Auto merge of #10702 - blyxyas:fix-let_underscore_untyped_help_message, r=Manishearth

Improve the help message + add a help span

This would close #10410, because it applies the general consensus achieved in that issue (that replacing `let _ = ...` to `_ = ...` doesn't present any benefits).

I also added a little help message span.

changelog:[`let_underscore_untyped`]: Fix the help message confusion + add a help message span.

Author: bors

clippy_lints/src/let_underscore.rs

@@ -6,6 +6,7 @@ use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::{BytePos, Span};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -205,8 +206,13 @@ impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
                     LET_UNDERSCORE_UNTYPED,
                     local.span,
                     "non-binding `let` without a type annotation",
-                    None,
-                    "consider adding a type annotation or removing the `let` keyword",
+                    Some(
+						Span::new(local.pat.span.hi(),
+						local.pat.span.hi() + BytePos(1),
+						local.pat.span.ctxt(),
+						local.pat.span.parent()
+					)),
+                    "consider adding a type annotation",
                 );
             }
         }

tests/ui/let_underscore_untyped.stderr

@@ -4,7 +4,11 @@ error: non-binding `let` without a type annotation
 LL |     let _ = a();
    |     ^^^^^^^^^^^^
    |
-   = help: consider adding a type annotation or removing the `let` keyword
+help: consider adding a type annotation
+  --> $DIR/let_underscore_untyped.rs:36:10
+   |
+LL |     let _ = a();
+   |          ^
    = note: `-D clippy::let-underscore-untyped` implied by `-D warnings`
 
 error: non-binding `let` without a type annotation
@@ -13,31 +17,47 @@ error: non-binding `let` without a type annotation
 LL |     let _ = b(1);
    |     ^^^^^^^^^^^^^
    |
-   = help: consider adding a type annotation or removing the `let` keyword
+help: consider adding a type annotation
+  --> $DIR/let_underscore_untyped.rs:37:10
+   |
+LL |     let _ = b(1);
+   |          ^
 
 error: non-binding `let` without a type annotation
   --> $DIR/let_underscore_untyped.rs:39:5
    |
 LL |     let _ = d(&1);
    |     ^^^^^^^^^^^^^^
    |
-   = help: consider adding a type annotation or removing the `let` keyword
+help: consider adding a type annotation
+  --> $DIR/let_underscore_untyped.rs:39:10
+   |
+LL |     let _ = d(&1);
+   |          ^
 
 error: non-binding `let` without a type annotation
   --> $DIR/let_underscore_untyped.rs:40:5
    |
 LL |     let _ = e();
    |     ^^^^^^^^^^^^
    |
-   = help: consider adding a type annotation or removing the `let` keyword
+help: consider adding a type annotation
+  --> $DIR/let_underscore_untyped.rs:40:10
+   |
+LL |     let _ = e();
+   |          ^
 
 error: non-binding `let` without a type annotation
   --> $DIR/let_underscore_untyped.rs:41:5
    |
 LL |     let _ = f();
    |     ^^^^^^^^^^^^
    |
-   = help: consider adding a type annotation or removing the `let` keyword
+help: consider adding a type annotation
+  --> $DIR/let_underscore_untyped.rs:41:10
+   |
+LL |     let _ = f();
+   |          ^
 
 error: aborting due to 5 previous errors