Add the let_underscore_untyped lint

Author: JirkaVebr

CHANGELOG.md

@@ -4494,6 +4494,7 @@ Released 2018-09-13
 [`let_underscore_future`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_future
 [`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_underscore_untyped`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_untyped
 [`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
 [`logic_bug`]: https://rust-lang.github.io/rust-clippy/master/index.html#logic_bug

clippy_lints/src/declared_lints.rs

@@ -224,6 +224,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::let_underscore::LET_UNDERSCORE_FUTURE_INFO,
     crate::let_underscore::LET_UNDERSCORE_LOCK_INFO,
     crate::let_underscore::LET_UNDERSCORE_MUST_USE_INFO,
+    crate::let_underscore::LET_UNDERSCORE_UNTYPED_INFO,
     crate::lifetimes::EXTRA_UNUSED_LIFETIMES_INFO,
     crate::lifetimes::NEEDLESS_LIFETIMES_INFO,
     crate::literal_representation::DECIMAL_LITERAL_REPRESENTATION_INFO,

clippy_lints/src/let_underscore.rs

@@ -90,7 +90,45 @@ declare_clippy_lint! {
     "non-binding `let` on a future"
 }
 
-declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_FUTURE]);
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for `let _ = <expr>` without a type annotation, and suggests to either provide one,
+    /// or remove the `let` keyword altogether.
+    ///
+    /// ### Why is this bad?
+    /// The `let _ = <expr>` expression ignores the value of `<expr>` but will remain doing so even
+    /// if the type were to change, thus potentially introducing subtle bugs. By supplying a type
+    /// annotation, one will be forced to re-visit the decision to ignore the value in such cases.
+    ///
+    /// ### Known problems
+    /// The `_ = <expr>` is not properly supported by some tools (e.g. IntelliJ) and may seem odd
+    /// to many developers. This lint also partially overlaps with the other `let_underscore_*`
+    /// lints.
+    ///
+    /// ### Example
+    /// ```rust
+    /// fn foo() -> Result<u32, ()> {
+    ///     Ok(123)
+    /// }
+    /// let _ = foo();
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// fn foo() -> Result<u32, ()> {
+    ///     Ok(123)
+    /// }
+    /// // Either provide a type annotation:
+    /// let _: Result<u32, ()> = foo();
+    /// // …or drop the let keyword:
+    /// _ = foo();
+    /// ```
+    #[clippy::version = "1.69.0"]
+    pub LET_UNDERSCORE_UNTYPED,
+    pedantic,
+    "non-binding `let` without a type annotation"
+}
+
+declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_FUTURE, LET_UNDERSCORE_UNTYPED]);
 
 const SYNC_GUARD_PATHS: [&[&str]; 3] = [
     &paths::PARKING_LOT_MUTEX_GUARD,
@@ -148,6 +186,18 @@ impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
                     "consider explicitly using function result",
                 );
             }
+
+            if local.pat.default_binding_modes && local.ty.is_none() {
+                // When `default_binding_modes` is true, the `let` keyword is present.
+                span_lint_and_help(
+                    cx,
+                    LET_UNDERSCORE_UNTYPED,
+                    local.span,
+                    "non-binding `let` without a type annotation",
+                    None,
+                    "consider adding a type annotation or removing the `let` keyword",
+                );
+            }
         }
     }
 }

tests/ui/let_underscore_untyped.rs

@@ -0,0 +1,54 @@
+#![allow(unused)]
+#![warn(clippy::let_underscore_untyped)]
+
+use std::future::Future;
+use std::{boxed::Box, fmt::Display};
+
+fn a() -> u32 {
+    1
+}
+
+fn b<T>(x: T) -> T {
+    x
+}
+
+fn c() -> impl Display {
+    1
+}
+
+fn d(x: &u32) -> &u32 {
+    x
+}
+
+fn e() -> Result<u32, ()> {
+    Ok(1)
+}
+
+fn f() -> Box<dyn Display> {
+    Box::new(1)
+}
+
+fn main() {
+    let _ = a();
+    let _ = b(1);
+    let _ = c();
+    let _ = d(&1);
+    let _ = e();
+    let _ = f();
+
+    _ = a();
+    _ = b(1);
+    _ = c();
+    _ = d(&1);
+    _ = e();
+    _ = f();
+
+    let _: u32 = a();
+    let _: u32 = b(1);
+    let _: &u32 = d(&1);
+    let _: Result<_, _> = e();
+    let _: Box<_> = f();
+
+    #[allow(clippy::let_underscore_untyped)]
+    let _ = a();
+}

tests/ui/let_underscore_untyped.stderr

@@ -0,0 +1,51 @@
+error: non-binding `let` without a type annotation
+  --> $DIR/let_underscore_untyped.rs:32:5
+   |
+LL |     let _ = a();
+   |     ^^^^^^^^^^^^
+   |
+   = help: consider adding a type annotation or removing the `let` keyword
+   = note: `-D clippy::let-underscore-untyped` implied by `-D warnings`
+
+error: non-binding `let` without a type annotation
+  --> $DIR/let_underscore_untyped.rs:33:5
+   |
+LL |     let _ = b(1);
+   |     ^^^^^^^^^^^^^
+   |
+   = help: consider adding a type annotation or removing the `let` keyword
+
+error: non-binding `let` without a type annotation
+  --> $DIR/let_underscore_untyped.rs:34:5
+   |
+LL |     let _ = c();
+   |     ^^^^^^^^^^^^
+   |
+   = help: consider adding a type annotation or removing the `let` keyword
+
+error: non-binding `let` without a type annotation
+  --> $DIR/let_underscore_untyped.rs:35:5
+   |
+LL |     let _ = d(&1);
+   |     ^^^^^^^^^^^^^^
+   |
+   = help: consider adding a type annotation or removing the `let` keyword
+
+error: non-binding `let` without a type annotation
+  --> $DIR/let_underscore_untyped.rs:36:5
+   |
+LL |     let _ = e();
+   |     ^^^^^^^^^^^^
+   |
+   = help: consider adding a type annotation or removing the `let` keyword
+
+error: non-binding `let` without a type annotation
+  --> $DIR/let_underscore_untyped.rs:37:5
+   |
+LL |     let _ = f();
+   |     ^^^^^^^^^^^^
+   |
+   = help: consider adding a type annotation or removing the `let` keyword
+
+error: aborting due to 6 previous errors
+

