Auto merge of rust-lang#10701 - blyxyas:fix-fp-let_underscore_untyped, r=llogiq

Bugfix: Ignore `impl Trait`(s) @ `let_underscore_untyped`

Fixes rust-lang#10411

changelog:[`let_underscore_untyped`]: Ignore `impl Trait`(s) that caused false positives.

Author: bors

clippy_lints/src/let_underscore.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_help;
 use clippy_utils::ty::{implements_trait, is_must_use_ty, match_type};
 use clippy_utils::{is_must_use_func_call, paths};
-use rustc_hir::{Local, PatKind};
+use rustc_hir::{ExprKind, Local, PatKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::subst::GenericArgKind;
@@ -189,7 +189,18 @@ impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
 
             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(
+
+				// Ignore function calls that return impl traits...
+				if let Some(init) = local.init &&
+				matches!(init.kind, ExprKind::Call(_, _) | ExprKind::MethodCall(_, _, _, _)) {
+					let expr_ty = cx.typeck_results().expr_ty(init);
+					if expr_ty.is_impl_trait() {
+						return;
+					}
+				}
+
+
+				span_lint_and_help(
                     cx,
                     LET_UNDERSCORE_UNTYPED,
                     local.span,

tests/ui/let_underscore_untyped.rs

@@ -28,13 +28,18 @@ fn f() -> Box<dyn Display> {
     Box::new(1)
 }
 
+fn g() -> impl Fn() {
+    || {}
+}
+
 fn main() {
     let _ = a();
     let _ = b(1);
     let _ = c();
     let _ = d(&1);
     let _ = e();
     let _ = f();
+    let _ = g();
 
     _ = a();
     _ = b(1);

tests/ui/let_underscore_untyped.stderr

@@ -1,5 +1,5 @@
 error: non-binding `let` without a type annotation
-  --> $DIR/let_underscore_untyped.rs:32:5
+  --> $DIR/let_underscore_untyped.rs:36:5
    |
 LL |     let _ = a();
    |     ^^^^^^^^^^^^
@@ -8,44 +8,36 @@ LL |     let _ = a();
    = 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
+  --> $DIR/let_underscore_untyped.rs:37:5
    |
 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
+  --> $DIR/let_underscore_untyped.rs:39:5
    |
 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
+  --> $DIR/let_underscore_untyped.rs:40:5
    |
 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
+  --> $DIR/let_underscore_untyped.rs:41:5
    |
 LL |     let _ = f();
    |     ^^^^^^^^^^^^
    |
    = help: consider adding a type annotation or removing the `let` keyword
 
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors