[refurb] Fix false negative for underscores before sign in Decimal constructor (FURB157) (#21190)

## Summary

Fixes FURB157 false negative where `Decimal("_-1")` was not flagged as
verbose when underscores precede the sign character. This fixes #21186.

## Problem Analysis

The `verbose-decimal-constructor` (FURB157) rule failed to detect
verbose `Decimal` constructors when the sign character (`+` or `-`) was
preceded by underscores. For example, `Decimal("_-1")` was not flagged,
even though it can be simplified to `Decimal(-1)`.

The bug occurred because the rule checked for the sign character at the
start of the string before stripping leading underscores. According to
Python's `Decimal` parser behavior (as documented in CPython's
`_pydecimal.py`), underscores are removed before parsing the sign. The
rule's logic didn't match this behavior, causing a false negative for
cases like `"_-1"` where the underscore came before the sign.

This was a regression introduced in version 0.14.3, as these cases were
correctly flagged in version 0.14.2.

## Approach

The fix updates the sign extraction logic to:
1. Strip leading underscores first (matching Python's Decimal parser
behavior)
2. Extract the sign from the underscore-stripped string
3. Preserve the string after the sign for normalization purposes

This ensures that cases like `Decimal("_-1")`, `Decimal("_+1")`, and
`Decimal("_-1_000")` are correctly detected and flagged. The
normalization logic was also updated to use the string after the sign
(without underscores) to avoid double signs in the replacement output.

Author: danparizher

crates/ruff_linter/resources/test/fixtures/refurb/FURB157.py

@@ -85,3 +85,9 @@
 Decimal("0001_2345")
 Decimal("000_1_2345")
 Decimal("000_000")
+
+# Test cases for underscores before sign
+# https://github.com/astral-sh/ruff/issues/21186
+Decimal("_-1")      # Should flag as verbose
+Decimal("_+1")      # Should flag as verbose
+Decimal("_-1_000")  # Should flag as verbose

crates/ruff_linter/src/rules/refurb/rules/verbose_decimal_constructor.rs

@@ -93,16 +93,21 @@ pub(crate) fn verbose_decimal_constructor(checker: &Checker, call: &ast::ExprCal
             // https://github.com/python/cpython/blob/ac556a2ad1213b8bb81372fe6fb762f5fcb076de/Lib/_pydecimal.py#L6060-L6077
             // _after_ trimming whitespace from the string and removing all occurrences of "_".
             let original_str = str_literal.to_str().trim_whitespace();
+            // Strip leading underscores before extracting the sign, as Python's Decimal parser
+            // removes underscores before parsing the sign.
+            let sign_check_str = original_str.trim_start_matches('_');
             // Extract the unary sign, if any.
-            let (unary, original_str) = if let Some(trimmed) = original_str.strip_prefix('+') {
+            let (unary, sign_check_str) = if let Some(trimmed) = sign_check_str.strip_prefix('+') {
                 ("+", trimmed)
-            } else if let Some(trimmed) = original_str.strip_prefix('-') {
+            } else if let Some(trimmed) = sign_check_str.strip_prefix('-') {
                 ("-", trimmed)
             } else {
-                ("", original_str)
+                ("", sign_check_str)
             };
-            let mut rest = Cow::from(original_str);
-            let has_digit_separators = memchr::memchr(b'_', rest.as_bytes()).is_some();
+            // Save the string after the sign for normalization (before removing underscores)
+            let str_after_sign_for_normalization = sign_check_str;
+            let mut rest = Cow::from(sign_check_str);
+            let has_digit_separators = memchr::memchr(b'_', original_str.as_bytes()).is_some();
             if has_digit_separators {
                 rest = Cow::from(rest.replace('_', ""));
             }
@@ -123,7 +128,7 @@ pub(crate) fn verbose_decimal_constructor(checker: &Checker, call: &ast::ExprCal
 
