src/tools/clippy/clippy_lints/src/let_if_seq.rs

use clippy_utils::diagnostics::span_lint_hir_and_then;
use clippy_utils::path_to_local_id;
use clippy_utils::source::snippet;
use clippy_utils::visitors::is_local_used;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_hir::{BindingMode, Mutability};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::declare_lint_pass;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for variable declarations immediately followed by a
    /// conditional affectation.
    ///
    /// ### Why is this bad?
    /// This is not idiomatic Rust.
    ///
    /// ### Example
    /// ```rust,ignore
    /// let foo;
    ///
    /// if bar() {
    ///     foo = 42;
    /// } else {
    ///     foo = 0;
    /// }
    ///
    /// let mut baz = None;
    ///
    /// if bar() {
    ///     baz = Some(42);
    /// }
    /// ```
    ///
    /// should be written
    ///
    /// ```rust,ignore
    /// let foo = if bar() {
    ///     42
    /// } else {
    ///     0
    /// };
    ///
    /// let baz = if bar() {
    ///     Some(42)
    /// } else {
    ///     None
    /// };
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub USELESS_LET_IF_SEQ,
    nursery,
    "unidiomatic `let mut` declaration followed by initialization in `if`"
}

declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);

impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
    fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'_>) {
        for [stmt, next] in block.stmts.array_windows::<2>() {
            if let hir::StmtKind::Let(local) = stmt.kind
                && let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind
                && let hir::StmtKind::Expr(if_) = next.kind
                && let hir::ExprKind::If(
                    hir::Expr {
                        kind: hir::ExprKind::DropTemps(cond),
                        ..
                    },
                    then,
                    else_,
                ) = if_.kind
                && !is_local_used(cx, *cond, canonical_id)
                && let hir::ExprKind::Block(then, _) = then.kind
                && let Some(value) = check_assign(cx, canonical_id, then)
                && !is_local_used(cx, value, canonical_id)
            {
                let span = stmt.span.to(if_.span);

                let has_interior_mutability = !cx
                    .typeck_results()
                    .node_type(canonical_id)
                    .is_freeze(cx.tcx, cx.param_env);
                if has_interior_mutability {
                    return;
                }

                let (default_multi_stmts, default) = if let Some(else_) = else_ {
                    if let hir::ExprKind::Block(else_, _) = else_.kind {
                        if let Some(default) = check_assign(cx, canonical_id, else_) {
                            (else_.stmts.len() > 1, default)
                        } else if let Some(default) = local.init {
                            (true, default)
                        } else {
                            continue;
                        }
                    } else {
                        continue;
                    }
                } else if let Some(default) = local.init {
                    (false, default)
                } else {
                    continue;
                };

                let mutability = match mode {
                    BindingMode(_, Mutability::Mut) => "<mut> ",
                    _ => "",
                };

                // FIXME: this should not suggest `mut` if we can detect that the variable is not
                // use mutably after the `if`

                let sug = format!(
                    "let {mutability}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
                    name=ident.name,
                    cond=snippet(cx, cond.span, "_"),
                    then=if then.stmts.len() > 1 { " ..;" } else { "" },
                    else=if default_multi_stmts { " ..;" } else { "" },
                    value=snippet(cx, value.span, "<value>"),
                    default=snippet(cx, default.span, "<default>"),
                );
                span_lint_hir_and_then(
                    cx,
                    USELESS_LET_IF_SEQ,
                    local.hir_id,
                    span,
                    "`if _ { .. } else { .. }` is an expression",
                    |diag| {
                        diag.span_suggestion(
                            span,
                            "it is more idiomatic to write",
                            sug,
                            Applicability::HasPlaceholders,
                        );
                        if !mutability.is_empty() {
                            diag.note("you might not need `mut` at all");
                        }
                    },
                );
            }
        }
    }
}

fn check_assign<'tcx>(
    cx: &LateContext<'tcx>,
    decl: hir::HirId,
    block: &'tcx hir::Block<'_>,
) -> Option<&'tcx hir::Expr<'tcx>> {
    if block.expr.is_none()
        && let Some(expr) = block.stmts.iter().last()
        && let hir::StmtKind::Semi(expr) = expr.kind
        && let hir::ExprKind::Assign(var, value, _) = expr.kind
        && path_to_local_id(var, decl)
    {
        if block
            .stmts
            .iter()
            .take(block.stmts.len() - 1)
            .any(|stmt| is_local_used(cx, stmt, decl))
        {
            None
        } else {
            Some(value)
        }
    } else {
        None
    }
}