Add more cases to the `useless_conversion` lint (#13756)

The new cases are the application of `Into::into` or `From::from`
through the following functions with no effect:
- `Option::map()`
- `Result::map()` and `Result::map_err()`
- `ControlFlow::map_break()` and `ControlFlow::map_err()`
- `Iterator::map()`

changelog: [`useless_conversion`]: detect useless calls to `Into::into`
and `From::from` application through `map*` methods

5 files changed, 240 insertions, 42 deletions

diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
index 7d1d5d69c99..941b086ab2e 100644
--- a/clippy_lints/src/methods/mod.rs
+++ b/clippy_lints/src/methods/mod.rs
@@ -4982,6 +4982,10 @@ impl Methods {
                     }
                     map_identity::check(cx, expr, recv, m_arg, name, span);
                     manual_inspect::check(cx, expr, m_arg, name, span, &self.msrv);
+                    crate::useless_conversion::check_function_application(cx, expr, recv, m_arg);
+                },
+                ("map_break" | "map_continue", [m_arg]) => {
+                    crate::useless_conversion::check_function_application(cx, expr, recv, m_arg);
                 },
                 ("map_or", [def, map]) => {
                     option_map_or_none::check(cx, expr, recv, def, map);
diff --git a/clippy_lints/src/useless_conversion.rs b/clippy_lints/src/useless_conversion.rs
index 3b05abc546f..ccf04ab50f2 100644
--- a/clippy_lints/src/useless_conversion.rs
+++ b/clippy_lints/src/useless_conversion.rs
@@ -1,8 +1,10 @@
 use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg, span_lint_and_then};
 use clippy_utils::source::{snippet, snippet_with_applicability, snippet_with_context};
-use clippy_utils::sugg::Sugg;
+use clippy_utils::sugg::{DiagExt as _, Sugg};
 use clippy_utils::ty::{is_copy, is_type_diagnostic_item, same_type_and_consts};
-use clippy_utils::{get_parent_expr, is_trait_method, is_ty_alias, path_to_local};
+use clippy_utils::{
+    get_parent_expr, is_inherent_method_call, is_trait_item, is_trait_method, is_ty_alias, path_to_local,
+};
 use rustc_errors::Applicability;
 use rustc_hir::def_id::DefId;
 use rustc_hir::{BindingMode, Expr, ExprKind, HirId, MatchSource, Node, PatKind};
@@ -10,9 +12,9 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::traits::Obligation;
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::traits::ObligationCause;
-use rustc_middle::ty::{self, EarlyBinder, GenericArg, GenericArgsRef, Ty, TypeVisitableExt};
+use rustc_middle::ty::{self, AdtDef, EarlyBinder, GenericArg, GenericArgsRef, Ty, TypeVisitableExt};
 use rustc_session::impl_lint_pass;
-use rustc_span::{Span, sym};
+use rustc_span::{Span, Symbol, sym};
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 
 declare_clippy_lint! {
@@ -382,3 +384,57 @@ impl<'tcx> LateLintPass<'tcx> for UselessConversion {
         }
     }
 }
+
+/// Check if `arg` is a `Into::into` or `From::from` applied to `receiver` to give `expr`, through a
+/// higher-order mapping function.
+pub fn check_function_application(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>, arg: &Expr<'_>) {
+    if has_eligible_receiver(cx, recv, expr)
+        && (is_trait_item(cx, arg, sym::Into) || is_trait_item(cx, arg, sym::From))
+        && let ty::FnDef(_, args) = cx.typeck_results().expr_ty(arg).kind()
+        && let &[from_ty, to_ty] = args.into_type_list(cx.tcx).as_slice()
+        && same_type_and_consts(from_ty, to_ty)
+    {
+        span_lint_and_then(
+            cx,
+            USELESS_CONVERSION,
+            expr.span.with_lo(recv.span.hi()),
+            format!("useless conversion to the same type: `{from_ty}`"),
+            |diag| {
+                diag.suggest_remove_item(
+                    cx,
+                    expr.span.with_lo(recv.span.hi()),
+                    "consider removing",
+                    Applicability::MachineApplicable,
+                );
+            },
+        );
+    }
+}
+
+fn has_eligible_receiver(cx: &LateContext<'_>, recv: &Expr<'_>, expr: &Expr<'_>) -> bool {
+    let recv_ty = cx.typeck_results().expr_ty(recv);
+    if is_inherent_method_call(cx, expr)
+        && let Some(recv_ty_defid) = recv_ty.ty_adt_def().map(AdtDef::did)
+    {
+        if let Some(diag_name) = cx.tcx.get_diagnostic_name(recv_ty_defid)
+            && matches!(diag_name, sym::Option | sym::Result)
+        {
+            return true;
+        }
+
+        // FIXME: Add ControlFlow diagnostic item
+        let def_path = cx.get_def_path(recv_ty_defid);
+        if def_path
+            .iter()
+            .map(Symbol::as_str)
+            .zip(["core", "ops", "control_flow", "ControlFlow"])
+            .all(|(sym, s)| sym == s)
+        {
+            return true;
+        }
+    }
+    if is_trait_method(cx, expr, sym::Iterator) {
+        return true;
+    }
+    false
+}
diff --git a/tests/ui/useless_conversion.fixed b/tests/ui/useless_conversion.fixed
index eff617a8016..2f7edd92bb7 100644
--- a/tests/ui/useless_conversion.fixed
+++ b/tests/ui/useless_conversion.fixed
@@ -3,6 +3,8 @@
 // FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
 #![allow(static_mut_refs)]
 
