, TextRange};
+use biome_rule_options::no_duplicate_else_if::NoDuplicateElseIfOptions;
+
+declare_lint_rule! {
+    /// Disallow duplicate conditions in Svelte `{#if}` / `{:else if}` chains.
+    ///
+    /// If an `{:else if}` condition is textually identical to a previous condition in the same
+    /// chain, it can never execute, making it dead code.
+    ///
+    /// This rule only applies to Svelte templates. It does not detect duplicate conditions in Vue or Astro templates.
+    ///
+    /// ## Examples
+    ///
+    /// ### Invalid
+    ///
+    /// ```svelte,expect_diagnostic
+    /// {#if a}
+    ///   <div>a</div>
+    /// {:else if b}
+    ///   <div>b</div>
+    /// {:else if a}
+    ///   <div>a again</div>
+    /// {/if}
+    /// ```
+    ///
+    /// ### Valid
+    ///
+    /// ```svelte
+    /// {#if a}
+    ///   <div>a</div>
+    /// {:else if b}
+    ///   <div>b</div>
+    /// {:else if c}
+    ///   <div>c</div>
+    /// {/if}
+    /// ```
+    ///
+    pub NoDuplicateElseIf {
+        version: "next",
+        name: "noDuplicateElseIf",
+        language: "html",
+        domains: &[RuleDomain::Svelte],
+        recommended: true,
+        sources: &[RuleSource::EslintSvelte("no-dupe-else-if-blocks").same()],
+    }
+}
+
+pub struct State {
+    /// Range of the duplicate condition.
+    duplicate_range: TextRange,
+    /// Range of the first occurrence.
+    original_range: TextRange,
+    /// The condition text.
+    condition: String,
+}
+
+impl Rule for NoDuplicateElseIf {
+    type Query = Ast<SvelteIfBlock>;
+    type State = State;
+    type Signals = Box<[Self::State]>;
+    type Options = NoDuplicateElseIfOptions;
+
+    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
+        let node = ctx.query();
+
+        // Collect (text, range) for each condition in the chain.
+        let mut conditions: Vec<(String, TextRange)> = Vec::new();
+        let mut violations: Vec<State> = Vec::new();
+
+        // Opening block condition.
+        if let Ok(opening) = node.opening_block()
+            && let Ok(expr) = opening.expression()
+            && let Ok(token) = expr.html_literal_token()
+        {
+            let text = token.text_trimmed().to_string();
+            let range = token.text_trimmed_range();
+            conditions.push((text, range));
+        }
+
+        // Else-if clause conditions.
+        for clause in node.else_if_clauses().iter() {
+            let Ok(expr) = clause.expression() else {
+                continue;
+            };
+            let Ok(token) = expr.html_literal_token() else {
+                continue;
+            };
+            let text = token.text_trimmed().to_string();
+            let range = token.text_trimmed_range();
+
+            if let Some((_, original_range)) =
+                conditions.iter().find(|(prev_text, _)| prev_text == &text)
+            {
+                violations.push(State {
+                    duplicate_range: clause.range(),
+                    original_range: *original_range,
+                    condition: text.clone(),
+                });
+            }
+
+            conditions.push((text, range));
+        }
+
+        violations.into_boxed_slice()
+    }
+
+    fn diagnostic(_ctx: &RuleContext<Self>, state: &Self::State) -> Option<RuleDiagnostic> {
+        let condition = state.condition.as_str();
+        Some(
+            RuleDiagnostic::new(
+                rule_category!(),
+                state.duplicate_range,
+                markup! {
+                    "This branch can never execute. Its condition "<Emphasis>{condition}</Emphasis>" is a duplicate of a previous condition."
+                },
+            )
+            .detail(
+                state.original_range,
+                "This is the first occurrence of the condition.",
+            )
+            .note(markup! {
+                "Remove the duplicate "<Emphasis>"{:else if}"</Emphasis>" branch or change its condition to make it reachable."
+            }),
+        )
+    }
+}