clippy_lints/src/strings.rs

use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg, span_lint_and_then};
use clippy_utils::source::{snippet, snippet_with_applicability};
use clippy_utils::ty::is_type_lang_item;
use clippy_utils::{
    SpanlessEq, get_expr_use_or_unification_node, get_parent_expr, is_lint_allowed, method_calls, path_def_id,
    peel_blocks, sym,
};
use rustc_errors::Applicability;
use rustc_hir::def_id::DefId;
use rustc_hir::{BinOpKind, BorrowKind, Expr, ExprKind, LangItem, Node, QPath};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::ty;
use rustc_session::declare_lint_pass;
use rustc_span::source_map::Spanned;

use std::ops::ControlFlow;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for string appends of the form `x = x + y` (without
    /// `let`!).
    ///
    /// ### Why is this bad?
    /// It's not really bad, but some people think that the
    /// `.push_str(_)` method is more readable.
    ///
    /// ### Example
    /// ```no_run
    /// let mut x = "Hello".to_owned();
    /// x = x + ", World";
    ///
    /// // More readable
    /// x += ", World";
    /// x.push_str(", World");
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub STRING_ADD_ASSIGN,
    pedantic,
    "using `x = x + ..` where x is a `String` instead of `push_str()`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for all instances of `x + _` where `x` is of type
    /// `String`, but only if [`string_add_assign`](#string_add_assign) does *not*
    /// match.
    ///
    /// ### Why restrict this?
    /// This particular
    /// `Add` implementation is asymmetric (the other operand need not be `String`,
    /// but `x` does), while addition as mathematically defined is symmetric, and
    /// the `String::push_str(_)` function is a perfectly good replacement.
    /// Therefore, some dislike it and wish not to have it in their code.
    ///
    /// That said, other people think that string addition, having a long tradition
    /// in other languages is actually fine, which is why we decided to make this
    /// particular lint `allow` by default.
    ///
    /// ### Example
    /// ```no_run
    /// let x = "Hello".to_owned();
    /// x + ", World";
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// let mut x = "Hello".to_owned();
    /// x.push_str(", World");
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub STRING_ADD,
    restriction,
    "using `x + ..` where x is a `String` instead of `push_str()`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for the `as_bytes` method called on string literals
    /// that contain only ASCII characters.
    ///
    /// ### Why is this bad?
    /// Byte string literals (e.g., `b"foo"`) can be used
    /// instead. They are shorter but less discoverable than `as_bytes()`.
    ///
    /// ### Known problems
    /// `"str".as_bytes()` and the suggested replacement of `b"str"` are not
    /// equivalent because they have different types. The former is `&[u8]`
    /// while the latter is `&[u8; 3]`. That means in general they will have a
    /// different set of methods and different trait implementations.
    ///
    /// ```compile_fail
    /// fn f(v: Vec<u8>) {}
    ///
    /// f("...".as_bytes().to_owned()); // works
    /// f(b"...".to_owned()); // does not work, because arg is [u8; 3] not Vec<u8>
    ///
    /// fn g(r: impl std::io::Read) {}
    ///
    /// g("...".as_bytes()); // works
    /// g(b"..."); // does not work
    /// ```
    ///
    /// The actual equivalent of `"str".as_bytes()` with the same type is not
    /// `b"str"` but `&b"str"[..]`, which is a great deal of punctuation and not
    /// more readable than a function call.
    ///
    /// ### Example
    /// ```no_run
    /// let bstr = "a byte string".as_bytes();
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// let bstr = b"a byte string";
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub STRING_LIT_AS_BYTES,
    nursery,
    "calling `as_bytes` on a string literal instead of using a byte string literal"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for slice operations on strings
    ///
    /// ### Why restrict this?
    /// UTF-8 characters span multiple bytes, and it is easy to inadvertently confuse character
    /// counts and string indices. This may lead to panics, and should warrant some test cases
    /// containing wide UTF-8 characters. This lint is most useful in code that should avoid
    /// panics at all costs.
    ///
    /// ### Known problems
    /// Probably lots of false positives. If an index comes from a known valid position (e.g.
    /// obtained via `char_indices` over the same string), it is totally OK.
    ///
    /// ### Example
    /// ```rust,should_panic
    /// &"Ölkanne"[1..];
    /// ```
    #[clippy::version = "1.58.0"]
    pub STRING_SLICE,
    restriction,
    "slicing a string"
}