tests/ui/map_flatten_fixable.fixed

@@ -1,6 +1,7 @@
 // run-rustfix
 
 #![warn(clippy::all, clippy::pedantic)]
+#![allow(clippy::let_underscore_untyped)]
 #![allow(clippy::missing_docs_in_private_items)]
 #![allow(clippy::map_identity)]
 #![allow(clippy::redundant_closure)]

tests/ui/map_flatten_fixable.rs

@@ -1,6 +1,7 @@
 // run-rustfix
 
 #![warn(clippy::all, clippy::pedantic)]
+#![allow(clippy::let_underscore_untyped)]
 #![allow(clippy::missing_docs_in_private_items)]
 #![allow(clippy::map_identity)]
 #![allow(clippy::redundant_closure)]

tests/ui/map_flatten_fixable.stderr

@@ -1,49 +1,49 @@
 error: called `map(..).flatten()` on `Iterator`
-  --> $DIR/map_flatten_fixable.rs:17:47
+  --> $DIR/map_flatten_fixable.rs:18:47
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id).flatten().collect();
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^^ help: try replacing `map` with `filter_map` and remove the `.flatten()`: `filter_map(option_id)`
    |
    = note: `-D clippy::map-flatten` implied by `-D warnings`
 
 error: called `map(..).flatten()` on `Iterator`
-  --> $DIR/map_flatten_fixable.rs:18:47
+  --> $DIR/map_flatten_fixable.rs:19:47
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_ref).flatten().collect();
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try replacing `map` with `filter_map` and remove the `.flatten()`: `filter_map(option_id_ref)`
 
 error: called `map(..).flatten()` on `Iterator`
-  --> $DIR/map_flatten_fixable.rs:19:47
+  --> $DIR/map_flatten_fixable.rs:20:47
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_closure).flatten().collect();
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try replacing `map` with `filter_map` and remove the `.flatten()`: `filter_map(option_id_closure)`
 
 error: called `map(..).flatten()` on `Iterator`
-  --> $DIR/map_flatten_fixable.rs:20:47
+  --> $DIR/map_flatten_fixable.rs:21:47
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| x.checked_add(1)).flatten().collect();
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try replacing `map` with `filter_map` and remove the `.flatten()`: `filter_map(|x| x.checked_add(1))`
 
 error: called `map(..).flatten()` on `Iterator`
-  --> $DIR/map_flatten_fixable.rs:23:47
+  --> $DIR/map_flatten_fixable.rs:24:47
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect();
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^ help: try replacing `map` with `flat_map` and remove the `.flatten()`: `flat_map(|x| 0..x)`
 
 error: called `map(..).flatten()` on `Option`
-  --> $DIR/map_flatten_fixable.rs:26:40
+  --> $DIR/map_flatten_fixable.rs:27:40
    |
 LL |     let _: Option<_> = (Some(Some(1))).map(|x| x).flatten();
    |                                        ^^^^^^^^^^^^^^^^^^^^ help: try replacing `map` with `and_then` and remove the `.flatten()`: `and_then(|x| x)`
 
 error: called `map(..).flatten()` on `Result`
-  --> $DIR/map_flatten_fixable.rs:29:42
+  --> $DIR/map_flatten_fixable.rs:30:42
    |
 LL |     let _: Result<_, &str> = (Ok(Ok(1))).map(|x| x).flatten();
    |                                          ^^^^^^^^^^^^^^^^^^^^ help: try replacing `map` with `and_then` and remove the `.flatten()`: `and_then(|x| x)`
 
 error: called `map(..).flatten()` on `Iterator`
-  --> $DIR/map_flatten_fixable.rs:38:10
+  --> $DIR/map_flatten_fixable.rs:39:10
    |
 LL |           .map(|n| match n {
    |  __________^
@@ -72,7 +72,7 @@ LL ~         });
    |
 
 error: called `map(..).flatten()` on `Option`
-  --> $DIR/map_flatten_fixable.rs:58:10
+  --> $DIR/map_flatten_fixable.rs:59:10
    |
 LL |           .map(|_| {
    |  __________^

tests/ui/methods.rs

@@ -4,6 +4,7 @@
 #![allow(
     clippy::disallowed_names,
     clippy::default_trait_access,
+    clippy::let_underscore_untyped,
     clippy::missing_docs_in_private_items,
     clippy::missing_safety_doc,
     clippy::non_ascii_literal,

tests/ui/methods.stderr

@@ -1,5 +1,5 @@
 error: methods called `new` usually return `Self`
-  --> $DIR/methods.rs:104:5
+  --> $DIR/methods.rs:105:5
    |
 LL | /     fn new() -> i32 {
 LL | |         0
@@ -9,7 +9,7 @@ LL | |     }
    = note: `-D clippy::new-ret-no-self` implied by `-D warnings`
 
 error: called `filter(..).next()` on an `Iterator`. This is more succinctly expressed by calling `.find(..)` instead
-  --> $DIR/methods.rs:125:13
+  --> $DIR/methods.rs:126:13
    |
 LL |       let _ = v.iter().filter(|&x| {
    |  _____________^