Auto merge of #11669 - y21:issue11577, r=Jarcho

new lint: `unnecessary_fallible_conversions`

Closes #11577

A new lint that looks for calls such as `i64::try_from(1i32)` and suggests `i64::from(1i32)`. See lint description (and linked issue) for more details for why.

There's a tiny bit of overlap with the `useless_conversion` lint, in that the other one warns `T::try_from(T)` (i.e., fallibly converting to the same type), so this lint ignores cases like `i32::try_from(1i32)` to avoid emitting two warnings for the same expression.

Also, funnily enough, with this one exception, this lint would warn on exactly every case in the `useless_conversion_try` ui test that `useless_conversion` didn't cover (but never two warnings at the same time), which is neat. I did add an `#![allow]` though since we don't want interleaved warnings from multiple lints in the same uitest.

changelog: new lint: `unnecessary_fallible_conversions`

tests/ui/manual_string_new.fixed

@@ -1,4 +1,5 @@
 #![warn(clippy::manual_string_new)]
+#![allow(clippy::unnecessary_fallible_conversions)]
 
 macro_rules! create_strings_from_macro {
     // When inside a macro, nothing should warn to prevent false positives.

tests/ui/manual_string_new.rs

@@ -1,4 +1,5 @@
 #![warn(clippy::manual_string_new)]
+#![allow(clippy::unnecessary_fallible_conversions)]
 
 macro_rules! create_strings_from_macro {
     // When inside a macro, nothing should warn to prevent false positives.

tests/ui/manual_string_new.stderr

@@ -1,5 +1,5 @@
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:13:13
+  --> $DIR/manual_string_new.rs:14:13
    |
 LL |     let _ = "".to_string();
    |             ^^^^^^^^^^^^^^ help: consider using: `String::new()`
@@ -8,49 +8,49 @@ LL |     let _ = "".to_string();
    = help: to override `-D warnings` add `#[allow(clippy::manual_string_new)]`
 
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:16:13
+  --> $DIR/manual_string_new.rs:17:13
    |
 LL |     let _ = "".to_owned();
    |             ^^^^^^^^^^^^^ help: consider using: `String::new()`
 
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:19:21
+  --> $DIR/manual_string_new.rs:20:21
    |
 LL |     let _: String = "".into();
    |                     ^^^^^^^^^ help: consider using: `String::new()`
 
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:26:13
+  --> $DIR/manual_string_new.rs:27:13
    |
 LL |     let _ = String::from("");
    |             ^^^^^^^^^^^^^^^^ help: consider using: `String::new()`
 
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:27:13
+  --> $DIR/manual_string_new.rs:28:13
    |
 LL |     let _ = <String>::from("");
    |             ^^^^^^^^^^^^^^^^^^ help: consider using: `String::new()`
 
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:32:13
+  --> $DIR/manual_string_new.rs:33:13
    |
 LL |     let _ = String::try_from("").unwrap();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `String::new()`
 
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:38:21
+  --> $DIR/manual_string_new.rs:39:21
    |
 LL |     let _: String = From::from("");
    |                     ^^^^^^^^^^^^^^ help: consider using: `String::new()`
 
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:43:21
+  --> $DIR/manual_string_new.rs:44:21
    |
 LL |     let _: String = TryFrom::try_from("").unwrap();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `String::new()`
 
 error: empty String is being created manually
-  --> $DIR/manual_string_new.rs:46:21
+  --> $DIR/manual_string_new.rs:47:21
    |
 LL |     let _: String = TryFrom::try_from("").expect("this should warn");
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `String::new()`

tests/ui/unnecessary_fallible_conversions.fixed

@@ -0,0 +1,6 @@
+#![warn(clippy::unnecessary_fallible_conversions)]
+
+fn main() {
+    let _: i64 = 0i32.into();
+    let _: i64 = 0i32.into();
+}

tests/ui/unnecessary_fallible_conversions.rs

@@ -0,0 +1,6 @@
+#![warn(clippy::unnecessary_fallible_conversions)]
+
+fn main() {
+    let _: i64 = 0i32.try_into().unwrap();
+    let _: i64 = 0i32.try_into().expect("can't happen");
+}

tests/ui/unnecessary_fallible_conversions.stderr

@@ -0,0 +1,17 @@
+error: use of a fallible conversion when an infallible one could be used
+  --> $DIR/unnecessary_fallible_conversions.rs:4:23
+   |
+LL |     let _: i64 = 0i32.try_into().unwrap();
+   |                       ^^^^^^^^^^^^^^^^^^^ help: use: `into()`
+   |
+   = note: `-D clippy::unnecessary-fallible-conversions` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::unnecessary_fallible_conversions)]`
+
+error: use of a fallible conversion when an infallible one could be used
+  --> $DIR/unnecessary_fallible_conversions.rs:5:23
+   |
+LL |     let _: i64 = 0i32.try_into().expect("can't happen");
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `into()`
+
+error: aborting due to 2 previous errors
+