             // If the original string had digit separators, normalize them
             let rest = if has_digit_separators {
-                Cow::from(normalize_digit_separators(original_str))
+                Cow::from(normalize_digit_separators(str_after_sign_for_normalization))
             } else {
                 Cow::from(rest)
             };

crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB157_FURB157.py.snap

@@ -669,6 +669,7 @@ help: Replace with `1_2345`
 85 + Decimal(1_2345)
 86 | Decimal("000_1_2345")
 87 | Decimal("000_000")
+88 | 
 
 FURB157 [*] Verbose expression in `Decimal` constructor
   --> FURB157.py:86:9
@@ -686,6 +687,8 @@ help: Replace with `1_2345`
    - Decimal("000_1_2345")
 86 + Decimal(1_2345)
 87 | Decimal("000_000")
+88 | 
+89 | # Test cases for underscores before sign
 
 FURB157 [*] Verbose expression in `Decimal` constructor
   --> FURB157.py:87:9
@@ -694,10 +697,66 @@ FURB157 [*] Verbose expression in `Decimal` constructor
 86 | Decimal("000_1_2345")
 87 | Decimal("000_000")
    |         ^^^^^^^^^
+88 |
+89 | # Test cases for underscores before sign
    |
 help: Replace with `0`
 84 | # Separators _and_ leading zeros
 85 | Decimal("0001_2345")
 86 | Decimal("000_1_2345")
    - Decimal("000_000")
 87 + Decimal(0)
+88 | 
+89 | # Test cases for underscores before sign
+90 | # https://github.com/astral-sh/ruff/issues/21186
+
+FURB157 [*] Verbose expression in `Decimal` constructor
+  --> FURB157.py:91:9
+   |
+89 | # Test cases for underscores before sign
+90 | # https://github.com/astral-sh/ruff/issues/21186
+91 | Decimal("_-1")      # Should flag as verbose
+   |         ^^^^^
+92 | Decimal("_+1")      # Should flag as verbose
+93 | Decimal("_-1_000")  # Should flag as verbose
+   |
+help: Replace with `-1`
+88 | 
+89 | # Test cases for underscores before sign
+90 | # https://github.com/astral-sh/ruff/issues/21186
+   - Decimal("_-1")      # Should flag as verbose
+91 + Decimal(-1)      # Should flag as verbose
+92 | Decimal("_+1")      # Should flag as verbose
+93 | Decimal("_-1_000")  # Should flag as verbose
+
+FURB157 [*] Verbose expression in `Decimal` constructor
+  --> FURB157.py:92:9
+   |
+90 | # https://github.com/astral-sh/ruff/issues/21186
+91 | Decimal("_-1")      # Should flag as verbose
+92 | Decimal("_+1")      # Should flag as verbose
+   |         ^^^^^
+93 | Decimal("_-1_000")  # Should flag as verbose
+   |
+help: Replace with `+1`
+89 | # Test cases for underscores before sign
+90 | # https://github.com/astral-sh/ruff/issues/21186
+91 | Decimal("_-1")      # Should flag as verbose
+   - Decimal("_+1")      # Should flag as verbose
+92 + Decimal(+1)      # Should flag as verbose
+93 | Decimal("_-1_000")  # Should flag as verbose
+
+FURB157 [*] Verbose expression in `Decimal` constructor
+  --> FURB157.py:93:9
+   |
+91 | Decimal("_-1")      # Should flag as verbose
+92 | Decimal("_+1")      # Should flag as verbose
+93 | Decimal("_-1_000")  # Should flag as verbose
+   |         ^^^^^^^^^
+   |
+help: Replace with `-1_000`
+90 | # https://github.com/astral-sh/ruff/issues/21186
+91 | Decimal("_-1")      # Should flag as verbose
+92 | Decimal("_+1")      # Should flag as verbose
+   - Decimal("_-1_000")  # Should flag as verbose
+93 + Decimal(-1_000)  # Should flag as verbose