Add new lint: manual_slice_match

Add a new pedantic lint that detects `if`/`else if` chains whose
conditions only inspect the length/emptiness of one and the same slice
or `Vec` (via `.is_empty()` or `.len()` compared against an integer
literal) and rewrites them as a `match` on a slice pattern.

The lint is conservative: it requires at least two conditions over the
same receiver and an explicit final `else`, and rejects chains that mix
in value guards. It generates concrete arms (`[]`, `[_]`, `[_, _, ..]`)
where the length predicates are expressible as slice patterns, and falls
back to a help-only diagnostic for comparisons that cannot be a single
pattern set (`<`, `<=`, `!=`). Arrays are not linted, since their length
is a compile-time constant. The suggestion is `MaybeIncorrect` (bodies
are copied verbatim and matching a `Vec` borrows the receiver across the
arms). It is gated on the slice-patterns MSRV (1.42).

Author: sylvestre

CHANGELOG.md

@@ -7025,6 +7025,7 @@ Released 2018-09-13
 [`manual_rotate`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_rotate
 [`manual_saturating_arithmetic`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_saturating_arithmetic
 [`manual_slice_fill`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_fill
+[`manual_slice_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_match
 [`manual_slice_size_calculation`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_size_calculation
 [`manual_split_once`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_split_once
 [`manual_str_repeat`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_str_repeat

clippy_lints/src/declared_lints.rs

@@ -322,6 +322,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::manual_rem_euclid::MANUAL_REM_EUCLID_INFO,
     crate::manual_retain::MANUAL_RETAIN_INFO,
     crate::manual_rotate::MANUAL_ROTATE_INFO,
+    crate::manual_slice_match::MANUAL_SLICE_MATCH_INFO,
     crate::manual_slice_size_calculation::MANUAL_SLICE_SIZE_CALCULATION_INFO,
     crate::manual_string_new::MANUAL_STRING_NEW_INFO,
     crate::manual_strip::MANUAL_STRIP_INFO,

clippy_lints/src/lib.rs

@@ -221,6 +221,7 @@ mod manual_range_patterns;
 mod manual_rem_euclid;
 mod manual_retain;
 mod manual_rotate;
+mod manual_slice_match;
 mod manual_slice_size_calculation;
 mod manual_string_new;
 mod manual_strip;
@@ -862,6 +863,7 @@ rustc_lint::late_lint_methods!(
         ByteCharSlice: byte_char_slices::ByteCharSlice = byte_char_slices::ByteCharSlice,
         ManualAssertEq: manual_assert_eq::ManualAssertEq = manual_assert_eq::ManualAssertEq,
         WithCapacityZero: with_capacity_zero::WithCapacityZero = with_capacity_zero::WithCapacityZero,
+        ManualSliceMatch: manual_slice_match::ManualSliceMatch = manual_slice_match::ManualSliceMatch::new(conf),
         // add late passes here, used by `cargo dev new_lint`
     ]]
 );

