Always allow explicit multi-line concatenations when implicit are banned (#12532)

## Summary

Closes https://github.com/astral-sh/ruff/issues/11582.

crates/ruff_linter/src/checkers/ast/analyze/expression.rs

@@ -1197,9 +1197,11 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             op: Operator::Add, ..
         }) => {
             if checker.enabled(Rule::ExplicitStringConcatenation) {
-                if let Some(diagnostic) =
-                    flake8_implicit_str_concat::rules::explicit(expr, checker.locator)
-                {
+                if let Some(diagnostic) = flake8_implicit_str_concat::rules::explicit(
+                    expr,
+                    checker.locator,
+                    checker.settings,
+                ) {
                     checker.diagnostics.push(diagnostic);
                 }
             }

crates/ruff_linter/src/checkers/tokens.rs

@@ -125,9 +125,9 @@ pub(crate) fn check_tokens(
         flake8_implicit_str_concat::rules::implicit(
             &mut diagnostics,
             tokens,
-            settings,
             locator,
             indexer,
+            settings,
         );
     }
 

crates/ruff_linter/src/rules/flake8_implicit_str_concat/rules/explicit.rs

@@ -1,5 +1,6 @@
 use ruff_python_ast::{self as ast, Expr, Operator};
 
+use crate::settings::LinterSettings;
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_source_file::Locator;
@@ -40,7 +41,19 @@ impl Violation for ExplicitStringConcatenation {
 }
 
 /// ISC003
-pub(crate) fn explicit(expr: &Expr, locator: &Locator) -> Option<Diagnostic> {
+pub(crate) fn explicit(
+    expr: &Expr,
+    locator: &Locator,
+    settings: &LinterSettings,
+) -> Option<Diagnostic> {
+    // If the user sets `allow-multiline` to `false`, then we should allow explicitly concatenated
+    // strings that span multiple lines even if this rule is enabled. Otherwise, there's no way
+    // for the user to write multiline strings, and that setting is "more explicit" than this rule
+    // being enabled.
+    if !settings.flake8_implicit_str_concat.allow_multiline {
+        return None;
+    }
+
     if let Expr::BinOp(ast::ExprBinOp {
         left,
         op,

crates/ruff_linter/src/rules/flake8_implicit_str_concat/rules/implicit.rs

@@ -93,9 +93,9 @@ impl Violation for MultiLineImplicitStringConcatenation {
 pub(crate) fn implicit(
     diagnostics: &mut Vec<Diagnostic>,
     tokens: &Tokens,
-    settings: &LinterSettings,
     locator: &Locator,
     indexer: &Indexer,
+    settings: &LinterSettings,
 ) {
     for (a_token, b_token) in tokens
         .iter()

crates/ruff_linter/src/rules/flake8_implicit_str_concat/snapshots/ruff_linter__rules__flake8_implicit_str_concat__tests__multiline_ISC003_ISC.py.snap

@@ -1,55 +1,4 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_implicit_str_concat/mod.rs
 ---
-ISC.py:9:3: ISC003 Explicitly concatenated string should be implicitly concatenated
-   |
- 8 |   _ = (
- 9 |     "abc" +
-   |  ___^
-10 | |   "def"
-   | |_______^ ISC003
-11 |   )
-   |
-
-ISC.py:14:3: ISC003 Explicitly concatenated string should be implicitly concatenated
-   |
-13 |   _ = (
-14 |     f"abc" +
-   |  ___^
-15 | |   "def"
-   | |_______^ ISC003
-16 |   )
-   |
-
-ISC.py:19:3: ISC003 Explicitly concatenated string should be implicitly concatenated
-   |
-18 |   _ = (
-19 |     b"abc" +
-   |  ___^
-20 | |   b"def"
-   | |________^ ISC003
-21 |   )
-   |
-
-ISC.py:78:10: ISC003 Explicitly concatenated string should be implicitly concatenated
-   |
-77 |   # Explicitly concatenated nested f-strings
-78 |   _ = f"a {f"first"
-   |  __________^
-79 | |     + f"second"} d"
-   | |_______________^ ISC003
-80 |   _ = f"a {f"first {f"middle"}"
-81 |       + f"second"} d"
-   |
-
-ISC.py:80:10: ISC003 Explicitly concatenated string should be implicitly concatenated
-   |
-78 |   _ = f"a {f"first"
-79 |       + f"second"} d"
-80 |   _ = f"a {f"first {f"middle"}"
-   |  __________^
-81 | |     + f"second"} d"
-   | |_______________^ ISC003
-   |
-
 

crates/ruff_workspace/src/options.rs

@@ -1260,10 +1260,11 @@ pub struct Flake8ImplicitStrConcatOptions {
     /// allowed (but continuation lines, delimited with a backslash, are
     /// prohibited).
     ///
-    /// Note that setting `allow-multiline = false` should typically be coupled
-    /// with disabling `explicit-string-concatenation` (`ISC003`). Otherwise,
-    /// both explicit and implicit multiline string concatenations will be seen
-    /// as violations.
+    /// Setting `allow-multiline = false` will automatically disable the
+    /// `explicit-string-concatenation` (`ISC003`) rule. Otherwise, both
+    /// implicit and explicit multiline string concatenations would be seen
+    /// as violations, making it impossible to write a linter-compliant multiline
+    /// string.
     #[option(
         default = r#"true"#,
         value_type = "bool",

ruff.schema.json

@@ -1033,7 +1033,7 @@
       "type": "object",
       "properties": {
         "allow-multiline": {
-          "description": "Whether to allow implicit string concatenations for multiline strings. By default, implicit concatenations of multiline strings are allowed (but continuation lines, delimited with a backslash, are prohibited).\n\nNote that setting `allow-multiline = false` should typically be coupled with disabling `explicit-string-concatenation` (`ISC003`). Otherwise, both explicit and implicit multiline string concatenations will be seen as violations.",
+          "description": "Whether to allow implicit string concatenations for multiline strings. By default, implicit concatenations of multiline strings are allowed (but continuation lines, delimited with a backslash, are prohibited).\n\nSetting `allow-multiline = false` will automatically disable the `explicit-string-concatenation` (`ISC003`) rule. Otherwise, both implicit and explicit multiline string concatenations would be seen as violations, making it impossible to write a linter-compliant multiline string.",
           "type": [
             "boolean",
             "null"