tests/ui/unnecessary_fallible_conversions_unfixable.rs

@@ -0,0 +1,43 @@
+//@aux-build:proc_macros.rs
+//@no-rustfix
+#![warn(clippy::unnecessary_fallible_conversions)]
+
+extern crate proc_macros;
+
+struct Foo;
+impl TryFrom<i32> for Foo {
+    type Error = ();
+    fn try_from(_: i32) -> Result<Self, Self::Error> {
+        Ok(Foo)
+    }
+}
+impl From<i64> for Foo {
+    fn from(_: i64) -> Self {
+        Foo
+    }
+}
+
+fn main() {
+    // `Foo` only implements `TryFrom<i32>` and not `From<i32>`, so don't lint
+    let _: Result<Foo, _> = 0i32.try_into();
+    let _: Result<Foo, _> = i32::try_into(0i32);
+    let _: Result<Foo, _> = Foo::try_from(0i32);
+
+    // ... it does impl From<i64> however
+    let _: Result<Foo, _> = 0i64.try_into();
+    //~^ ERROR: use of a fallible conversion when an infallible one could be used
+    let _: Result<Foo, _> = i64::try_into(0i64);
+    //~^ ERROR: use of a fallible conversion when an infallible one could be used
+    let _: Result<Foo, _> = Foo::try_from(0i64);
+    //~^ ERROR: use of a fallible conversion when an infallible one could be used
+
+    let _: Result<i64, _> = 0i32.try_into();
+    //~^ ERROR: use of a fallible conversion when an infallible one could be used
+    let _: Result<i64, _> = i32::try_into(0i32);
+    //~^ ERROR: use of a fallible conversion when an infallible one could be used
+    let _: Result<i64, _> = <_>::try_from(0i32);
+    //~^ ERROR: use of a fallible conversion when an infallible one could be used
+
+    // From a macro
+    let _: Result<i64, _> = proc_macros::external!(0i32).try_into();
+}

tests/ui/unnecessary_fallible_conversions_unfixable.stderr

@@ -0,0 +1,41 @@
+error: use of a fallible conversion when an infallible one could be used
+  --> $DIR/unnecessary_fallible_conversions_unfixable.rs:27:34
+   |
+LL |     let _: Result<Foo, _> = 0i64.try_into();
+   |                                  ^^^^^^^^ help: use: `into`
+   |
+   = note: `-D clippy::unnecessary-fallible-conversions` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::unnecessary_fallible_conversions)]`
+
+error: use of a fallible conversion when an infallible one could be used
+  --> $DIR/unnecessary_fallible_conversions_unfixable.rs:29:29
+   |
+LL |     let _: Result<Foo, _> = i64::try_into(0i64);
+   |                             ^^^^^^^^^^^^^ help: use: `Into::into`
+
+error: use of a fallible conversion when an infallible one could be used
+  --> $DIR/unnecessary_fallible_conversions_unfixable.rs:31:29
+   |
+LL |     let _: Result<Foo, _> = Foo::try_from(0i64);
+   |                             ^^^^^^^^^^^^^ help: use: `From::from`
+
+error: use of a fallible conversion when an infallible one could be used
+  --> $DIR/unnecessary_fallible_conversions_unfixable.rs:34:34
+   |
+LL |     let _: Result<i64, _> = 0i32.try_into();
+   |                                  ^^^^^^^^ help: use: `into`
+
+error: use of a fallible conversion when an infallible one could be used
+  --> $DIR/unnecessary_fallible_conversions_unfixable.rs:36:29
+   |
+LL |     let _: Result<i64, _> = i32::try_into(0i32);
+   |                             ^^^^^^^^^^^^^ help: use: `Into::into`
+
+error: use of a fallible conversion when an infallible one could be used
+  --> $DIR/unnecessary_fallible_conversions_unfixable.rs:38:29
+   |
+LL |     let _: Result<i64, _> = <_>::try_from(0i32);
+   |                             ^^^^^^^^^^^^^ help: use: `From::from`
+
+error: aborting due to 6 previous errors
+

tests/ui/useless_conversion_try.rs

@@ -1,5 +1,5 @@
 #![deny(clippy::useless_conversion)]
-#![allow(clippy::needless_if)]
+#![allow(clippy::needless_if, clippy::unnecessary_fallible_conversions)]
 
 fn test_generic<T: Copy>(val: T) -> T {
     let _ = T::try_from(val).unwrap();