8733: Suggest `str.lines` when splitting at hard-coded newlines

Adds a new `splitting_strings_at_newlines` lint that suggests to use
`str.lines` instead of splitting a trimmed string at hard-coded
newlines.

2 files changed, 73 insertions, 0 deletions

diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
index 0cde17ef5ad..711cad1dcf6 100644
--- a/clippy_lints/src/methods/mod.rs
+++ b/clippy_lints/src/methods/mod.rs
@@ -94,6 +94,7 @@ mod single_char_pattern;
 mod single_char_push_string;
 mod skip_while_next;
 mod stable_sort_primitive;
+mod str_split;
 mod str_splitn;
 mod string_extend_chars;
 mod string_lit_chars_any;
@@ -3856,6 +3857,36 @@ declare_clippy_lint! {
     "using `.map(f).unwrap_or_default()`, which is more succinctly expressed as `is_some_and(f)` or `is_ok_and(f)`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for usages of `str.trim().split("\n")` and `str.trim().split("\r\n")`.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// Hard-coding the line endings makes the code less compatible. `str.lines` should be used instead.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// "some\ntext\nwith\nnewlines\n".trim().split('\n');
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// "some\ntext\nwith\nnewlines\n".lines();
+    /// ```
+    ///
+    /// ### Known Problems
+    ///
+    /// This lint cannot detect if the split is intentionally restricted to a single type of newline (`"\n"` or
+    /// `"\r\n"`), for example during the parsing of a specific file format in which precisely one newline type is
+    /// valid.
+    /// ```
+    #[clippy::version = "1.76.0"]
+    pub STR_SPLIT_AT_NEWLINE,
+    pedantic,
+    "splitting a trimmed string at hard-coded newlines"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Msrv,
@@ -4011,6 +4042,7 @@ impl_lint_pass!(Methods => [
     ITER_FILTER_IS_SOME,
     ITER_FILTER_IS_OK,
     MANUAL_IS_VARIANT_AND,
+    STR_SPLIT_AT_NEWLINE,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -4597,6 +4629,9 @@ impl Methods {
                 ("sort_unstable_by", [arg]) => {
                     unnecessary_sort_by::check(cx, expr, recv, arg, true);
                 },
+                ("split", [arg]) => {
+                    str_split::check(cx, expr, recv, arg);
+                },
                 ("splitn" | "rsplitn", [count_arg, pat_arg]) => {
                     if let Some(Constant::Int(count)) = constant(cx, cx.typeck_results(), count_arg) {
                         suspicious_splitn::check(cx, name, expr, recv, count);
diff --git a/clippy_lints/src/methods/str_split.rs b/clippy_lints/src/methods/str_split.rs
new file mode 100644
index 00000000000..3586e11f56a
--- /dev/null
+++ b/clippy_lints/src/methods/str_split.rs
@@ -0,0 +1,38 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_with_context;
+use clippy_utils::visitors::is_const_evaluatable;
+use rustc_ast::ast::LitKind;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+
+use super::STR_SPLIT_AT_NEWLINE;
+
+pub(super) fn check<'a>(cx: &LateContext<'a>, expr: &'_ Expr<'_>, split_recv: &'a Expr<'_>, split_arg: &'_ Expr<'_>) {
+    // We're looking for `A.trim().split(B)`, where the adjusted type of `A` is `&str` (e.g. an
+    // expression returning `String`), and `B` is a `Pattern` that hard-codes a newline (either `"\n"`
+    // or `"\r\n"`). There are a lot of ways to specify a pattern, and this lint only checks the most
+    // basic ones: a `'\n'`, `"\n"`, and `"\r\n"`.
+    if let ExprKind::MethodCall(trim_method_name, trim_recv, [], _) = split_recv.kind
+        && trim_method_name.ident.as_str() == "trim"
+        && cx.typeck_results().expr_ty_adjusted(trim_recv).peel_refs().is_str()
+        && !is_const_evaluatable(cx, trim_recv)
+        && let ExprKind::Lit(split_lit) = split_arg.kind
+        && (matches!(split_lit.node, LitKind::Char('\n'))
+            || matches!(split_lit.node, LitKind::Str(sym, _) if (sym.as_str() == "\n" || sym.as_str() == "\r\n")))
+    {
+        let mut app = Applicability::MaybeIncorrect;
+        span_lint_and_sugg(
+            cx,
+            STR_SPLIT_AT_NEWLINE,
+            expr.span,
+            "using `str.trim().split()` with hard-coded newlines",
+            "use `str.lines()` instead",
+            format!(
+                "{}.lines()",
+                snippet_with_context(cx, trim_recv.span, expr.span.ctxt(), "..", &mut app).0
+            ),
+            app,
+        );
+    }
+}