Auto merge of rust-lang#10467 - blyxyas:underscore_typed, r=Jarcho

Add `let_with_type_underscore` lint

Fixes rust-lang#10463
changelog: [`let_with_type_underscore`]: Add the lint.

CHANGELOG.md

@@ -4643,6 +4643,7 @@ Released 2018-09-13
 [`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
+[`let_with_type_underscore`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_with_type_underscore
 [`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
 [`lossy_float_literal`]: https://rust-lang.github.io/rust-clippy/master/index.html#lossy_float_literal

clippy_lints/src/declared_lints.rs

@@ -227,6 +227,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::let_underscore::LET_UNDERSCORE_LOCK_INFO,
     crate::let_underscore::LET_UNDERSCORE_MUST_USE_INFO,
     crate::let_underscore::LET_UNDERSCORE_UNTYPED_INFO,
+    crate::let_with_type_underscore::LET_WITH_TYPE_UNDERSCORE_INFO,
     crate::lifetimes::EXTRA_UNUSED_LIFETIMES_INFO,
     crate::lifetimes::NEEDLESS_LIFETIMES_INFO,
     crate::literal_representation::DECIMAL_LITERAL_REPRESENTATION_INFO,

clippy_lints/src/let_with_type_underscore.rs

@@ -0,0 +1,45 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use rustc_hir::*;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::lint::in_external_macro;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Detects when a variable is declared with an explicit type of `_`.
+    /// ### Why is this bad?
+    /// It adds noise, `: _` provides zero clarity or utility.
+    /// ### Example
+    /// ```rust,ignore
+    /// let my_number: _ = 1;
+    /// ```
+    /// Use instead:
+    /// ```rust,ignore
+    /// let my_number = 1;
+    /// ```
+    #[clippy::version = "1.69.0"]
+    pub LET_WITH_TYPE_UNDERSCORE,
+    complexity,
+    "unneeded underscore type (`_`) in a variable declaration"
+}
+declare_lint_pass!(UnderscoreTyped => [LET_WITH_TYPE_UNDERSCORE]);
+
+impl LateLintPass<'_> for UnderscoreTyped {
+    fn check_local<'tcx>(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'tcx>) {
+        if_chain! {
+            if !in_external_macro(cx.tcx.sess, local.span);
+            if let Some(ty) = local.ty; // Ensure that it has a type defined
+            if let TyKind::Infer = &ty.kind; // that type is '_'
+            if local.span.ctxt() == ty.span.ctxt();
+            then {
+                span_lint_and_help(cx,
+                    LET_WITH_TYPE_UNDERSCORE,
+                    local.span,
+                    "variable declared with type underscore",
+                    Some(ty.span.with_lo(local.pat.span.hi())),
+                    "remove the explicit type `_` declaration"
+                )
+            }
+        };
+    }
+}

clippy_lints/src/lib.rs

@@ -167,6 +167,7 @@ mod large_stack_arrays;
 mod len_zero;
 mod let_if_seq;
 mod let_underscore;
+mod let_with_type_underscore;
 mod lifetimes;
 mod literal_representation;
 mod loops;
@@ -930,6 +931,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(collection_is_never_read::CollectionIsNeverRead));
     store.register_late_pass(|_| Box::new(missing_assert_message::MissingAssertMessage));
     store.register_early_pass(|| Box::new(redundant_async_block::RedundantAsyncBlock));
+    store.register_late_pass(|_| Box::new(let_with_type_underscore::UnderscoreTyped));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

tests/ui/crashes/ice-6179.rs

@@ -2,7 +2,7 @@
 //! The ICE is mainly caused by using `hir_ty_to_ty`. See the discussion in the PR for details.
 
 #![warn(clippy::use_self)]
-#![allow(dead_code)]
+#![allow(dead_code, clippy::let_with_type_underscore)]
 
 struct Foo;
 

tests/ui/default_numeric_fallback_f64.fixed

@@ -9,7 +9,8 @@
     clippy::unnecessary_operation,
     clippy::branches_sharing_code,
     clippy::match_single_binding,
-    clippy::let_unit_value
+    clippy::let_unit_value,
+    clippy::let_with_type_underscore
 )]
 
 #[macro_use]

tests/ui/default_numeric_fallback_f64.rs

@@ -9,7 +9,8 @@
     clippy::unnecessary_operation,
     clippy::branches_sharing_code,
     clippy::match_single_binding,
-    clippy::let_unit_value
+    clippy::let_unit_value,
+    clippy::let_with_type_underscore
 )]
 