+use std::ops::ControlFlow;
+
 fn test_generic<T: Copy>(val: T) -> T {
     let _ = val;
     val
@@ -297,3 +299,46 @@ impl From<Foo<'a'>> for Foo<'b'> {
         Foo
     }
 }
+
+fn direct_application() {
+    let _: Result<(), std::io::Error> = test_issue_3913();
+    //~^ useless_conversion
+    let _: Result<(), std::io::Error> = test_issue_3913();
+    //~^ useless_conversion
+    let _: Result<(), std::io::Error> = test_issue_3913();
+    //~^ useless_conversion
+    let _: Result<(), std::io::Error> = test_issue_3913();
+    //~^ useless_conversion
+
+    let c: ControlFlow<()> = ControlFlow::Continue(());
+    let _: ControlFlow<()> = c;
+    //~^ useless_conversion
+    let c: ControlFlow<()> = ControlFlow::Continue(());
+    let _: ControlFlow<()> = c;
+    //~^ useless_conversion
+
+    struct Absorb;
+    impl From<()> for Absorb {
+        fn from(_: ()) -> Self {
+            Self
+        }
+    }
+    impl From<std::io::Error> for Absorb {
+        fn from(_: std::io::Error) -> Self {
+            Self
+        }
+    }
+    let _: Vec<u32> = [1u32].into_iter().collect();
+    //~^ useless_conversion
+
+    // No lint for those
+    let _: Result<Absorb, std::io::Error> = test_issue_3913().map(Into::into);
+    let _: Result<(), Absorb> = test_issue_3913().map_err(Into::into);
+    let _: Result<Absorb, std::io::Error> = test_issue_3913().map(From::from);
+    let _: Result<(), Absorb> = test_issue_3913().map_err(From::from);
+}
+
+fn gen_identity<T>(x: [T; 3]) -> Vec<T> {
+    x.into_iter().collect()
+    //~^ useless_conversion
+}
diff --git a/tests/ui/useless_conversion.rs b/tests/ui/useless_conversion.rs
index 64b06620789..eacdf77f905 100644
--- a/tests/ui/useless_conversion.rs
+++ b/tests/ui/useless_conversion.rs
@@ -3,6 +3,8 @@
 // FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
 #![allow(static_mut_refs)]
 