declare_lint_pass!(StringAdd => [STRING_ADD, STRING_ADD_ASSIGN, STRING_SLICE]);

impl<'tcx> LateLintPass<'tcx> for StringAdd {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
        if e.span.in_external_macro(cx.sess().source_map()) {
            return;
        }
        match e.kind {
            ExprKind::Binary(
                Spanned {
                    node: BinOpKind::Add, ..
                },
                left,
                _,
            ) => {
                if is_string(cx, left) {
                    if !is_lint_allowed(cx, STRING_ADD_ASSIGN, e.hir_id) {
                        let parent = get_parent_expr(cx, e);
                        if let Some(p) = parent
                            && let ExprKind::Assign(target, _, _) = p.kind
                                // avoid duplicate matches
                                && SpanlessEq::new(cx).eq_expr(target, left)
                        {
                            return;
                        }
                    }
                    span_lint(
                        cx,
                        STRING_ADD,
                        e.span,
                        "you added something to a string. Consider using `String::push_str()` instead",
                    );
                }
            },
            ExprKind::Assign(target, src, _) => {
                if is_string(cx, target) && is_add(cx, src, target) {
                    span_lint(
                        cx,
                        STRING_ADD_ASSIGN,
                        e.span,
                        "you assigned the result of adding something to this string. Consider using \
                         `String::push_str()` instead",
                    );
                }
            },
            ExprKind::Index(target, _idx, _) => {
                let e_ty = cx.typeck_results().expr_ty_adjusted(target).peel_refs();
                if e_ty.is_str() || is_type_lang_item(cx, e_ty, LangItem::String) {
                    span_lint(
                        cx,
                        STRING_SLICE,
                        e.span,
                        "indexing into a string may panic if the index is within a UTF-8 character",
                    );
                }
            },
            _ => {},
        }
    }
}

fn is_string(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
    is_type_lang_item(cx, cx.typeck_results().expr_ty(e).peel_refs(), LangItem::String)
}

fn is_add(cx: &LateContext<'_>, src: &Expr<'_>, target: &Expr<'_>) -> bool {
    match peel_blocks(src).kind {
        ExprKind::Binary(
            Spanned {
                node: BinOpKind::Add, ..
            },
            left,
            _,
        ) => SpanlessEq::new(cx).eq_expr(target, left),
        _ => false,
    }
}

declare_clippy_lint! {
    /// ### What it does
    /// Check if the string is transformed to byte array and casted back to string.
    ///
    /// ### Why is this bad?
    /// It's unnecessary, the string can be used directly.
    ///
    /// ### Example
    /// ```no_run
    /// std::str::from_utf8(&"Hello World!".as_bytes()[6..11]).unwrap();
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// &"Hello World!"[6..11];
    /// ```
    #[clippy::version = "1.50.0"]
    pub STRING_FROM_UTF8_AS_BYTES,
    complexity,
    "casting string slices to byte slices and back"
}

// Max length a b"foo" string can take
const MAX_LENGTH_BYTE_STRING_LIT: usize = 32;

declare_lint_pass!(StringLitAsBytes => [STRING_LIT_AS_BYTES, STRING_FROM_UTF8_AS_BYTES]);

impl<'tcx> LateLintPass<'tcx> for StringLitAsBytes {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
        use rustc_ast::LitKind;

        if let ExprKind::Call(fun, [bytes_arg]) = e.kind
            // Find `std::str::converts::from_utf8` or `std::primitive::str::from_utf8`
            && let Some(sym::str_from_utf8 | sym::str_inherent_from_utf8) =
                path_def_id(cx, fun).and_then(|id| cx.tcx.get_diagnostic_name(id))