 #[macro_use]

tests/ui/default_numeric_fallback_f64.stderr

@@ -1,145 +1,145 @@
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:21:17
+  --> $DIR/default_numeric_fallback_f64.rs:22:17
    |
 LL |         let x = 0.12;
    |                 ^^^^ help: consider adding suffix: `0.12_f64`
    |
    = note: `-D clippy::default-numeric-fallback` implied by `-D warnings`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:22:18
+  --> $DIR/default_numeric_fallback_f64.rs:23:18
    |
 LL |         let x = [1., 2., 3.];
    |                  ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:22:22
+  --> $DIR/default_numeric_fallback_f64.rs:23:22
    |
 LL |         let x = [1., 2., 3.];
    |                      ^^ help: consider adding suffix: `2.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:22:26
+  --> $DIR/default_numeric_fallback_f64.rs:23:26
    |
 LL |         let x = [1., 2., 3.];
    |                          ^^ help: consider adding suffix: `3.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:23:28
+  --> $DIR/default_numeric_fallback_f64.rs:24:28
    |
 LL |         let x = if true { (1., 2.) } else { (3., 4.) };
    |                            ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:23:32
+  --> $DIR/default_numeric_fallback_f64.rs:24:32
    |
 LL |         let x = if true { (1., 2.) } else { (3., 4.) };
    |                                ^^ help: consider adding suffix: `2.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:23:46
+  --> $DIR/default_numeric_fallback_f64.rs:24:46
    |
 LL |         let x = if true { (1., 2.) } else { (3., 4.) };
    |                                              ^^ help: consider adding suffix: `3.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:23:50
+  --> $DIR/default_numeric_fallback_f64.rs:24:50
    |
 LL |         let x = if true { (1., 2.) } else { (3., 4.) };
    |                                                  ^^ help: consider adding suffix: `4.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:24:23
+  --> $DIR/default_numeric_fallback_f64.rs:25:23
    |
 LL |         let x = match 1. {
    |                       ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:25:18
+  --> $DIR/default_numeric_fallback_f64.rs:26:18
    |
 LL |             _ => 1.,
    |                  ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:44:21
+  --> $DIR/default_numeric_fallback_f64.rs:45:21
    |
 LL |             let y = 1.;
    |                     ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:52:21
+  --> $DIR/default_numeric_fallback_f64.rs:53:21
    |
 LL |             let y = 1.;
    |                     ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:58:21
+  --> $DIR/default_numeric_fallback_f64.rs:59:21
    |
 LL |             let y = 1.;
    |                     ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:66:21
+  --> $DIR/default_numeric_fallback_f64.rs:67:21
    |
 LL |             let y = 1.;
    |                     ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:78:9
+  --> $DIR/default_numeric_fallback_f64.rs:79:9
    |
 LL |         1.
    |         ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:84:27
+  --> $DIR/default_numeric_fallback_f64.rs:85:27
    |
 LL |         let f = || -> _ { 1. };
    |                           ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:88:29
+  --> $DIR/default_numeric_fallback_f64.rs:89:29
    |
 LL |         let f = || -> f64 { 1. };
    |                             ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:102:21
+  --> $DIR/default_numeric_fallback_f64.rs:103:21
    |
 LL |         generic_arg(1.);
    |                     ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:105:32
+  --> $DIR/default_numeric_fallback_f64.rs:106:32
    |
 LL |         let x: _ = generic_arg(1.);
    |                                ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:123:28
+  --> $DIR/default_numeric_fallback_f64.rs:124:28
    |
 LL |         GenericStruct { x: 1. };
    |                            ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:126:36
+  --> $DIR/default_numeric_fallback_f64.rs:127:36
    |
 LL |         let _ = GenericStruct { x: 1. };
    |                                    ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:144:24
+  --> $DIR/default_numeric_fallback_f64.rs:145:24
    |
 LL |         GenericEnum::X(1.);
    |                        ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:164:23
+  --> $DIR/default_numeric_fallback_f64.rs:165:23
    |
 LL |         s.generic_arg(1.);
    |                       ^^ help: consider adding suffix: `1.0_f64`
 
 error: default numeric fallback might occur
-  --> $DIR/default_numeric_fallback_f64.rs:171:21
+  --> $DIR/default_numeric_fallback_f64.rs:172:21
    |
 LL |             let x = 22.;
    |                     ^^^ help: consider adding suffix: `22.0_f64`

tests/ui/default_numeric_fallback_i32.fixed

@@ -9,7 +9,8 @@
     clippy::no_effect,
     clippy::unnecessary_operation,
     clippy::branches_sharing_code,
-    clippy::let_unit_value
+    clippy::let_unit_value,
+    clippy::let_with_type_underscore
 )]
 
 #[macro_use]

tests/ui/default_numeric_fallback_i32.rs

@@ -9,7 +9,8 @@
     clippy::no_effect,
     clippy::unnecessary_operation,
     clippy::branches_sharing_code,
-    clippy::let_unit_value
+    clippy::let_unit_value,
+    clippy::let_with_type_underscore
 )]
 
 #[macro_use]