Repository: ota-meshi/eslint-plugin-regexp (ts)

Issue:
Add suggestions for `no-useless-assertion`
**Description**
Since we know when an assertion always accepts/rejects, we can add a suggestion that removes the asserion/the branch of the assertion.

E.g. `/a\Bb/` -> `/ab/` and `/a|$b/` -> `/a/`


Pull request:
Add suggestions for `regexp/no-useless-assertion`
Fixes #662.

Patch:
diff --git a/.changeset/unlucky-humans-fry.md b/.changeset/unlucky-humans-fry.md
new file mode 100644
index 0000000..7ac5cb1
--- /dev/null
+++ b/.changeset/unlucky-humans-fry.md
@@ -0,0 +1,5 @@
+---
+"eslint-plugin-regexp": minor
+---
+
+Add suggestions for `regexp/no-useless-assertion`
diff --git a/README.md b/README.md
index 14c627a..7d7b4b3 100644
--- a/README.md
+++ b/README.md
@@ -128,7 +128,7 @@ The `plugin:regexp/all` config enables all rules. It's meant for testing, not fo
 | [no-potentially-useless-backreference](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-potentially-useless-backreference.html) | disallow backreferences that reference a group that might not be matched                |    | ✅  |    |    |
 | [no-super-linear-backtracking](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-super-linear-backtracking.html)                 | disallow exponential and polynomial backtracking                                        | ✅  |    | 🔧 |    |
 | [no-super-linear-move](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-super-linear-move.html)                                 | disallow quantifiers that cause quadratic moves                                         |    |    |    |    |
-| [no-useless-assertions](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-assertions.html)                               | disallow assertions that are known to always accept (or reject)                         | ✅  |    |    |    |
+| [no-useless-assertions](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-assertions.html)                               | disallow assertions that are known to always accept (or reject)                         | ✅  |    |    | 💡 |
 | [no-useless-backreference](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-backreference.html)                         | disallow useless backreferences in regular expressions                                  | ✅  |    |    |    |
 | [no-useless-dollar-replacements](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-dollar-replacements.html)             | disallow useless `$` replacements in replacement string                                 | ✅  |    |    |    |
 | [strict](https://ota-meshi.github.io/eslint-plugin-regexp/rules/strict.html)                                                             | disallow not strictly valid regular expressions                                         | ✅  |    | 🔧 | 💡 |
diff --git a/docs/rules/index.md b/docs/rules/index.md
index 535d969..6216701 100644
--- a/docs/rules/index.md
+++ b/docs/rules/index.md
@@ -34,7 +34,7 @@ sidebarDepth: 0
 | [no-potentially-useless-backreference](no-potentially-useless-backreference.md) | disallow backreferences that reference a group that might not be matched                |    | ✅  |    |    |
 | [no-super-linear-backtracking](no-super-linear-backtracking.md)                 | disallow exponential and polynomial backtracking                                        | ✅  |    | 🔧 |    |
 | [no-super-linear-move](no-super-linear-move.md)                                 | disallow quantifiers that cause quadratic moves                                         |    |    |    |    |
-| [no-useless-assertions](no-useless-assertions.md)                               | disallow assertions that are known to always accept (or reject)                         | ✅  |    |    |    |
+| [no-useless-assertions](no-useless-assertions.md)                               | disallow assertions that are known to always accept (or reject)                         | ✅  |    |    | 💡 |
 | [no-useless-backreference](no-useless-backreference.md)                         | disallow useless backreferences in regular expressions                                  | ✅  |    |    |    |
 | [no-useless-dollar-replacements](no-useless-dollar-replacements.md)             | disallow useless `$` replacements in replacement string                                 | ✅  |    |    |    |
 | [strict](strict.md)                                                             | disallow not strictly valid regular expressions                                         | ✅  |    | 🔧 | 💡 |
diff --git a/docs/rules/no-useless-assertions.md b/docs/rules/no-useless-assertions.md
index 5eecad9..4185f8f 100644
--- a/docs/rules/no-useless-assertions.md
+++ b/docs/rules/no-useless-assertions.md
@@ -9,13 +9,15 @@ since: "v0.9.0"
 
 💼 This rule is enabled in the ✅ `plugin:regexp/recommended` config.
 
