clippy_lints/src/let_if_seq.rs

use clippy_utils::diagnostics::span_lint_hir_and_then;
use clippy_utils::res::MaybeResPath;
use clippy_utils::source::snippet_with_context;
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::Expr(if_) = next.kind {
                check_block_inner(cx, stmt, if_);
            }
        }

        if let Some(expr) = block.expr
            && let Some(stmt) = block.stmts.last()
        {
            check_block_inner(cx, stmt, expr);
        }
    }
}

fn check_block_inner<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx hir::Stmt<'tcx>, if_: &'tcx hir::Expr<'tcx>) {
    if let hir::StmtKind::Let(local) = stmt.kind
        && let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind
        && let hir::ExprKind::If(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.typing_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 {
                    return;
                }
            } else {
                return;
            }
        } else if let Some(default) = local.init {
            (false, default)
        } else {
            return;
        };

        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 mut applicability = Applicability::HasPlaceholders;
        let (cond_snip, _) = snippet_with_context(cx, cond.span, if_.span.ctxt(), "_", &mut applicability);
        let (value_snip, _) = snippet_with_context(cx, value.span, if_.span.ctxt(), "<value>", &mut applicability);
        let (default_snip, _) =
            snippet_with_context(cx, default.span, if_.span.ctxt(), "<default>", &mut applicability);
        let sug = format!(
            "let {mutability}{name} = if {cond_snip} {{{then} {value_snip} }} else {{{else} {default_snip} }};",
            name=ident.name,
            then=if then.stmts.len() > 1 { " ..;" } else { "" },
            else=if default_multi_stmts { " ..;" } else { "" },
        );
        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);
                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
        && var.res_local_id() == Some(decl)
    {
        if block
            .stmts
            .iter()
            .take(block.stmts.len() - 1)
            .any(|stmt| is_local_used(cx, stmt, decl))
        {
            None
        } else {
            Some(value)
        }
    } else {
        None
    }
}