            // Find string::as_bytes
            && let ExprKind::AddrOf(BorrowKind::Ref, _, args) = bytes_arg.kind
            && let ExprKind::Index(left, right, _) = args.kind
            && let (method_names, expressions, _) = method_calls(left, 1)
            && method_names == [sym::as_bytes]
            && expressions.len() == 1
            && expressions[0].1.is_empty()

            // Check for slicer
            && let ExprKind::Struct(QPath::LangItem(LangItem::Range, ..), _, _) = right.kind
        {
            let mut applicability = Applicability::MachineApplicable;
            let string_expression = &expressions[0].0;

            let snippet_app = snippet_with_applicability(cx, string_expression.span, "..", &mut applicability);

            span_lint_and_sugg(
                cx,
                STRING_FROM_UTF8_AS_BYTES,
                e.span,
                "calling a slice of `as_bytes()` with `from_utf8` should be not necessary",
                "try",
                format!("Some(&{snippet_app}[{}])", snippet(cx, right.span, "..")),
                applicability,
            );
        }

        if !e.span.in_external_macro(cx.sess().source_map())
            && let ExprKind::MethodCall(path, receiver, ..) = &e.kind
            && path.ident.name == sym::as_bytes
            && let ExprKind::Lit(lit) = &receiver.kind
            && let LitKind::Str(lit_content, _) = &lit.node
        {
            let callsite = snippet(cx, receiver.span.source_callsite(), r#""foo""#);
            let mut applicability = Applicability::MachineApplicable;
            if callsite.starts_with("include_str!") {
                span_lint_and_sugg(
                    cx,
                    STRING_LIT_AS_BYTES,
                    e.span,
                    "calling `as_bytes()` on `include_str!(..)`",
                    "consider using `include_bytes!(..)` instead",
                    snippet_with_applicability(cx, receiver.span.source_callsite(), r#""foo""#, &mut applicability)
                        .replacen("include_str", "include_bytes", 1),
                    applicability,
                );
            } else if lit_content.as_str().is_ascii()
                && lit_content.as_str().len() <= MAX_LENGTH_BYTE_STRING_LIT
                && !receiver.span.from_expansion()
            {
                if let Some((parent, id)) = get_expr_use_or_unification_node(cx.tcx, e)
                    && let Node::Expr(parent) = parent
                    && let ExprKind::Match(scrutinee, ..) = parent.kind
                    && scrutinee.hir_id == id
                {
                    // Don't lint. Byte strings produce `&[u8; N]` whereas `as_bytes()` produces
                    // `&[u8]`. This change would prevent matching with different sized slices.
                } else if !callsite.starts_with("env!") {
                    span_lint_and_sugg(
                        cx,
                        STRING_LIT_AS_BYTES,
                        e.span,
                        "calling `as_bytes()` on a string literal",
                        "consider using a byte string literal instead",
                        format!(
                            "b{}",
                            snippet_with_applicability(cx, receiver.span, r#""foo""#, &mut applicability)
                        ),
                        applicability,
                    );
                }
            }
        }

        if let ExprKind::MethodCall(path, recv, [], _) = &e.kind
            && path.ident.name == sym::into_bytes
            && let ExprKind::MethodCall(path, recv, [], _) = &recv.kind
            && matches!(path.ident.name, sym::to_owned | sym::to_string)
            && let ExprKind::Lit(lit) = &recv.kind
            && let LitKind::Str(lit_content, _) = &lit.node
            && lit_content.as_str().is_ascii()
            && lit_content.as_str().len() <= MAX_LENGTH_BYTE_STRING_LIT
            && !recv.span.from_expansion()
        {
            let mut applicability = Applicability::MachineApplicable;

            span_lint_and_sugg(
                cx,
                STRING_LIT_AS_BYTES,
                e.span,
                "calling `into_bytes()` on a string literal",
                "consider using a byte string literal instead",
                format!(
                    "b{}.to_vec()",
                    snippet_with_applicability(cx, recv.span, r#""..""#, &mut applicability)
                ),
                applicability,
            );
        }
    }
}