+💡 This rule is manually fixable by [editor suggestions](https://eslint.org/docs/developer-guide/working-with-rules#providing-suggestions).
+
 <!-- end auto-generated rule header -->
 
 > disallow assertions that are known to always accept (or reject)
 
 ## :book: Rule Details
 
-Some assertion are unnecessary because the rest of the pattern forces them to
+Some assertions are unnecessary because the rest of the pattern forces them to
 always be accept (or reject).
 
 <eslint-code-block>
diff --git a/lib/rules/no-useless-assertions.ts b/lib/rules/no-useless-assertions.ts
index 808b147..6d8268d 100644
--- a/lib/rules/no-useless-assertions.ts
+++ b/lib/rules/no-useless-assertions.ts
@@ -1,10 +1,12 @@
 import type { RegExpVisitor } from "@eslint-community/regexpp/visitor"
 import type {
+    Alternative,
     Assertion,
     EdgeAssertion,
     Element,
     LookaroundAssertion,
     Node,
+    Pattern,
     WordBoundaryAssertion,
 } from "@eslint-community/regexpp/ast"
 import type { RegExpContext } from "../utils"
@@ -203,6 +205,73 @@ function createReorderingGetFirstCharAfter(
     }
 }
 
+function removeAlternative(
+    alternative: Alternative,
+): [Element | Pattern, string] {
+    const parent = alternative.parent
+    if (parent.alternatives.length > 1) {
+        // we can just remove the alternative
+        let { start, end } = alternative
+        if (parent.alternatives[0] === alternative) {
+            end++
+        } else {
+            start--
+        }
+        const before = parent.raw.slice(0, start - parent.start)
+        const after = parent.raw.slice(end - parent.start)
+        return [parent, before + after]
+    }
+
+    // we have to remove the parent as well
+
+    switch (parent.type) {
+        case "Pattern":
+            return [parent, "[]"]
+
+        case "Assertion": {
+            // the inner part of the assertion always rejects
+            const assertionParent = parent.parent
+            if (parent.negate) {
+                // the assertion always accepts
+                return [
+                    assertionParent.type === "Quantifier"
+                        ? assertionParent
+                        : parent,
+                    "",
+                ]
+            }
+            if (assertionParent.type === "Quantifier") {
+                if (assertionParent.min === 0) {
+                    return [assertionParent, ""]
+                }
+                return removeAlternative(assertionParent.parent)
+            }
+            return removeAlternative(assertionParent)
+        }
+
+        case "CapturingGroup": {
+            // we don't remove capturing groups
+            const before = parent.raw.slice(0, alternative.start - parent.start)
+            const after = parent.raw.slice(alternative.end - parent.start)
+            return [parent, `${before}[]${after}`]
+        }
+
+        case "Group": {
+            const groupParent = parent.parent
+            if (groupParent.type === "Quantifier") {
+                if (groupParent.min === 0) {
+                    return [groupParent, ""]
+                }
+                return removeAlternative(groupParent.parent)
+            }
+            return removeAlternative(groupParent)
+        }
+
+        default:
+            return assertNever(parent)
+    }
+}
+
 const messages = {
     alwaysRejectByChar:
         "{{assertion}} will always reject because it is {{followedOrPreceded}} by a character.",
@@ -226,6 +295,10 @@ const messages = {
         "The {{kind}} {{assertion}} will always {{acceptOrReject}}.",
     alwaysForNegativeLookaround:
         "The negative {{kind}} {{assertion}} will always {{acceptOrReject}}.",
+
+    acceptSuggestion: "Remove the assertion. (Replace with empty string.)",
+    rejectSuggestion:
+        "Remove branch of the assertion. (Replace with empty set.)",
 }
 
 export default createRule("no-useless-assertions", {
@@ -236,6 +309,7 @@ export default createRule("no-useless-assertions", {
             category: "Possible Errors",
             recommended: true,
         },
+        hasSuggestions: true,
         schema: [],
         messages,
         type: "problem",
@@ -245,15 +319,44 @@ export default createRule("no-useless-assertions", {
             node,
             flags,
             getRegexpLocation,
+            fixReplaceNode,
         }: RegExpContext): RegExpVisitor.Handlers {
             const reported = new Set<Assertion>()
 