+use std::ops::ControlFlow;
+
 fn test_generic<T: Copy>(val: T) -> T {
     let _ = T::from(val);
     val.into()
@@ -297,3 +299,46 @@ impl From<Foo<'a'>> for Foo<'b'> {
         Foo
     }
 }
+
+fn direct_application() {
+    let _: Result<(), std::io::Error> = test_issue_3913().map(Into::into);
+    //~^ useless_conversion
+    let _: Result<(), std::io::Error> = test_issue_3913().map_err(Into::into);
+    //~^ useless_conversion
+    let _: Result<(), std::io::Error> = test_issue_3913().map(From::from);
+    //~^ useless_conversion
+    let _: Result<(), std::io::Error> = test_issue_3913().map_err(From::from);
+    //~^ useless_conversion
+
+    let c: ControlFlow<()> = ControlFlow::Continue(());
+    let _: ControlFlow<()> = c.map_break(Into::into);
+    //~^ useless_conversion
+    let c: ControlFlow<()> = ControlFlow::Continue(());
+    let _: ControlFlow<()> = c.map_continue(Into::into);
+    //~^ useless_conversion
+
+    struct Absorb;
+    impl From<()> for Absorb {
+        fn from(_: ()) -> Self {
+            Self
+        }
+    }
+    impl From<std::io::Error> for Absorb {
+        fn from(_: std::io::Error) -> Self {
+            Self
+        }
+    }
+    let _: Vec<u32> = [1u32].into_iter().map(Into::into).collect();
+    //~^ useless_conversion
+
+    // No lint for those
+    let _: Result<Absorb, std::io::Error> = test_issue_3913().map(Into::into);
+    let _: Result<(), Absorb> = test_issue_3913().map_err(Into::into);
+    let _: Result<Absorb, std::io::Error> = test_issue_3913().map(From::from);
+    let _: Result<(), Absorb> = test_issue_3913().map_err(From::from);
+}
+
+fn gen_identity<T>(x: [T; 3]) -> Vec<T> {
+    x.into_iter().map(Into::into).collect()
+    //~^ useless_conversion
+}
diff --git a/tests/ui/useless_conversion.stderr b/tests/ui/useless_conversion.stderr
index b149357bcf4..6aeb382902b 100644
--- a/tests/ui/useless_conversion.stderr
+++ b/tests/ui/useless_conversion.stderr
@@ -1,5 +1,5 @@
 error: useless conversion to the same type: `T`
-  --> tests/ui/useless_conversion.rs:7:13
+  --> tests/ui/useless_conversion.rs:9:13
    |
 LL |     let _ = T::from(val);
    |             ^^^^^^^^^^^^ help: consider removing `T::from()`: `val`
@@ -11,220 +11,268 @@ LL | #![deny(clippy::useless_conversion)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: useless conversion to the same type: `T`
-  --> tests/ui/useless_conversion.rs:8:5
+  --> tests/ui/useless_conversion.rs:10:5
    |
 LL |     val.into()
    |     ^^^^^^^^^^ help: consider removing `.into()`: `val`
 
 error: useless conversion to the same type: `i32`
-  --> tests/ui/useless_conversion.rs:20:22
+  --> tests/ui/useless_conversion.rs:22:22
    |
 LL |         let _: i32 = 0i32.into();
    |                      ^^^^^^^^^^^ help: consider removing `.into()`: `0i32`
 
 error: useless conversion to the same type: `std::str::Lines<'_>`
-  --> tests/ui/useless_conversion.rs:50:22
+  --> tests/ui/useless_conversion.rs:52:22
    |
 LL |     if Some("ok") == lines.into_iter().next() {}
    |                      ^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `lines`
 
 error: useless conversion to the same type: `std::str::Lines<'_>`
-  --> tests/ui/useless_conversion.rs:55:21
+  --> tests/ui/useless_conversion.rs:57:21
    |
 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<'_>`
-  --> tests/ui/useless_conversion.rs:61:22
+  --> tests/ui/useless_conversion.rs:63:22
    |
 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>`
-  --> tests/ui/useless_conversion.rs:67:13
+  --> tests/ui/useless_conversion.rs:69:13
    |
 LL |     let _ = NUMBERS.into_iter().next();
    |             ^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `NUMBERS`
 
