Auto merge of #4809 - iankronquist:patch-1, r=flip1995

[rust.git] / clippy_lints / src / let_underscore.rs

diff --git a/clippy_lints/src/let_underscore.rs b/clippy_lints/src/let_underscore.rs
index 4933b3049c28166f22d4ab41061418db3c7695f9..c2a404ebee7cc0830296946a00f52b6ee3149777 100644 (file)
--- a/clippy_lints/src/let_underscore.rs
+++ b/clippy_lints/src/let_underscore.rs
@@ -1,10 +1,10 @@
 use if_chain::if_chain;
-use rustc::declare_lint_pass;
-use rustc::hir::*;
-use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
-use rustc_session::declare_tool_lint;
+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, span_help_and_lint};
+use crate::utils::{is_must_use_func_call, is_must_use_ty, match_type, paths, span_lint_and_help};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for `let _ = <expr>`
@@ -27,32 +27,79 @@
     /// ```
     pub LET_UNDERSCORE_MUST_USE,
     restriction,
-    "non-binding let on a #[must_use] expression"
+    "non-binding let on a `#[must_use]` expression"
 }
 
-declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE]);
+declare_clippy_lint! {
+    /// **What it does:** Checks for `let _ = sync_lock`
+    ///
+    /// **Why is this bad?** This statement immediately drops the lock instead of
+    /// extending it's lifetime to the end of the scope, which is often not intended.
+    /// To extend lock lifetime to the end of the scope, use an underscore-prefixed
+    /// name instead (i.e. _lock). If you want to explicitly drop the lock,
+    /// `std::mem::drop` conveys your intention better and is less error-prone.
+    ///
+    /// **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 SYNC_GUARD_PATHS: [&[&str]; 3] = [
+    &paths::MUTEX_GUARD,
+    &paths::RWLOCK_READ_GUARD,
+    &paths::RWLOCK_WRITE_GUARD,
+];
 
 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 is_must_use_ty(cx, cx.tables.expr_ty(init)) {
-                   span_help_and_lint(
+                let check_ty = |ty| SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, ty, path));
+                if cx.tables.expr_ty(init).walk().any(check_ty) {
+                    span_lint_and_help(
+                        cx,
+                        LET_UNDERSCORE_LOCK,
+                        stmt.span,
+                        "non-binding let on a synchronization lock",
+                        "consider using an underscore-prefixed named \
+                            binding or dropping explicitly with `std::mem::drop`"
+                    )
+                } 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",
+                        "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_help_and_lint(
+                    span_lint_and_help(
                         cx,
                         LET_UNDERSCORE_MUST_USE,
                         stmt.span,
-                        "non-binding let on a result of a #[must_use] function",
+                        "non-binding let on a result of a `#[must_use]` function",
                         "consider explicitly using function result"
                     )
                 }