declare_clippy_lint! {
    /// ### What it does
    /// This lint checks for `.to_string()` method calls on values of type `&str`.
    ///
    /// ### Why restrict this?
    /// The `to_string` method is also used on other types to convert them to a string.
    /// When called on a `&str` it turns the `&str` into the owned variant `String`, which can be
    /// more specifically expressed with `.to_owned()`.
    ///
    /// ### Example
    /// ```no_run
    /// let _ = "str".to_string();
    /// ```
    /// Use instead:
    /// ```no_run
    /// let _ = "str".to_owned();
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub STR_TO_STRING,
    restriction,
    "using `to_string()` on a `&str`, which should be `to_owned()`"
}

declare_lint_pass!(StrToString => [STR_TO_STRING]);

impl<'tcx> LateLintPass<'tcx> for StrToString {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'_>) {
        if expr.span.from_expansion() {
            return;
        }

        if let ExprKind::MethodCall(path, self_arg, [], _) = &expr.kind
            && path.ident.name == sym::to_string
            && let ty = cx.typeck_results().expr_ty(self_arg)
            && let ty::Ref(_, ty, ..) = ty.kind()
            && ty.is_str()
        {
            span_lint_and_then(
                cx,
                STR_TO_STRING,
                expr.span,
                "`to_string()` called on a `&str`",
                |diag| {
                    let mut applicability = Applicability::MachineApplicable;
                    let snippet = snippet_with_applicability(cx, self_arg.span, "..", &mut applicability);
                    diag.span_suggestion(expr.span, "try", format!("{snippet}.to_owned()"), applicability);
                },
            );
        }
    }
}

declare_clippy_lint! {
    /// ### What it does
    /// This lint checks for `.to_string()` method calls on values of type `String`.
    ///
    /// ### Why restrict this?
    /// The `to_string` method is also used on other types to convert them to a string.
    /// When called on a `String` it only clones the `String`, which can be more specifically
    /// expressed with `.clone()`.
    ///
    /// ### Example
    /// ```no_run
    /// let msg = String::from("Hello World");
    /// let _ = msg.to_string();
    /// ```
    /// Use instead:
    /// ```no_run
    /// let msg = String::from("Hello World");
    /// let _ = msg.clone();
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub STRING_TO_STRING,
    restriction,
    "using `to_string()` on a `String`, which should be `clone()`"
}

declare_lint_pass!(StringToString => [STRING_TO_STRING]);

fn is_parent_map_like(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<rustc_span::Span> {
    if let Some(parent_expr) = get_parent_expr(cx, expr)
        && let ExprKind::MethodCall(name, _, _, parent_span) = parent_expr.kind
        && name.ident.name == sym::map
        && let Some(caller_def_id) = cx.typeck_results().type_dependent_def_id(parent_expr.hir_id)
        && (clippy_utils::is_diag_item_method(cx, caller_def_id, sym::Result)
            || clippy_utils::is_diag_item_method(cx, caller_def_id, sym::Option)
            || clippy_utils::is_diag_trait_item(cx, caller_def_id, sym::Iterator))
    {
        Some(parent_span)
    } else {
        None
    }
}

fn is_called_from_map_like(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<rustc_span::Span> {
    // Look for a closure as parent of `expr`, discarding simple blocks
    let parent_closure = cx
        .tcx
        .hir_parent_iter(expr.hir_id)
        .try_fold(expr.hir_id, |child_hir_id, (_, node)| match node {
            // Check that the child expression is the only expression in the block
            Node::Block(block) if block.stmts.is_empty() && block.expr.map(|e| e.hir_id) == Some(child_hir_id) => {
                ControlFlow::Continue(block.hir_id)
            },
            Node::Expr(expr) if matches!(expr.kind, ExprKind::Block(..)) => ControlFlow::Continue(expr.hir_id),
            Node::Expr(expr) if matches!(expr.kind, ExprKind::Closure(_)) => ControlFlow::Break(Some(expr)),
            _ => ControlFlow::Break(None),
        })
        .break_value()?;
    is_parent_map_like(cx, parent_closure?)
}

fn suggest_cloned_string_to_string(cx: &LateContext<'_>, span: rustc_span::Span) {
    span_lint_and_sugg(
        cx,
        STRING_TO_STRING,
        span,
        "`to_string()` called on a `String`",
        "try",
        "cloned()".to_string(),
        Applicability::MachineApplicable,
    );
}

