feat(linter): implement @typescript-eslint/prefer-as-const

crates/oxc_linter/src/rules.rs

@@ -85,6 +85,7 @@ mod typescript {
     pub mod no_this_alias;
     pub mod no_unnecessary_type_constraint;
     pub mod no_var_requires;
+    pub mod prefer_as_const;
 }
 
 mod jest {
@@ -167,6 +168,7 @@ oxc_macros::declare_all_lint_rules! {
     typescript::no_this_alias,
     typescript::no_namespace,
     typescript::no_var_requires,
+    typescript::prefer_as_const,
     jest::no_disabled_tests,
     jest::no_test_prefixes,
     jest::no_focused_tests,

crates/oxc_linter/src/rules/typescript/prefer_as_const.rs

@@ -0,0 +1,190 @@
+use oxc_ast::ast::{Expression, TSLiteral, TSType};
+use oxc_ast::AstKind;
+
+use oxc_diagnostics::{
+    miette::{self, Diagnostic},
+    thiserror::Error,
+};
+use oxc_macros::declare_oxc_lint;
+use oxc_span::Span;
+
+use crate::{context::LintContext, fixer::Fix, rule::Rule, AstNode};
+
+#[derive(Debug, Error, Diagnostic)]
+#[error("Expected a `const` assertion instead of a literal type annotation.")]
+#[diagnostic(severity(warning), help("You should use `as const` instead of type annotation."))]
+struct PreferAsConstDiagnostic(#[label] pub Span);
+
+#[derive(Debug, Default, Clone)]
+pub struct PreferAsConst;
+
+declare_oxc_lint!(
+    /// ### What it does
+    /// Enforce the use of as const over literal type.
+    /// ### Why is this bad?
+    /// There are two common ways to tell TypeScript that a literal value should be interpreted as its literal type (e.g. 2) rather than general primitive type (e.g. number);
+    ///
+    /// as const: telling TypeScript to infer the literal type automatically
+    /// as with the literal type: explicitly telling the literal type to TypeScript
+    ///
+    /// as const is generally preferred, as it doesn't require re-typing the literal value.
+    /// This rule reports when an as with an explicit literal type can be replaced with an as const.
+    ///
+    /// ### Example
+    /// ```javascript
+    /// let bar: 2 = 2;
+    /// let foo = { bar: 'baz' as 'baz' };
+    /// ```
+    PreferAsConst,
+    correctness
+);
+
+impl Rule for PreferAsConst {
+    fn run<'a>(&self, node: &AstNode<'a>, ctx: &LintContext<'a>) {
+        match node.kind() {
+            AstKind::VariableDeclarator(variable_declarator) => {
+                let Some(type_annotation) = &variable_declarator.id.type_annotation else { return; };
+                let Some(initial_value_expression) = &variable_declarator.init else { return; };
+                check_and_report(
+                    &type_annotation.type_annotation,
+                    initial_value_expression,
+                    ctx,
+                    false,
+                );
+            }
+            AstKind::PropertyDefinition(property_definition) => {
+                let Some(type_annotation) = &property_definition.type_annotation else { return; };
+                let Some(initial_value_expression) = &property_definition.value else { return; };
+                check_and_report(
+                    &type_annotation.type_annotation,
+                    initial_value_expression,
+                    ctx,
+                    false,
+                );
+            }
+            AstKind::TSAsExpression(as_expression) => {
+                check_and_report(
+                    &as_expression.type_annotation,
+                    &as_expression.expression,
+                    ctx,
+                    true,
+                );
+            }
+            _ => {}
+        }
+    }
+}
+
+fn check_and_report(
+    ts_type: &TSType,
+    initial_value_expression: &Expression,
+    ctx: &LintContext,
+    can_fix: bool,
+) {
+    if let TSType::TSLiteralType(literal_type) = &ts_type {
+        let error_span = match &literal_type.literal {
+            TSLiteral::StringLiteral(string_literal) => match initial_value_expression {
+                Expression::StringLiteral(initial_string) => {
+                    if string_literal.value.eq(&initial_string.value) {
+                        Some(string_literal.span)
+                    } else {
+                        None
+                    }
+                }
+                _ => None,
+            },
+            TSLiteral::NullLiteral(null_literal) => match initial_value_expression {
+                Expression::NullLiteral(_) => Some(null_literal.span),
+                _ => None,
+            },
+            TSLiteral::NumberLiteral(number_literal) => match initial_value_expression {
+                Expression::NumberLiteral(initial_number) => {
+                    if (number_literal.value - initial_number.value).abs() < f64::EPSILON {
+                        Some(number_literal.span)
+                    } else {
+                        None
+                    }
+                }
+                _ => None,
+            },
+            _ => None,
+        };
+        if let Some(span) = error_span {
+            if can_fix {
+                ctx.diagnostic_with_fix(PreferAsConstDiagnostic(span), || {
+                    let start = span.start;
+                    let end = span.end;
+                    Fix::new("const", Span { start, end })
+                });
+            } else {
+                ctx.diagnostic(PreferAsConstDiagnostic(span));
+            }
+        }
+    }
+}
+
+#[test]
+fn test() {
+    use crate::tester::Tester;
+
+    let pass = vec![
+        "let foo = 'baz' as const;",
+        "let foo = 1 as const;",
+        "let foo = { bar: 'baz' as const };",
+        "let foo = { bar: 1 as const };",
+        "let foo = { bar: 'baz' };",
+        "let foo = { bar: 2 };",
+        "let foo = 'bar' as string;",
+        "let foo = `bar` as `bar`;",
+        "let foo = `bar` as `foo`;",
+        "let foo = `bar` as 'bar';",
+        "let foo: string = 'bar';",
+        "let foo: number = 1;",
+        "let foo: 'bar' = baz;",
+        "let foo = 'bar';",
+        "let foo: 'bar';",
+        "let foo = { bar };",
+        "let foo: 'baz' = 'baz' as const;",
+        "class foo { bar = 'baz'; }",
+        "class foo { bar: 'baz'; }",
+        "class foo { bar; }",
+        "class foo { bar: string = 'baz'; }",
+        "class foo { bar: number = 1; }",
+        "class foo { bar = 'baz' as const; }",
+        "class foo { bar = 2 as const; }",
+        "class foo { get bar(): 'bar' {} set bar(bar: 'bar') {} }",
+        "class foo { bar = () => 'bar' as const; }",
+        "type BazFunction = () => 'baz'; class foo { bar: BazFunction = () => 'bar'; }",
+        "class foo { bar(): void {} }",
+        // NOTE: OXC does not parse these format yet.
+        // "let foo = <bar>'bar';",
+        // "let foo = <string>'bar';",
+        // "class foo { bar = <baz>'baz'; }",
+    ];
+
+    let fail = vec![
+        "let []: 'bar' = 'bar';",
+        "let foo: 'bar' = 'bar';",
+        "let foo: 2 = 2;",
+        "class foo { bar: 'baz' = 'baz';}",
+        "class foo { bar: 2 = 2;}",
+    ];
+
+    let fix = vec![
+        ("let foo = { bar: 'baz' as 'baz' };", "let foo = { bar: 'baz' as const };", None),
+        ("let foo = { bar: 1 as 1 };", "let foo = { bar: 1 as const };", None),
+        ("let foo: 'bar' = 'bar' as 'bar';", "let foo: 'bar' = 'bar' as const;", None),
+        ("let foo = 'bar' as 'bar';", "let foo = 'bar' as const;", None),
+        ("let foo = 5 as 5;", "let foo = 5 as const;", None),
+        ("class foo { foo = 'bar' as 'bar'; }", "class foo { foo = 'bar' as const; }", None),
+        ("class foo { foo = 5 as 5; }", "class foo { foo = 5 as const; }", None),
+        // NOTE: OXC does not parse these format yet.
+        // ("let foo = <4>4;", "let foo = <const>4;", None),
+        // ("let foo = <'bar'>'bar';", "let foo = <const>'bar';", None),
+        // ("class foo { foo = <'bar'>'bar'; }", "class foo { foo = <const>'bar'; }", None),
+    ];
+
+    let mut tester = Tester::new_without_config(PreferAsConst::NAME, pass, fail);
+    tester.test_and_snapshot();
+    tester.test_fix(fix);
+}

