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

Issue:
Make `no-useless-assertion` detect `(?!$)$`
**Description**
While looking at #659, I noticed that `no-useless-assertion` did not detect that `(?!$)$` will always reject. We should fix that.


Pull request:
Improve detection for `regexp/no-useless-assertion`
Fixes #661.

The new condition for always accepting is relatively simple. It's just the previous check, but it accounts for edges properly and allows more assertions. 

The complex part of this PR is the `isSingleCharacterAssertion` function. Implementing this function was surprisingly complex. Since we can't modify the AST, we have to handle some pretty complex cases. But even with those complex cases handled, `isSingleCharacterAssertion` is still not perfect and will return false negatives at times. Fortunately, false negatives aren't a problem for us.

Patch:
diff --git a/.changeset/odd-oranges-matter.md b/.changeset/odd-oranges-matter.md
new file mode 100644
index 0000000..2c3d260
--- /dev/null
+++ b/.changeset/odd-oranges-matter.md
@@ -0,0 +1,5 @@
+---
+"eslint-plugin-regexp": minor
+---
+
+Improve detection of useless assertions for `regexp/no-useless-assertion`
diff --git a/lib/rules/no-useless-assertions.ts b/lib/rules/no-useless-assertions.ts
index 422fdcb..808b147 100644
--- a/lib/rules/no-useless-assertions.ts
+++ b/lib/rules/no-useless-assertions.ts
@@ -4,6 +4,7 @@ import type {
     EdgeAssertion,
     Element,
     LookaroundAssertion,
+    Node,
     WordBoundaryAssertion,
 } from "@eslint-community/regexpp/ast"
 import type { RegExpContext } from "../utils"
@@ -23,10 +24,106 @@ import {
     isPotentiallyEmpty,
     isZeroLength,
     FirstConsumedChars,
+    invertMatchingDirection,
 } from "regexp-ast-analysis"
 import { mention } from "../utils/mention"
 import { assertNever } from "../utils/util"
 
+function containsAssertion(n: Node): boolean {
+    return hasSomeDescendant(n, (d) => d.type === "Assertion")
+}
+
+/**
+ * Returns whether the given lookaround asserts exactly one character in the given direction.
+ */
+function isSingleCharacterAssertion(
+    assertion: Assertion,
+    direction: MatchingDirection,
+    flags: ReadonlyFlags,
+): boolean {
+    switch (assertion.kind) {
+        case "word":
+            // \b and \B assert one character in BOTH directions
+            return false
+
+        case "start":
+            return direction === "rtl"
+        case "end":
+            return direction === "ltr"
+
+        default:
+            break
+    }
+
+    if (getMatchingDirectionFromAssertionKind(assertion.kind) !== direction) {
+        return false
+    }
+
+    return assertion.alternatives.every((alt) => {
+        if (!containsAssertion(alt)) {
+            // if we don't contains assertions, then we can only need to check
+            // the length range of the consumed characters
+            const range = getLengthRange(alt, flags)
+            return range.min === 1 && range.max === 1
+        }
+
+        // now it gets tricky
+        // e.g. (?=(?=[a-z])\w(?<=[a-g])) is a single character assertion
+
+        let consumed = false
+        let asserted = false
+        const elements =
+            direction === "ltr" ? alt.elements : [...alt.elements].reverse()
+        for (const e of elements) {
+            if (!consumed) {
+                // before we consumed our first (and only) character, we can
+                // have single character assertions
+
+                if (
+                    e.type === "Assertion" &&
+                    isSingleCharacterAssertion(e, direction, flags)
+                ) {
+                    asserted = true
+                    continue
+                }
+
+                if (containsAssertion(e)) {
+                    // too complex to reason about, so we just give up
+                    return false
+                }
+
+                const range = getLengthRange(e, flags)
+                if (range.max === 0) {
+                    // we haven't consumed anything, so onto the next one
+                    continue
+                } else if (range.min === 1 && range.max === 1) {
+                    // finally, a character
+                    consumed = true
+                } else {
+                    // it's not exactly a single character
+                    return false
+                }
+            } else {
+                // since we already consumed our first (and only) character, we
+                // can at most have assertions in the direction of the
+                // first character
+
+                const otherDir = invertMatchingDirection(direction)
+                if (
+                    e.type === "Assertion" &&
+                    isSingleCharacterAssertion(e, otherDir, flags)
+                ) {
+                    continue
+                }
+
+                return false
+            }
+        }
+
+        return consumed || asserted
+    })
+}
+
 /**
  * Combines 2 look chars such that the result is equivalent to 2 adjacent
  * assertions `(?=a)(?=b)`.
@@ -356,40 +453,31 @@ export default createRule("no-useless-assertions", {
                     )
                 }
 
