Don't emit clippy::useless_conversion on type aliases

clippy_lints/src/useless_conversion.rs

@@ -1,4 +1,5 @@
 use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
+use clippy_utils::is_ty_alias;
 use clippy_utils::source::{snippet, snippet_with_context};
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::{is_copy, is_type_diagnostic_item, same_type_and_consts};
@@ -138,6 +139,7 @@ impl<'tcx> LateLintPass<'tcx> for UselessConversion {
                 if_chain! {
                     if let ExprKind::Path(ref qpath) = path.kind;
                     if let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id();
+                    if !is_ty_alias(qpath);
                     then {
                         let a = cx.typeck_results().expr_ty(e);
                         let b = cx.typeck_results().expr_ty(arg);

clippy_utils/src/lib.rs

@@ -1,5 +1,6 @@
 #![feature(array_chunks)]
 #![feature(box_patterns)]
+#![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(lint_reasons)]
 #![feature(never_type)]
@@ -282,6 +283,15 @@ pub fn is_wild(pat: &Pat<'_>) -> bool {
     matches!(pat.kind, PatKind::Wild)
 }
 
+/// Checks if the given `QPath` belongs to a type alias.
+pub fn is_ty_alias(qpath: &QPath<'_>) -> bool {
+    match *qpath {
+        QPath::Resolved(_, path) => matches!(path.res, Res::Def(DefKind::TyAlias, ..)),
+        QPath::TypeRelative(ty, _) if let TyKind::Path(qpath) = ty.kind => { is_ty_alias(&qpath) },
+        _ => false,
+    }
+}
+
 /// Checks if the method call given in `expr` belongs to the given trait.
 /// This is a deprecated function, consider using [`is_trait_method`].
 pub fn match_trait_method(cx: &LateContext<'_>, expr: &Expr<'_>, path: &[&str]) -> bool {

tests/ui/useless_conversion.fixed

@@ -33,6 +33,11 @@ fn test_issue_3913() -> Result<(), std::io::Error> {
     Ok(())
 }
 
+fn dont_lint_on_type_alias() {
+    type A = i32;
+    _ = A::from(0i32);
+}
+
 fn dont_lint_into_iter_on_immutable_local_implementing_iterator_in_expr() {
     let text = "foo\r\nbar\n\nbaz\n";
     let lines = text.lines();
@@ -106,6 +111,7 @@ fn main() {
     test_questionmark().unwrap();
     test_issue_3913().unwrap();
 
+    dont_lint_on_type_alias();
     dont_lint_into_iter_on_immutable_local_implementing_iterator_in_expr();
     lint_into_iter_on_mutable_local_implementing_iterator_in_expr();
     lint_into_iter_on_expr_implementing_iterator();

tests/ui/useless_conversion.rs

@@ -33,6 +33,11 @@ fn test_issue_3913() -> Result<(), std::io::Error> {
     Ok(())
 }
 
+fn dont_lint_on_type_alias() {
+    type A = i32;
+    _ = A::from(0i32);
+}
+
 fn dont_lint_into_iter_on_immutable_local_implementing_iterator_in_expr() {
     let text = "foo\r\nbar\n\nbaz\n";
     let lines = text.lines();
@@ -106,6 +111,7 @@ fn main() {
     test_questionmark().unwrap();
     test_issue_3913().unwrap();
 
+    dont_lint_on_type_alias();
     dont_lint_into_iter_on_immutable_local_implementing_iterator_in_expr();
     lint_into_iter_on_mutable_local_implementing_iterator_in_expr();
     lint_into_iter_on_expr_implementing_iterator();

tests/ui/useless_conversion.stderr

@@ -23,97 +23,97 @@ LL |         let _: i32 = 0i32.into();
    |                      ^^^^^^^^^^^ help: consider removing `.into()`: `0i32`
 
 error: useless conversion to the same type: `std::str::Lines<'_>`
-  --> $DIR/useless_conversion.rs:45:22
+  --> $DIR/useless_conversion.rs:50:22
    |
 LL |     if Some("ok") == lines.into_iter().next() {}
    |                      ^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `lines`
 