clippy_lints/src/manual_slice_match.rs

@@ -0,0 +1,310 @@
+use clippy_config::Conf;
+use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
+use clippy_utils::msrvs::{self, Msrv};
+use clippy_utils::res::MaybeDef;
+use clippy_utils::source::{snippet_indent, snippet_opt};
+use clippy_utils::sugg::Sugg;
+use clippy_utils::{SpanlessEq, if_sequence, is_else_clause, is_in_const_context, sym};
+use rustc_ast::LitKind;
+use rustc_data_structures::packed::Pu128;
+use rustc_errors::Applicability;
+use rustc_hir::{BinOpKind, Block, Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::{self, Ty};
+use rustc_session::impl_lint_pass;
+use rustc_span::SyntaxContext;
+use std::fmt::Write;
+
+const MSG: &str = "`if` chain checking only the length can be rewritten with a `match` on a slice pattern";
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for `if`/`else if` chains whose conditions only inspect the
+    /// length of one and the same slice, `Vec` or array (via `.is_empty()` or
+    /// `.len()` compared against an integer literal) and which can be rewritten
+    /// as a `match` on a slice pattern.
+    ///
+    /// ### Why is this bad?
+    /// `if` chains are not checked for exhaustiveness and the length checks tend
+    /// to be repetitive. A `match` on a slice pattern is exhaustive and makes the
+    /// handled shapes explicit, and the bound elements can be named instead of
+    /// being indexed (which avoids potential out-of-bounds panics).
+    ///
+    /// ### Example
+    /// ```no_run
+    /// # let v: Vec<u32> = vec![];
+    /// if v.is_empty() {
+    ///     // ...
+    /// } else if v.len() == 1 {
+    ///     println!("{}", v[0]);
+    /// } else {
+    ///     // ...
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// # let v: Vec<u32> = vec![];
+    /// match v.as_slice() {
+    ///     [] => { /* ... */ }
+    ///     [single] => println!("{single}"),
+    ///     _ => { /* ... */ }
+    /// }
+    /// ```
+    #[clippy::version = "1.98.0"]
+    pub MANUAL_SLICE_MATCH,
+    pedantic,
+    "`if` length-check chains that can be rewritten as a `match` on a slice pattern"
+}
+
+impl_lint_pass!(ManualSliceMatch => [MANUAL_SLICE_MATCH]);
+
+pub struct ManualSliceMatch {
+    msrv: Msrv,
+}
+
+impl ManualSliceMatch {
+    pub fn new(conf: &'static Conf) -> Self {
+        Self { msrv: conf.msrv }
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for ManualSliceMatch {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if expr.span.from_expansion() {
+            return;
+        }
+
+        // Only look at the top-most `if` in the chain.
+        if is_else_clause(cx.tcx, expr) {
+            return;
+        }
+
+        if is_in_const_context(cx) {
+            return;
+        }
+
+        if !self.msrv.meets(cx, msrvs::SLICE_PATTERNS) {
+            return;
+        }
+
+        let (conds, blocks) = if_sequence(expr);
+
+        // Require at least two conditions and an explicit final `else`, so the
+        // chain is a genuine partition over the collection's shape.
+        if conds.len() < 2 || blocks.len() != conds.len() + 1 {
+            return;
+        }
+
+        // Every condition must be a length/emptiness predicate over the *same*
+        // receiver. The first condition fixes the receiver, the rest must match.
+        let Some((recv, _, _)) = len_predicate(conds[0]) else {
+            return;
+        };
+
+        let mut spanless_eq = SpanlessEq::new(cx);
+        for cond in &conds[1..] {
+            match len_predicate(cond) {
+                Some((other, _, _)) if spanless_eq.eq_expr(SyntaxContext::root(), recv, other) => {},
+                _ => return,
+            }
+        }
+
+        // The receiver has to be something we can match on as a slice.
+        let ty = cx.typeck_results().expr_ty(recv).peel_refs();
+        let Some(scrutinee) = slice_scrutinee(cx, ty, recv) else {
+            return;
+        };
+
+        // Try to build the full `match` with concrete slice-pattern arms. If the
+        // chain uses comparisons we cannot express as a single set of patterns
+        // (e.g. `len() < n` or `len() != n`), fall back to a help-only diagnostic.
+        if let Some(sugg) = build_match(cx, &conds, &blocks, &scrutinee, expr.span) {
+            span_lint_and_sugg(
+                cx,
+                MANUAL_SLICE_MATCH,
+                expr.span,
+                MSG,
+                "consider rewriting the `if` chain with a `match`",
+                sugg,
+                // The bodies are copied verbatim (still using indexing), and matching a
+                // `Vec` via `as_slice()` holds a borrow across the arms, so a body that
+                // moves out of the receiver would not compile. Leave it to the user.
+                Applicability::MaybeIncorrect,
+            );
+        } else {
+            span_lint_and_help(
+                cx,
+                MANUAL_SLICE_MATCH,
+                expr.span,
+                MSG,
+                None,
+                format!("rewrite this as a `match` on a slice pattern, e.g. `match {scrutinee} {{ .. }}`"),
+            );
+        }
+    }
+}
+
+/// Builds the full `match` replacement text with one arm per `if`/`else if`
+/// branch plus a wildcard arm for the final `else`. Returns `None` if any
+/// condition cannot be turned into a slice pattern.
+fn build_match<'tcx>(
+    cx: &LateContext<'tcx>,
+    conds: &[&'tcx Expr<'tcx>],
+    blocks: &[&'tcx Block<'tcx>],
+    scrutinee: &str,
+    span: rustc_span::Span,
+) -> Option<String> {
+    let indent = snippet_indent(cx, span).unwrap_or_default();
+    let arm_indent = format!("{indent}    ");
+    let last = conds.len() - 1;
