Auto merge of rust-lang#8596 - Jaic1:unnecessary_cast, r=flip1995

Fix unnecessary_cast suggestion for type aliasses

Fix rust-lang#6923. The [`unnecessary_cast`] lint now will skip casting to non-primitive type.

changelog: fix lint [`unnecessary_cast `]

clippy_lints/src/casts/unnecessary_cast.rs

@@ -4,7 +4,8 @@ use clippy_utils::source::snippet_opt;
 use if_chain::if_chain;
 use rustc_ast::{LitFloatType, LitIntType, LitKind};
 use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind, Lit, UnOp};
+use rustc_hir::def::Res;
+use rustc_hir::{Expr, ExprKind, Lit, QPath, TyKind, UnOp};
 use rustc_lint::{LateContext, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::{self, FloatTy, InferTy, Ty};
@@ -18,6 +19,17 @@ pub(super) fn check(
     cast_from: Ty<'_>,
     cast_to: Ty<'_>,
 ) -> bool {
+    // skip non-primitive type cast
+    if_chain! {
+        if let ExprKind::Cast(_, cast_to) = expr.kind;
+        if let TyKind::Path(QPath::Resolved(_, path)) = &cast_to.kind;
+        if let Res::PrimTy(_) = path.res;
+        then {}
+        else {
+            return false
+        }
+    }
+
     if let Some(lit) = get_numeric_literal(cast_expr) {
         let literal_str = snippet_opt(cx, cast_expr.span).unwrap_or_default();
 

tests/ui/unnecessary_cast.rs

@@ -30,4 +30,10 @@ fn main() {
 
     // do not lint cast to cfg-dependant type
     1 as std::os::raw::c_char;
+
+    // do not lint cast to alias type
+    1 as I32Alias;
+    &1 as &I32Alias;
 }
+
+type I32Alias = i32;

tests/ui/unnecessary_cast_fixable.fixed

@@ -42,4 +42,9 @@ fn main() {
 
     let _ = -1_i32;
     let _ = -1.0_f32;
+
+    let _ = 1 as I32Alias;
+    let _ = &1 as &I32Alias;
 }
+
+type I32Alias = i32;

tests/ui/unnecessary_cast_fixable.rs

@@ -42,4 +42,9 @@ fn main() {
 
     let _ = -1 as i32;
     let _ = -1.0 as f32;
+
+    let _ = 1 as I32Alias;
+    let _ = &1 as &I32Alias;
 }
+
+type I32Alias = i32;