feat(es/lints): Implement no-var (#4485)

crates/swc/tests/errors/lints/no-var/default/.swcrc

@@ -0,0 +1,9 @@
+{
+    "jsc": {
+        "lints": {
+            "no-var": [
+                "error"
+            ]
+        }
+    }
+}

crates/swc/tests/errors/lints/no-var/default/input.js

@@ -0,0 +1,4 @@
+var a = 1;
+
+let b = 1;
+const c = 1;

crates/swc/tests/errors/lints/no-var/default/output.swc-stderr

@@ -0,0 +1,6 @@
+
+  x Unexpected var, use let or const instead
+   ,----
+ 1 | var a = 1;
+   : ^^^^^^^^^^
+   `----

crates/swc_ecma_lints/src/config.rs

@@ -165,6 +165,10 @@ pub struct LintConfig {
     #[serde(default, alias = "noObjCalls")]
     pub no_obj_calls: RuleConfig<()>,
 
+    #[cfg(feature = "non_critical_lints")]
+    #[serde(default, alias = "noVar")]
+    pub no_var: RuleConfig<()>,
+
     #[cfg(feature = "non_critical_lints")]
     #[serde(default, alias = "noThrowLiteral")]
     pub no_throw_literal: RuleConfig<()>,

crates/swc_ecma_lints/src/rules/mod.rs

@@ -32,6 +32,7 @@ pub(crate) mod non_critical_lints {
     pub mod no_restricted_syntax;
     pub mod no_throw_literal;
     pub mod no_use_before_define;
+    pub mod no_var;
     pub mod prefer_regex_literals;
     pub mod quotes;
     pub mod radix;
@@ -159,7 +160,9 @@ pub fn all(lint_params: LintParams) -> Vec<Box<dyn Rule>> {
 
         rules.extend(no_throw_literal::no_throw_literal(
             &lint_config.no_throw_literal,
-        ))
+        ));
+
+        rules.extend(no_var::no_var(&lint_config.no_var));
     }
 
     rules

crates/swc_ecma_lints/src/rules/no_var.rs

@@ -0,0 +1,52 @@
+use swc_common::{errors::HANDLER, Span};
+use swc_ecma_ast::*;
+use swc_ecma_visit::{Visit, VisitWith};
+
+use crate::{
+    config::{LintRuleReaction, RuleConfig},
+    rule::{visitor_rule, Rule},
+};
+
+const MESSAGE: &str = "Unexpected var, use let or const instead";
+
+pub fn no_var(config: &RuleConfig<()>) -> Option<Box<dyn Rule>> {
+    let rule_reaction = config.get_rule_reaction();
+
+    match rule_reaction {
+        LintRuleReaction::Off => None,
+        _ => Some(visitor_rule(NoVar::new(rule_reaction))),
+    }
+}
+
+#[derive(Debug, Default)]
+struct NoVar {
+    expected_reaction: LintRuleReaction,
+}
+
+impl NoVar {
+    fn new(expected_reaction: LintRuleReaction) -> Self {
+        Self { expected_reaction }
+    }
+
+    fn emit_error(&self, span: Span) {
+        HANDLER.with(|handler| match self.expected_reaction {
+            LintRuleReaction::Error => {
+                handler.struct_span_err(span, MESSAGE).emit();
+            }
+            LintRuleReaction::Warning => {
+                handler.struct_span_warn(span, MESSAGE).emit();
+            }
+            _ => {}
+        });
+    }
+}
+
+impl Visit for NoVar {
+    fn visit_var_decl(&mut self, var_decl: &VarDecl) {
+        if let VarDeclKind::Var = var_decl.kind {
+            self.emit_error(var_decl.span);
+        }
+
+        var_decl.visit_children_with(self);
+    }
+}