-                if (after.edge) {
-                    return
-                }
-
-                // accept is harder because that can't generally be decided by the first character
-
-                // if this contains another assertion then that might reject. It's out of our control
+                // We can only decide the accept case for exact single-character assertions.
+                // We want the character after the assertion to be a subset of the asserted characters. For this to be
+                // correct, the set of assertion characters needs to be exact. We also have to consider edges. Edges
+                // can be thought of as a special character, so the same subset requirement applies.
+                const edgeSubset = firstOf.edge || !after.edge
                 if (
-                    !hasSomeDescendant(
+                    firstOf.exact &&
+                    edgeSubset &&
+                    after.char.isSubsetOf(firstOf.char) &&
+                    isSingleCharacterAssertion(
                         assertion,
-                        (d) => d !== assertion && d.type === "Assertion",
+                        getMatchingDirectionFromAssertionKind(assertion.kind),
+                        flags,
                     )
                 ) {
-                    const range = getLengthRange(assertion.alternatives, flags)
-                    // we only check the first character, so it's only correct if the assertion requires only one
-                    // character
-                    if (range.max === 1) {
-                        // require exactness
-                        if (
-                            firstOf.exact &&
-                            after.char.isSubsetOf(firstOf.char)
-                        ) {
-                            report(
-                                assertion,
-                                assertion.negate
-                                    ? "alwaysForNegativeLookaround"
-                                    : "alwaysForLookaround",
-                                {
-                                    kind: assertion.kind,
-                                    acceptOrReject: accept,
-                                },
-                            )
-                        }
-                    }
+                    report(
+                        assertion,
+                        assertion.negate
+                            ? "alwaysForNegativeLookaround"
+                            : "alwaysForLookaround",
+                        {
+                            kind: assertion.kind,
+                            acceptOrReject: accept,
+                        },
+                    )
                 }
             }
 


Test patch:
diff --git a/tests/lib/rules/no-useless-assertions.ts b/tests/lib/rules/no-useless-assertions.ts
index 5dedfa4..b7c755f 100644
--- a/tests/lib/rules/no-useless-assertions.ts
+++ b/tests/lib/rules/no-useless-assertions.ts
@@ -433,5 +433,26 @@ tester.run("no-useless-assertions", rule as any, {
                 "'\\B' will always accept because it is preceded by a non-word character and followed by a non-word character.",
             ],
         },
+
+        {
+            code: String.raw`/(?!$)$/`,
+            errors: ["The negative lookahead '(?!$)' will always reject."],
+        },
+        {
+            code: String.raw`/(?=a|$)a/`,
+            errors: ["The lookahead '(?=a|$)' will always accept."],
+        },
+        {
+            code: String.raw`/(?=(?=[a-f])(?=a|A)[\w%])a/`,
+            errors: [
+                "The lookahead '(?=(?=[a-f])(?=a|A)[\\w%])' will always accept.",
+            ],
+        },
+        {
+            code: String.raw`/(?=(?=[a-f])(?=[aA])\w(?<=[aA])(?<=[a-f]))a/`,
+            errors: [
+                "The lookahead '(?=(?=[a-f])(?=[aA])\\w(?<=[aA])(?<=[a-f]))' will always accept.",
+            ],
+        },
     ],
 })