 error: useless conversion to the same type: `std::str::Lines<'_>`
-  --> $DIR/useless_conversion.rs:50:21
+  --> $DIR/useless_conversion.rs:55:21
    |
 LL |     let mut lines = text.lines().into_iter();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `text.lines()`
 
 error: useless conversion to the same type: `std::str::Lines<'_>`
-  --> $DIR/useless_conversion.rs:56:22
+  --> $DIR/useless_conversion.rs:61:22
    |
 LL |     if Some("ok") == text.lines().into_iter().next() {}
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `text.lines()`
 
 error: useless conversion to the same type: `std::ops::Range<i32>`
-  --> $DIR/useless_conversion.rs:62:13
+  --> $DIR/useless_conversion.rs:67:13
    |
 LL |     let _ = NUMBERS.into_iter().next();
    |             ^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `NUMBERS`
 
 error: useless conversion to the same type: `std::ops::Range<i32>`
-  --> $DIR/useless_conversion.rs:67:17
+  --> $DIR/useless_conversion.rs:72:17
    |
 LL |     let mut n = NUMBERS.into_iter();
    |                 ^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `NUMBERS`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:128:21
+  --> $DIR/useless_conversion.rs:134:21
    |
 LL |     let _: String = "foo".to_string().into();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into()`: `"foo".to_string()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:129:21
+  --> $DIR/useless_conversion.rs:135:21
    |
 LL |     let _: String = From::from("foo".to_string());
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `From::from()`: `"foo".to_string()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:130:13
+  --> $DIR/useless_conversion.rs:136:13
    |
 LL |     let _ = String::from("foo".to_string());
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `String::from()`: `"foo".to_string()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:131:13
+  --> $DIR/useless_conversion.rs:137:13
    |
 LL |     let _ = String::from(format!("A: {:04}", 123));
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `String::from()`: `format!("A: {:04}", 123)`
 
 error: useless conversion to the same type: `std::str::Lines<'_>`
-  --> $DIR/useless_conversion.rs:132:13
+  --> $DIR/useless_conversion.rs:138:13
    |
 LL |     let _ = "".lines().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `"".lines()`
 
 error: useless conversion to the same type: `std::vec::IntoIter<i32>`
-  --> $DIR/useless_conversion.rs:133:13
+  --> $DIR/useless_conversion.rs:139:13
    |
 LL |     let _ = vec![1, 2, 3].into_iter().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![1, 2, 3].into_iter()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:134:21
+  --> $DIR/useless_conversion.rs:140:21
    |
 LL |     let _: String = format!("Hello {}", "world").into();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into()`: `format!("Hello {}", "world")`
 
 error: useless conversion to the same type: `i32`
-  --> $DIR/useless_conversion.rs:139:13
+  --> $DIR/useless_conversion.rs:145:13
    |
 LL |     let _ = i32::from(a + b) * 3;
    |             ^^^^^^^^^^^^^^^^ help: consider removing `i32::from()`: `(a + b)`
 
 error: useless conversion to the same type: `Foo<'a'>`
-  --> $DIR/useless_conversion.rs:145:23
+  --> $DIR/useless_conversion.rs:151:23
    |
 LL |     let _: Foo<'a'> = s2.into();
    |                       ^^^^^^^^^ help: consider removing `.into()`: `s2`
 
 error: useless conversion to the same type: `Foo<'a'>`
-  --> $DIR/useless_conversion.rs:147:13
+  --> $DIR/useless_conversion.rs:153:13
    |
 LL |     let _ = Foo::<'a'>::from(s3);
    |             ^^^^^^^^^^^^^^^^^^^^ help: consider removing `Foo::<'a'>::from()`: `s3`
 
 error: useless conversion to the same type: `std::vec::IntoIter<Foo<'a'>>`
-  --> $DIR/useless_conversion.rs:149:13
+  --> $DIR/useless_conversion.rs:155:13
    |
 LL |     let _ = vec![s4, s4, s4].into_iter().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![s4, s4, s4].into_iter()`