 error: useless conversion to the same type: `std::ops::Range<i32>`
-  --> tests/ui/useless_conversion.rs:72:17
+  --> tests/ui/useless_conversion.rs:74:17
    |
 LL |     let mut n = NUMBERS.into_iter();
    |                 ^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `NUMBERS`
 
 error: useless conversion to the same type: `std::string::String`
-  --> tests/ui/useless_conversion.rs:134:21
+  --> tests/ui/useless_conversion.rs:136:21
    |
 LL |     let _: String = "foo".to_string().into();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into()`: `"foo".to_string()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> tests/ui/useless_conversion.rs:135:21
+  --> tests/ui/useless_conversion.rs:137:21
    |
 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`
-  --> tests/ui/useless_conversion.rs:136:13
+  --> tests/ui/useless_conversion.rs:138:13
    |
 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`
-  --> tests/ui/useless_conversion.rs:137:13
+  --> tests/ui/useless_conversion.rs:139:13
    |
 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<'_>`
-  --> tests/ui/useless_conversion.rs:138:13
+  --> tests/ui/useless_conversion.rs:140:13
    |
 LL |     let _ = "".lines().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `"".lines()`
 
 error: useless conversion to the same type: `std::vec::IntoIter<i32>`
-  --> tests/ui/useless_conversion.rs:139:13
+  --> tests/ui/useless_conversion.rs:141:13
    |
 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`
-  --> tests/ui/useless_conversion.rs:140:21
+  --> tests/ui/useless_conversion.rs:142:21
    |
 LL |     let _: String = format!("Hello {}", "world").into();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into()`: `format!("Hello {}", "world")`
 
 error: useless conversion to the same type: `i32`
-  --> tests/ui/useless_conversion.rs:145:13
+  --> tests/ui/useless_conversion.rs:147:13
    |
 LL |     let _ = i32::from(a + b) * 3;
    |             ^^^^^^^^^^^^^^^^ help: consider removing `i32::from()`: `(a + b)`
 
 error: useless conversion to the same type: `Foo<'a'>`
-  --> tests/ui/useless_conversion.rs:151:23
+  --> tests/ui/useless_conversion.rs:153:23
    |
 LL |     let _: Foo<'a'> = s2.into();
    |                       ^^^^^^^^^ help: consider removing `.into()`: `s2`
 