+
+    let mut arms = String::new();
+    let mut exact_lengths = Vec::new();
+    for (i, cond) in conds.iter().enumerate() {
+        let (_, op, n) = len_predicate(cond)?;
+        let pat = arm_pattern(op, n, i == last)?;
+        // Two arms matching the same fixed length would make the second one
+        // unreachable; bail rather than emit a suggestion that warns.
+        if let Some(len) = pat.exact_len {
+            if exact_lengths.contains(&len) {
+                return None;
+            }
+            exact_lengths.push(len);
+        }
+        let body = reindent_block(&snippet_opt(cx, blocks[i].span)?);
+        let _ = writeln!(arms, "{arm_indent}{} => {body}", pat.text);
+    }
+
+    let else_body = reindent_block(&snippet_opt(cx, blocks[last + 1].span)?);
+    let _ = writeln!(arms, "{arm_indent}_ => {else_body}");
+
+    Some(format!("match {scrutinee} {{\n{arms}{indent}}}"))
+}
+
+/// Indents every line of a block snippet except the first by one level, so a body
+/// lifted from an `if` branch nests correctly under its new `match` arm.
+fn reindent_block(snippet: &str) -> String {
+    let mut out = String::new();
+    for (i, line) in snippet.lines().enumerate() {
+        if i != 0 {
+            out.push('\n');
+            if !line.is_empty() {
+                out.push_str("    ");
+            }
+        }
+        out.push_str(line);
+    }
+    out
+}
+
+struct ArmPat {
+    text: String,
+    /// `Some(n)` when the pattern matches exactly `n` elements.
+    exact_len: Option<u128>,
+}
+
+/// Maps a normalized `len <op> n` predicate to a slice pattern. Open-ended
+/// patterns (`> n`, `>= n`) are only allowed in the final condition, where the
+/// following wildcard arm keeps the `match` exhaustive without overlap.
+fn arm_pattern(op: BinOpKind, n: u128, is_last: bool) -> Option<ArmPat> {
+    let underscores = |k: u128| -> Option<String> { Some(vec!["_"; usize::try_from(k).ok()?].join(", ")) };
+    match op {
+        // `len() == n` -> exactly `n` elements.
+        BinOpKind::Eq => Some(ArmPat {
+            text: format!("[{}]", underscores(n)?),
+            exact_len: Some(n),
+        }),
+        // `len() > n` -> at least `n + 1` elements.
+        BinOpKind::Gt if is_last => Some(ArmPat {
+            text: format!("[{}, ..]", underscores(n + 1)?),
+            exact_len: None,
+        }),
+        // `len() >= n` -> at least `n` elements (`n == 0` would match everything).
+        BinOpKind::Ge if is_last && n >= 1 => Some(ArmPat {
+            text: format!("[{}, ..]", underscores(n)?),
+            exact_len: None,
+        }),
+        _ => None,
+    }
+}
+
+/// If `cond` is a length/emptiness predicate, returns its receiver together with
+/// the comparison normalized to `len <op> n` form (the literal on the right) and
+/// the literal value. `recv.is_empty()` is treated as `len == 0`.
+fn len_predicate<'tcx>(cond: &'tcx Expr<'tcx>) -> Option<(&'tcx Expr<'tcx>, BinOpKind, u128)> {
+    match cond.kind {
+        // `recv.is_empty()`
+        ExprKind::MethodCall(path, recv, [], _) if path.ident.name == sym::is_empty => Some((recv, BinOpKind::Eq, 0)),
+        // `recv.len() <cmp> <int>` or `<int> <cmp> recv.len()`
+        ExprKind::Binary(op, lhs, rhs) if is_len_cmp(op.node) => {
+            if let Some(recv) = len_receiver(lhs)
+                && let Some(n) = int_lit_val(rhs)
+            {
+                Some((recv, op.node, n))
+            } else if let Some(recv) = len_receiver(rhs)
+                && let Some(n) = int_lit_val(lhs)
+            {
+                // Flip the operator so the literal ends up on the right.
+                Some((recv, flip_cmp(op.node), n))
+            } else {
+                None
+            }
+        },
+        _ => None,
+    }
+}
+
+/// Returns the receiver of a `recv.len()` call.
+fn len_receiver<'tcx>(expr: &'tcx Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
+    if let ExprKind::MethodCall(path, recv, [], _) = expr.kind
+        && path.ident.name == sym::len
+    {
+        Some(recv)
+    } else {
+        None
+    }
+}
+
+fn int_lit_val(expr: &Expr<'_>) -> Option<u128> {
+    if let ExprKind::Lit(lit) = expr.kind
+        && let LitKind::Int(Pu128(n), _) = lit.node
+    {
+        Some(n)
+    } else {
+        None
+    }
+}
+
+/// Swaps the operands of a comparison: `a <op> b` is equivalent to `b <flipped> a`.
+fn flip_cmp(kind: BinOpKind) -> BinOpKind {
+    match kind {
+        BinOpKind::Lt => BinOpKind::Gt,
+        BinOpKind::Le => BinOpKind::Ge,
+        BinOpKind::Gt => BinOpKind::Lt,
+        BinOpKind::Ge => BinOpKind::Le,
+        other => other,
+    }
+}
+
+fn is_len_cmp(kind: BinOpKind) -> bool {
+    matches!(
+        kind,
+        BinOpKind::Eq | BinOpKind::Ne | BinOpKind::Lt | BinOpKind::Le | BinOpKind::Gt | BinOpKind::Ge
+    )
+}
+
+/// Builds the `match` scrutinee text for a sliceable receiver, or `None` if the
+/// receiver type cannot be matched on as a slice pattern.
+fn slice_scrutinee<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, recv: &'tcx Expr<'tcx>) -> Option<String> {
+    let sugg = Sugg::hir(cx, recv, "..");
+    match ty.kind() {
+        // Slices can be matched directly. Arrays are deliberately excluded: their
+        // length is a compile-time constant, so a length-check chain over an array
+        // is degenerate and a slice pattern of a different length would not even
+        // type-check.
+        ty::Slice(_) => Some(sugg.to_string()),
+        // `Vec` needs an explicit conversion to a slice.
+        _ if ty.is_diag_item(cx, sym::Vec) => Some(format!("{}.as_slice()", sugg.maybe_paren())),
+        _ => None,
+    }
+}