impl<'tcx> LateLintPass<'tcx> for StringToString {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'_>) {
        if expr.span.from_expansion() {
            return;
        }

        match &expr.kind {
            ExprKind::MethodCall(path, self_arg, [], _) => {
                if path.ident.name == sym::to_string
                    && let ty = cx.typeck_results().expr_ty(self_arg)
                    && is_type_lang_item(cx, ty.peel_refs(), LangItem::String)
                {
                    if let Some(parent_span) = is_called_from_map_like(cx, expr) {
                        suggest_cloned_string_to_string(cx, parent_span);
                    } else {
                        #[expect(clippy::collapsible_span_lint_calls, reason = "rust-clippy#7797")]
                        span_lint_and_then(
                            cx,
                            STRING_TO_STRING,
                            expr.span,
                            "`to_string()` called on a `String`",
                            |diag| {
                                diag.help("consider using `.clone()`");
                            },
                        );
                    }
                }
            },
            ExprKind::Path(QPath::TypeRelative(ty, segment)) => {
                if segment.ident.name == sym::to_string
                    && let rustc_hir::TyKind::Path(QPath::Resolved(_, path)) = ty.peel_refs().kind
                    && let rustc_hir::def::Res::Def(_, def_id) = path.res
                    && cx
                        .tcx
                        .lang_items()
                        .get(LangItem::String)
                        .is_some_and(|lang_id| lang_id == def_id)
                    && let Some(parent_span) = is_parent_map_like(cx, expr)
                {
                    suggest_cloned_string_to_string(cx, parent_span);
                }
            },
            _ => {},
        }
    }
}

declare_clippy_lint! {
    /// ### What it does
    /// Warns about calling `str::trim` (or variants) before `str::split_whitespace`.
    ///
    /// ### Why is this bad?
    /// `split_whitespace` already ignores leading and trailing whitespace.
    ///
    /// ### Example
    /// ```no_run
    /// " A B C ".trim().split_whitespace();
    /// ```
    /// Use instead:
    /// ```no_run
    /// " A B C ".split_whitespace();
    /// ```
    #[clippy::version = "1.62.0"]
    pub TRIM_SPLIT_WHITESPACE,
    style,
    "using `str::trim()` or alike before `str::split_whitespace`"
}
declare_lint_pass!(TrimSplitWhitespace => [TRIM_SPLIT_WHITESPACE]);

impl<'tcx> LateLintPass<'tcx> for TrimSplitWhitespace {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'_>) {
        let tyckres = cx.typeck_results();
        if let ExprKind::MethodCall(path, split_recv, [], split_ws_span) = expr.kind
            && path.ident.name == sym::split_whitespace
            && let Some(split_ws_def_id) = tyckres.type_dependent_def_id(expr.hir_id)
            && cx.tcx.is_diagnostic_item(sym::str_split_whitespace, split_ws_def_id)
            && let ExprKind::MethodCall(path, _trim_recv, [], trim_span) = split_recv.kind
            && let trim_fn_name @ (sym::trim | sym::trim_start | sym::trim_end) = path.ident.name
            && let Some(trim_def_id) = tyckres.type_dependent_def_id(split_recv.hir_id)
            && is_one_of_trim_diagnostic_items(cx, trim_def_id)
        {
            span_lint_and_sugg(
                cx,
                TRIM_SPLIT_WHITESPACE,
                trim_span.with_hi(split_ws_span.lo()),
                format!("found call to `str::{trim_fn_name}` before `str::split_whitespace`"),
                format!("remove `{trim_fn_name}()`"),
                String::new(),
                Applicability::MachineApplicable,
            );
        }
    }
}

fn is_one_of_trim_diagnostic_items(cx: &LateContext<'_>, trim_def_id: DefId) -> bool {
    cx.tcx.is_diagnostic_item(sym::str_trim, trim_def_id)
        || cx.tcx.is_diagnostic_item(sym::str_trim_start, trim_def_id)
        || cx.tcx.is_diagnostic_item(sym::str_trim_end, trim_def_id)
}