+            function replaceWithEmptyString(assertion: Assertion) {
+                if (assertion.parent.type === "Quantifier") {
+                    // the assertion always accepts does not consume characters, we can remove the quantifier as well.
+                    return fixReplaceNode(assertion.parent, "")
+                }
+                return fixReplaceNode(assertion, "")
+            }
+
+            function replaceWithEmptySet(assertion: Assertion) {
+                if (assertion.parent.type === "Quantifier") {
+                    if (assertion.parent.min === 0) {
+                        // the assertion always rejects does not consume characters, we can remove the quantifier as well.
+                        return fixReplaceNode(assertion.parent, "")
+                    }
+                    const [element, replacement] = removeAlternative(
+                        assertion.parent.parent,
+                    )
+                    return fixReplaceNode(element, replacement)
+                }
+                const [element, replacement] = removeAlternative(
+                    assertion.parent,
+                )
+                return fixReplaceNode(element, replacement)
+            }
+
             function report(
                 assertion: Assertion,
                 messageId: keyof typeof messages,
-                data: Record<string, string>,
+                data: Record<string, string> & {
+                    acceptOrReject: "accept" | "reject"
+                },
             ) {
                 reported.add(assertion)
+                const { acceptOrReject } = data
 
                 context.report({
                     node,
@@ -263,6 +366,15 @@ export default createRule("no-useless-assertions", {
                         assertion: mention(assertion),
                         ...data,
                     },
+                    suggest: [
+                        {
+                            messageId: `${acceptOrReject}Suggestion`,
+                            fix:
+                                acceptOrReject === "accept"
+                                    ? replaceWithEmptyString(assertion)
+                                    : replaceWithEmptySet(assertion),
+                        },
+                    ],
                 })
             }
 
@@ -295,6 +407,7 @@ export default createRule("no-useless-assertions", {
                         if (next.char.isEmpty) {
                             report(assertion, "alwaysAcceptByChar", {
                                 followedOrPreceded,
+                                acceptOrReject: "accept",
                             })
                         }
                     } else {
@@ -306,6 +419,7 @@ export default createRule("no-useless-assertions", {
                                 {
                                     followedOrPreceded,
                                     startOrEnd: assertion.kind,
+                                    acceptOrReject: "accept",
                                 },
                             )
                         }
@@ -317,6 +431,7 @@ export default createRule("no-useless-assertions", {
                         // since the m flag isn't present any character will result in trivial rejection
                         report(assertion, "alwaysRejectByChar", {
                             followedOrPreceded,
+                            acceptOrReject: "reject",
                         })
                     } else {
                         // only if the character is a sub set of /./, will the assertion trivially reject
@@ -325,11 +440,15 @@ export default createRule("no-useless-assertions", {
                             report(
                                 assertion,
                                 "alwaysRejectByNonLineTerminator",
-                                { followedOrPreceded },
+                                {
+                                    followedOrPreceded,
+                                    acceptOrReject: "reject",
+                                },
                             )
                         } else if (next.char.isSubsetOf(lineTerminator)) {
                             report(assertion, "alwaysAcceptByLineTerminator", {
                                 followedOrPreceded,
+                                acceptOrReject: "accept",
                             })
                         }
                     }