crates/oxc_linter/src/snapshots/prefer_as_const.snap

@@ -0,0 +1,41 @@
+---
+source: crates/oxc_linter/src/tester.rs
+assertion_line: 80
+expression: prefer_as_const
+---
+  ⚠ Expected a `const` assertion instead of a literal type annotation.
+   ╭─[prefer_as_const.tsx:1:1]
+ 1 │ let []: 'bar' = 'bar';
+   ·         ─────
+   ╰────
+  help: You should use `as const` instead of type annotation.
+
+  ⚠ Expected a `const` assertion instead of a literal type annotation.
+   ╭─[prefer_as_const.tsx:1:1]
+ 1 │ let foo: 'bar' = 'bar';
+   ·          ─────
+   ╰────
+  help: You should use `as const` instead of type annotation.
+
+  ⚠ Expected a `const` assertion instead of a literal type annotation.
+   ╭─[prefer_as_const.tsx:1:1]
+ 1 │ let foo: 2 = 2;
+   ·          ─
+   ╰────
+  help: You should use `as const` instead of type annotation.
+
+  ⚠ Expected a `const` assertion instead of a literal type annotation.
+   ╭─[prefer_as_const.tsx:1:1]
+ 1 │ class foo { bar: 'baz' = 'baz';}
+   ·                  ─────
+   ╰────
+  help: You should use `as const` instead of type annotation.
+
+  ⚠ Expected a `const` assertion instead of a literal type annotation.
+   ╭─[prefer_as_const.tsx:1:1]
+ 1 │ class foo { bar: 2 = 2;}
+   ·                  ─
+   ╰────
+  help: You should use `as const` instead of type annotation.
+
+