fix(linter): no unused errors should be warnings

crates/oxc_linter/src/rules/eslint/func_names.rs

@@ -28,7 +28,7 @@ impl TryFrom<&serde_json::Value> for FuncNamesConfig {
 
     fn try_from(raw: &serde_json::Value) -> Result<Self, Self::Error> {
         if !raw.is_string() {
-            return Err(OxcDiagnostic::error(format!(
+            return Err(OxcDiagnostic::warn(format!(
                 "Expecting string for eslint/func-names configuration, got {raw}"
             )));
         }
@@ -37,7 +37,7 @@ impl TryFrom<&serde_json::Value> for FuncNamesConfig {
             "always" => Ok(FuncNamesConfig::Always),
             "as-needed" => Ok(FuncNamesConfig::AsNeeded),
             "never" => Ok(FuncNamesConfig::Never),
-            v => Err(OxcDiagnostic::error(format!(
+            v => Err(OxcDiagnostic::warn(format!(
                 "Expecting always, as-needed or never for eslint/func-names configuration, got {v}"
             ))),
         }

crates/oxc_linter/src/rules/eslint/no_fallthrough.rs

@@ -21,15 +21,15 @@ use rustc_hash::{FxHashMap, FxHashSet};
 use crate::{context::LintContext, rule::Rule, AstNode};
 
 fn no_fallthrough_case_diagnostic(span: Span) -> OxcDiagnostic {
-    OxcDiagnostic::error("Expected a 'break' statement before 'case'.").with_label(span)
+    OxcDiagnostic::warn("Expected a 'break' statement before 'case'.").with_label(span)
 }
 
 fn no_fallthrough_default_diagnostic(span: Span) -> OxcDiagnostic {
-    OxcDiagnostic::error("Expected a 'break' statement before 'default'.").with_label(span)
+    OxcDiagnostic::warn("Expected a 'break' statement before 'default'.").with_label(span)
 }
 
 fn no_unused_fallthrough_diagnostic(span: Span) -> OxcDiagnostic {
-    OxcDiagnostic::error(
+    OxcDiagnostic::warn(
         "Found a comment that would permit fallthrough, but case cannot fall through.",
     )
     .with_label(span)

crates/oxc_linter/src/rules/eslint/no_unreachable.rs

@@ -13,7 +13,7 @@ use oxc_span::{GetSpan, Span};
 use crate::{context::LintContext, rule::Rule};
 
 fn no_unreachable_diagnostic(span: Span) -> OxcDiagnostic {
-    OxcDiagnostic::error("Unreachable code.").with_label(span)
+    OxcDiagnostic::warn("Unreachable code.").with_label(span)
 }
 
 /// <https://github.com/eslint/eslint/blob/069aa680c78b8516b9a1b568519f1d01e74fb2a2/lib/rules/no-unreachable.js#L196>

crates/oxc_linter/src/rules/eslint/no_unused_vars/diagnostic.rs

@@ -44,7 +44,7 @@ pub fn declared(symbol: &Symbol<'_, '_>) -> OxcDiagnostic {
     let pronoun = pronoun_for_symbol(symbol.flags());
     let name = symbol.name();
 
-    OxcDiagnostic::error(format!("{pronoun} '{name}' is {verb} but never used."))
+    OxcDiagnostic::warn(format!("{pronoun} '{name}' is {verb} but never used."))
         .with_label(symbol.span().label(format!("'{name}' is declared here")))
         .with_help(help)
 }
@@ -54,7 +54,7 @@ pub fn assign(symbol: &Symbol<'_, '_>, assign_span: Span) -> OxcDiagnostic {
     let pronoun = pronoun_for_symbol(symbol.flags());
     let name = symbol.name();
 
-    OxcDiagnostic::error(format!("{pronoun} '{name}' is assigned a value but never used."))
+    OxcDiagnostic::warn(format!("{pronoun} '{name}' is assigned a value but never used."))
         .with_labels([
             symbol.span().label(format!("'{name}' is declared here")),
             assign_span.label("it was last assigned here"),
@@ -66,7 +66,7 @@ pub fn assign(symbol: &Symbol<'_, '_>, assign_span: Span) -> OxcDiagnostic {
 pub fn param(symbol: &Symbol<'_, '_>) -> OxcDiagnostic {
     let name = symbol.name();
 
-    OxcDiagnostic::error(format!("Parameter '{name}' is declared but never used."))
+    OxcDiagnostic::warn(format!("Parameter '{name}' is declared but never used."))
         .with_label(symbol.span().label(format!("'{name}' is declared here")))
         .with_help("Consider removing this parameter.")
 }
@@ -76,7 +76,7 @@ pub fn imported(symbol: &Symbol<'_, '_>) -> OxcDiagnostic {
     let pronoun = pronoun_for_symbol(symbol.flags());
     let name = symbol.name();
 
-    OxcDiagnostic::error(format!("{pronoun} '{name}' is imported but never used."))
+    OxcDiagnostic::warn(format!("{pronoun} '{name}' is imported but never used."))
         .with_label(symbol.span().label(format!("'{name}' is imported here")))
         .with_help("Consider removing this import.")
 }