add lint

CHANGELOG.md

@@ -1153,6 +1153,7 @@ Released 2018-09-13
 [`len_without_is_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty
 [`len_zero`]: https://rust-lang.github.io/rust-clippy/master/index.html#len_zero
 [`let_and_return`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_and_return
+[`let_underscore_lock`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_lock
 [`let_underscore_must_use`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_must_use
 [`let_unit_value`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
 [`linkedlist`]: https://rust-lang.github.io/rust-clippy/master/index.html#linkedlist

README.md

@@ -6,7 +6,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 349 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 350 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/let_underscore.rs

@@ -4,7 +4,7 @@ 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_lint_and_help};
+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>`
@@ -30,7 +30,35 @@ declare_clippy_lint! {
     "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_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<'_>) {
@@ -43,22 +71,37 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetUnderscore {
             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_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"
-                    )
+                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 an 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"
+                            )
+                        }
+                    }
                 }
             }
         }

clippy_lints/src/lib.rs

@@ -565,6 +565,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &len_zero::LEN_WITHOUT_IS_EMPTY,
         &len_zero::LEN_ZERO,
         &let_if_seq::USELESS_LET_IF_SEQ,
+        &let_underscore::LET_UNDERSCORE_LOCK,
         &let_underscore::LET_UNDERSCORE_MUST_USE,
         &lifetimes::EXTRA_UNUSED_LIFETIMES,
         &lifetimes::NEEDLESS_LIFETIMES,
@@ -1168,6 +1169,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&len_zero::LEN_WITHOUT_IS_EMPTY),
         LintId::of(&len_zero::LEN_ZERO),
         LintId::of(&let_if_seq::USELESS_LET_IF_SEQ),
+        LintId::of(&let_underscore::LET_UNDERSCORE_LOCK),
         LintId::of(&lifetimes::EXTRA_UNUSED_LIFETIMES),
         LintId::of(&lifetimes::NEEDLESS_LIFETIMES),
         LintId::of(&literal_representation::INCONSISTENT_DIGIT_GROUPING),
@@ -1551,6 +1553,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&infinite_iter::INFINITE_ITER),
         LintId::of(&inherent_to_string::INHERENT_TO_STRING_SHADOW_DISPLAY),
         LintId::of(&inline_fn_without_body::INLINE_FN_WITHOUT_BODY),
+        LintId::of(&let_underscore::LET_UNDERSCORE_LOCK),
         LintId::of(&literal_representation::MISTYPED_LITERAL_SUFFIXES),
         LintId::of(&loops::FOR_LOOP_OVER_OPTION),
         LintId::of(&loops::FOR_LOOP_OVER_RESULT),

clippy_lints/src/utils/paths.rs

@@ -58,6 +58,7 @@ pub const MEM_REPLACE: [&str; 3] = ["core", "mem", "replace"];
 pub const MEM_UNINITIALIZED: [&str; 3] = ["core", "mem", "uninitialized"];
 pub const MEM_ZEROED: [&str; 3] = ["core", "mem", "zeroed"];
 pub const MUTEX: [&str; 4] = ["std", "sync", "mutex", "Mutex"];
+pub const MUTEX_LOCK: [&str; 5] = ["std", "sync", "mutex", "Mutex", "lock"];
 pub const OPEN_OPTIONS: [&str; 3] = ["std", "fs", "OpenOptions"];
 pub const OPS_MODULE: [&str; 2] = ["core", "ops"];
 pub const OPTION: [&str; 3] = ["core", "option", "Option"];
@@ -100,6 +101,8 @@ pub const REPEAT: [&str; 3] = ["core", "iter", "repeat"];
 pub const RESULT: [&str; 3] = ["core", "result", "Result"];
 pub const RESULT_ERR: [&str; 4] = ["core", "result", "Result", "Err"];
 pub const RESULT_OK: [&str; 4] = ["core", "result", "Result", "Ok"];
+pub const RWLOCK_READ: [&str; 5] = ["std", "sync", "rwlock", "RwLock", "read"];
+pub const RWLOCK_WRITE: [&str; 5] = ["std", "sync", "rwlock", "RwLock", "write"];
 pub const SERDE_DE_VISITOR: [&str; 3] = ["serde", "de", "Visitor"];
 pub const SLICE_INTO_VEC: [&str; 4] = ["alloc", "slice", "<impl [T]>", "into_vec"];
 pub const SLICE_ITER: [&str; 3] = ["core", "slice", "Iter"];

src/lintlist/mod.rs

@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 349] = [
+pub const ALL_LINTS: [Lint; 350] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -959,6 +959,13 @@ pub const ALL_LINTS: [Lint; 349] = [
         deprecation: None,
         module: "returns",
     },
+    Lint {
+        name: "let_underscore_lock",
+        group: "correctness",
+        desc: "non-binding let on a synchronization lock",
+        deprecation: None,
+        module: "let_underscore",
+    },
     Lint {
         name: "let_underscore_must_use",
         group: "restriction",

tests/ui/let_underscore.rs

@@ -88,4 +88,11 @@ fn main() {
     let _ = a.map(|_| ());
 
     let _ = a;
+
+    let m = std::sync::Mutex::new(());
+    let rw = std::sync::RwLock::new(());
+
+    let _ = m.lock();
+    let _ = rw.read();
+    let _ = rw.write();
 }

tests/ui/let_underscore.stderr

@@ -95,5 +95,30 @@ LL |     let _ = a;
    |
    = help: consider explicitly using expression value
 
-error: aborting due to 12 previous errors
+error: non-binding let on an a synchronization lock
+  --> $DIR/let_underscore.rs:95:5
+   |
+LL |     let _ = m.lock();
+   |     ^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_lock)]` on by default
+   = help: consider using an underscore-prefixed named binding
+
+error: non-binding let on an a synchronization lock
+  --> $DIR/let_underscore.rs:96:5
+   |
+LL |     let _ = rw.read();
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding
+
+error: non-binding let on an a synchronization lock
+  --> $DIR/let_underscore.rs:97:5
+   |
+LL |     let _ = rw.write();
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding
+
+error: aborting due to 15 previous errors