 error: useless conversion to the same type: `Foo<'a'>`
-  --> tests/ui/useless_conversion.rs:153:13
+  --> tests/ui/useless_conversion.rs:155:13
    |
 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'>>`
-  --> tests/ui/useless_conversion.rs:155:13
+  --> tests/ui/useless_conversion.rs:157:13
    |
 LL |     let _ = vec![s4, s4, s4].into_iter().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![s4, s4, s4].into_iter()`
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:187:7
+  --> tests/ui/useless_conversion.rs:189:7
    |
 LL |     b(vec![1, 2].into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:177:13
+  --> tests/ui/useless_conversion.rs:179:13
    |
 LL |     fn b<T: IntoIterator<Item = i32>>(_: T) {}
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:188:7
+  --> tests/ui/useless_conversion.rs:190:7
    |
 LL |     c(vec![1, 2].into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:178:18
+  --> tests/ui/useless_conversion.rs:180:18
    |
 LL |     fn c(_: impl IntoIterator<Item = i32>) {}
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:189:7
+  --> tests/ui/useless_conversion.rs:191:7
    |
 LL |     d(vec![1, 2].into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:181:12
+  --> tests/ui/useless_conversion.rs:183:12
    |
 LL |         T: IntoIterator<Item = i32>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:192:7
+  --> tests/ui/useless_conversion.rs:194:7
    |
 LL |     b(vec![1, 2].into_iter().into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`s: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:177:13
+  --> tests/ui/useless_conversion.rs:179:13
    |
 LL |     fn b<T: IntoIterator<Item = i32>>(_: T) {}
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:193:7
+  --> tests/ui/useless_conversion.rs:195:7
    |
 LL |     b(vec![1, 2].into_iter().into_iter().into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`s: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:177:13
+  --> tests/ui/useless_conversion.rs:179:13
    |
 LL |     fn b<T: IntoIterator<Item = i32>>(_: T) {}
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:239:24
+  --> tests/ui/useless_conversion.rs:241:24
    |
 LL |         foo2::<i32, _>([1, 2, 3].into_iter());
    |                        ^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `[1, 2, 3]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:218:12
+  --> tests/ui/useless_conversion.rs:220:12
    |
 LL |         I: IntoIterator<Item = i32> + Helper<X>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:247:14
+  --> tests/ui/useless_conversion.rs:249:14
    |
 LL |         foo3([1, 2, 3].into_iter());
    |              ^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `[1, 2, 3]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:227:12
+  --> tests/ui/useless_conversion.rs:229:12
    |
 LL |         I: IntoIterator<Item = i32>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:256:16
+  --> tests/ui/useless_conversion.rs:258:16
    |
 LL |         S1.foo([1, 2].into_iter());
    |                ^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `[1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:253:27
+  --> tests/ui/useless_conversion.rs:255:27
    |
 LL |             pub fn foo<I: IntoIterator>(&self, _: I) {}
    |                           ^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> tests/ui/useless_conversion.rs:275:44
+  --> tests/ui/useless_conversion.rs:277:44
    |
 LL |         v0.into_iter().interleave_shortest(v1.into_iter());
    |                                            ^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `v1`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> tests/ui/useless_conversion.rs:262:20
+  --> tests/ui/useless_conversion.rs:264:20
    |
 LL |                 J: IntoIterator,
    |                    ^^^^^^^^^^^^
 
-error: aborting due to 28 previous errors
+error: useless conversion to the same type: `()`
+  --> tests/ui/useless_conversion.rs:304:58
+   |
+LL |     let _: Result<(), std::io::Error> = test_issue_3913().map(Into::into);
+   |                                                          ^^^^^^^^^^^^^^^^ help: consider removing
+
+error: useless conversion to the same type: `std::io::Error`
+  --> tests/ui/useless_conversion.rs:306:58
+   |
+LL |     let _: Result<(), std::io::Error> = test_issue_3913().map_err(Into::into);
+   |                                                          ^^^^^^^^^^^^^^^^^^^^ help: consider removing
+
+error: useless conversion to the same type: `()`
+  --> tests/ui/useless_conversion.rs:308:58
+   |
+LL |     let _: Result<(), std::io::Error> = test_issue_3913().map(From::from);
+   |                                                          ^^^^^^^^^^^^^^^^ help: consider removing
+
+error: useless conversion to the same type: `std::io::Error`
+  --> tests/ui/useless_conversion.rs:310:58
+   |
+LL |     let _: Result<(), std::io::Error> = test_issue_3913().map_err(From::from);
+   |                                                          ^^^^^^^^^^^^^^^^^^^^ help: consider removing
+
+error: useless conversion to the same type: `()`
+  --> tests/ui/useless_conversion.rs:314:31
+   |
+LL |     let _: ControlFlow<()> = c.map_break(Into::into);
+   |                               ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing
+
+error: useless conversion to the same type: `()`
+  --> tests/ui/useless_conversion.rs:317:31
+   |
+LL |     let _: ControlFlow<()> = c.map_continue(Into::into);
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing
+
+error: useless conversion to the same type: `u32`
+  --> tests/ui/useless_conversion.rs:331:41
+   |
+LL |     let _: Vec<u32> = [1u32].into_iter().map(Into::into).collect();
+   |                                         ^^^^^^^^^^^^^^^^ help: consider removing
+
+error: useless conversion to the same type: `T`
+  --> tests/ui/useless_conversion.rs:342:18
+   |
+LL |     x.into_iter().map(Into::into).collect()
+   |                  ^^^^^^^^^^^^^^^^ help: consider removing
+
+error: aborting due to 36 previous errors