Add new lint, checking for .to_string().parse().unwrap()

CHANGELOG.md

@@ -5945,6 +5945,7 @@ Released 2018-09-13
 [`panic_params`]: https://rust-lang.github.io/rust-clippy/master/index.html#panic_params
 [`panicking_overflow_checks`]: https://rust-lang.github.io/rust-clippy/master/index.html#panicking_overflow_checks
 [`panicking_unwrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#panicking_unwrap
+[`parse_to_string`]: https://rust-lang.github.io/rust-clippy/master/index.html#parse_to_string
 [`partial_pub_fields`]: https://rust-lang.github.io/rust-clippy/master/index.html#partial_pub_fields
 [`partialeq_ne_impl`]: https://rust-lang.github.io/rust-clippy/master/index.html#partialeq_ne_impl
 [`partialeq_to_none`]: https://rust-lang.github.io/rust-clippy/master/index.html#partialeq_to_none

clippy_lints/src/declared_lints.rs

@@ -456,6 +456,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::methods::OPTION_MAP_OR_NONE_INFO,
     crate::methods::OR_FUN_CALL_INFO,
     crate::methods::OR_THEN_UNWRAP_INFO,
+    crate::methods::PARSE_TO_STRING_INFO,
     crate::methods::PATH_BUF_PUSH_OVERWRITE_INFO,
     crate::methods::PATH_ENDS_WITH_EXT_INFO,
     crate::methods::RANGE_ZIP_WITH_LEN_INFO,

clippy_lints/src/methods/mod.rs

@@ -86,6 +86,7 @@ mod option_map_or_none;
 mod option_map_unwrap_or;
 mod or_fun_call;
 mod or_then_unwrap;
+mod parse_to_string;
 mod path_buf_push_overwrite;
 mod path_ends_with_ext;
 mod range_zip_with_len;
@@ -4434,6 +4435,28 @@ declare_clippy_lint! {
     "calling .bytes() is very inefficient when data is not in memory"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Finds places where something is converted to string and then parsed.
+    ///
+    /// ### Why is this bad?
+    /// Converting between types through string is often lossy
+    /// and has usually worse performance than a direct approach.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let bad: u64 = 42_u32.to_string().parse().unwrap();
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let good: u64 = 42_u32.into();
+    /// ```
+    #[clippy::version = "1.86.0"]
+    pub PARSE_TO_STRING,
+    suspicious,
+    "parsing after converting to string"
+}
+
 #[expect(clippy::struct_excessive_bools)]
 pub struct Methods {
     avoid_breaking_exported_api: bool,
@@ -4609,6 +4632,7 @@ impl_lint_pass!(Methods => [
     SLICED_STRING_AS_BYTES,
     RETURN_AND_THEN,
     UNBUFFERED_BYTES,
+    PARSE_TO_STRING,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -5333,6 +5357,11 @@ impl Methods {
                         Some(("or", recv, [or_arg], or_span, _)) => {
                             or_then_unwrap::check(cx, expr, recv, or_arg, or_span);
                         },
+                        Some(("parse", recv, _, _, _)) => {
+                            if let Some(("to_string", _, _, _, span)) = method_call(recv) {
+                                parse_to_string::check(cx, expr, span);
+                            }
+                        },
                         _ => {},
                     }
                     unnecessary_literal_unwrap::check(cx, expr, recv, name, args);

clippy_lints/src/methods/parse_to_string.rs

@@ -0,0 +1,12 @@
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_span::Span;
+
+pub fn check(cx: &LateContext<'_>, expr: &Expr<'_>, span: Span) {
+    clippy_utils::diagnostics::span_lint(
+        cx,
+        super::PARSE_TO_STRING,
+        expr.span.with_lo(span.lo()),
+        "parsing a the output of `.to_string()`",
+    );
+}

tests/ui/parse_to_string.rs

@@ -0,0 +1,6 @@
+#![warn(clippy::parse_to_string)]
+
+fn main() {
+    let bad: u64 = 42_u32.to_string().parse().unwrap();
+    //~^ ERROR: parsing
+}

tests/ui/parse_to_string.stderr

@@ -0,0 +1,11 @@
+error: parsing a the output of `.to_string()`
+  --> tests/ui/parse_to_string.rs:4:27
+   |
+LL |     let bad: u64 = 42_u32.to_string().parse().unwrap();
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::parse-to-string` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::parse_to_string)]`
+
+error: aborting due to 1 previous error
+