Test patch:
diff --git a/tests/lib/rules/no-useless-assertions.ts b/tests/lib/rules/no-useless-assertions.ts
index b7c755f..7d50ebe 100644
--- a/tests/lib/rules/no-useless-assertions.ts
+++ b/tests/lib/rules/no-useless-assertions.ts
@@ -64,6 +64,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\b' will always reject because it is preceded by a word character and followed by a word character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -73,6 +79,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\b' will always reject because it is preceded by a word character and followed by a word character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/v`,
+                        },
+                    ],
                 },
             ],
         },
@@ -82,6 +94,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\b' will always reject because it is preceded by a non-word character and followed by a non-word character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -91,6 +109,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\b' will always accept because it is preceded by a non-word character and followed by a word character.",
+                    suggestions: [
+                        {
+                            messageId: "acceptSuggestion",
+                            output: String.raw`/,b/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -100,6 +124,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\b' will always accept because it is preceded by a word character and followed by a non-word character.",
+                    suggestions: [
+                        {
+                            messageId: "acceptSuggestion",
+                            output: String.raw`/a,/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -110,6 +140,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\B' will always accept because it is preceded by a word character and followed by a word character.",
+                    suggestions: [
+                        {
+                            messageId: "acceptSuggestion",
+                            output: String.raw`/ab/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -119,6 +155,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\B' will always accept because it is preceded by a non-word character and followed by a non-word character.",
+                    suggestions: [
+                        {
+                            messageId: "acceptSuggestion",
+                            output: String.raw`/,,/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -128,6 +170,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\B' will always reject because it is preceded by a non-word character and followed by a word character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -137,6 +185,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'\\B' will always reject because it is preceded by a word character and followed by a non-word character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -147,6 +201,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'^' will always reject because it is preceded by a non-line-terminator character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/m`,
+                        },
+                    ],
                 },
             ],
         },
@@ -156,6 +216,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'^' will always accept because it is preceded by a line-terminator character.",
+                    suggestions: [
+                        {
+                            messageId: "acceptSuggestion",
+                            output: String.raw`/\nfoo/m`,
+                        },
+                    ],
                 },
             ],
         },
@@ -165,6 +231,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'^' will always reject because it is preceded by a character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -174,6 +246,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'^' will always reject because it is preceded by a character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -184,6 +262,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'$' will always reject because it is followed by a non-line-terminator character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/m`,
+                        },
+                    ],
                 },
             ],
         },
@@ -202,6 +286,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'$' will always reject because it is followed by a character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -211,6 +301,12 @@ tester.run("no-useless-assertions", rule as any, {
                 {
                     message:
                         "'$' will always reject because it is followed by a character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/[]/`,
+                        },
+                    ],
                 },
             ],
         },
@@ -454,5 +550,105 @@ tester.run("no-useless-assertions", rule as any, {
                 "The lookahead '(?=(?=[a-f])(?=[aA])\\w(?<=[aA])(?<=[a-f]))' will always accept.",
             ],
         },
+
+        {
+            code: String.raw`/foo(?:\.(?!$)|$)$/`,
+            errors: [
+                {
+                    message:
+                        "The negative lookahead '(?!$)' will always reject.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/foo(?:$)$/`,
+                        },
+                    ],
+                },
+                {
+                    message:
+                        "'$' will always accept because it is never followed by a character.",
+                    suggestions: [
+                        {
+                            messageId: "acceptSuggestion",
+                            output: String.raw`/foo(?:\.(?!$)|)$/`,
+                        },
+                    ],
+                },
+            ],
+        },
+        {
+            code: String.raw`/(?=a)+a/`,
+            errors: [
+                {
+                    message: "The lookahead '(?=a)' will always accept.",
+                    suggestions: [
+                        {
+                            messageId: "acceptSuggestion",
+                            output: String.raw`/a/`,
+                        },
+                    ],
+                },
+            ],
+        },
+        {
+            code: String.raw`/(?!a)*a/`,
+            errors: [
+                {
+                    message:
+                        "The negative lookahead '(?!a)' will always reject.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/a/`,
+                        },
+                    ],
+                },
+            ],
+        },
+        {
+            code: String.raw`/a(\B)b/`,
+            errors: [
+                {
+                    message:
+                        "'\\B' will always accept because it is preceded by a word character and followed by a word character.",
+                    suggestions: [
+                        {
+                            messageId: "acceptSuggestion",
+                            output: String.raw`/a()b/`,
+                        },
+                    ],
+                },
+            ],
+        },
+        {
+            code: String.raw`/a(\b)b/`,
+            errors: [
+                {
+                    message:
+                        "'\\b' will always reject because it is preceded by a word character and followed by a word character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/a([])b/`,
+                        },
+                    ],
+                },
+            ],
+        },
+        {
+            code: String.raw`/foo|b(?:a(?:\br)|oo)|baz/`,
+            errors: [
+                {
+                    message:
+                        "'\\b' will always reject because it is preceded by a word character and followed by a word character.",
+                    suggestions: [
+                        {
+                            messageId: "rejectSuggestion",
+                            output: String.raw`/foo|b(?:oo)|baz/`,
+                        },
+                    ],
+                },
+            ],
+        },
     ],
 })