rust/clippy_lints/src/let_underscore.rs

use if_chain::if_chain;
use rustc::lint::in_external_macro;
use rustc_hir::*;
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};

use crate::utils::{is_must_use_func_call, is_must_use_ty, match_def_path, paths, span_lint_and_help};

declare_clippy_lint! {
    /// **What it does:** Checks for `let _ = <expr>`
    /// where expr is #[must_use]
    ///
    /// **Why is this bad?** It's better to explicitly
    /// handle the value of a #[must_use] expr
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust
    /// fn f() -> Result<u32, u32> {
    ///     Ok(0)
    /// }
    ///
    /// let _ = f();
    /// // is_ok() is marked #[must_use]
    /// let _ = f().is_ok();
    /// ```
    pub LET_UNDERSCORE_MUST_USE,
    restriction,
    "non-binding let on a `#[must_use]` expression"
}

declare_clippy_lint! {
    /// **What it does:** Checks for `let _ = sync_primitive.lock()`
    ///
    /// **Why is this bad?** This statement locks the synchronization
    /// primitive and immediately drops the lock, which is probably
    /// not intended. To extend lock lifetime to the end of the scope,
    /// use an underscore-prefixed name instead (i.e. _lock).
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    ///
    /// Bad:
    /// ```rust,ignore
    /// let _ = mutex.lock();
    /// ```
    ///
    /// Good:
    /// ```rust,ignore
    /// let _lock = mutex.lock();
    /// ```
    pub LET_UNDERSCORE_LOCK,
    correctness,
    "non-binding let on a synchronization lock"
}

declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK]);

const LOCK_METHODS_PATHS: [&[&str]; 3] = [&paths::MUTEX_LOCK, &paths::RWLOCK_READ, &paths::RWLOCK_WRITE];

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetUnderscore {
    fn check_stmt(&mut self, cx: &LateContext<'_, '_>, stmt: &Stmt<'_>) {
        if in_external_macro(cx.tcx.sess, stmt.span) {
            return;
        }

        if_chain! {
            if let StmtKind::Local(ref local) = stmt.kind;
            if let PatKind::Wild = local.pat.kind;
            if let Some(ref init) = local.init;
            then {
                if_chain! {
                    if let ExprKind::MethodCall(_, _, _) = init.kind;
                    let method_did = cx.tables.type_dependent_def_id(init.hir_id).unwrap();
                    if LOCK_METHODS_PATHS.iter().any(|path| match_def_path(cx, method_did, path));
                    then {
                        span_lint_and_help(
                            cx,
                            LET_UNDERSCORE_LOCK,
                            stmt.span,
                            "non-binding let on a synchronization lock",
                            "consider using an underscore-prefixed named binding"
                        )
                    } else {
                        if is_must_use_ty(cx, cx.tables.expr_ty(init)) {
                            span_lint_and_help(
                                cx,
                                LET_UNDERSCORE_MUST_USE,
                                stmt.span,
                                "non-binding let on an expression with `#[must_use]` type",
                                "consider explicitly using expression value"
                            )
                        } else if is_must_use_func_call(cx, init) {
                            span_lint_and_help(
                                cx,
                                LET_UNDERSCORE_MUST_USE,
                                stmt.span,
                                "non-binding let on a result of a `#[must_use]` function",
                                "consider explicitly using function result"
                            )
                        }
                    }
                }
            }
        }
    }
}