Auto merge of #89770 - jkugelman:must-use-from-and-into, r=joshtriplett

Add #[must_use] to From::from and Into::into

Risk of churn: **High**
Magic 8-Ball says: **Outlook not so good**

I figured I'd put this out there. If we don't do it now maybe we save it for a rainy day.

Parent issue: #89692

r? `@joshtriplett`

library/core/src/convert/mod.rs

@@ -273,6 +273,7 @@ pub trait AsMut<T: ?Sized> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Into<T>: Sized {
     /// Performs the conversion.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn into(self) -> T;
 }
@@ -367,6 +368,7 @@ pub trait Into<T>: Sized {
 pub trait From<T>: Sized {
     /// Performs the conversion.
     #[lang = "from"]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn from(_: T) -> Self;
 }

src/test/ui/never_type/never-value-fallback-issue-66757.nofallback.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `E: From<()>` is not satisfied
-  --> $DIR/never-value-fallback-issue-66757.rs:27:5
+  --> $DIR/never-value-fallback-issue-66757.rs:28:5
    |
 LL |     <E as From<_>>::from(never);
    |     ^^^^^^^^^^^^^^^^^^^^ the trait `From<()>` is not implemented for `E`

src/test/ui/never_type/never-value-fallback-issue-66757.rs

@@ -22,6 +22,7 @@ impl From<!> for E {
 
 #[allow(unreachable_code)]
 #[allow(dead_code)]
+#[allow(unused_must_use)]
 fn foo(never: !) {
     <E as From<!>>::from(never);  // Ok
     <E as From<_>>::from(never);  //[nofallback]~ ERROR trait bound `E: From<()>` is not satisfied

src/tools/clippy/tests/ui/cast_lossless_float.fixed

@@ -6,24 +6,24 @@
 fn main() {
     // Test clippy::cast_lossless with casts to floating-point types
     let x0 = 1i8;
-    f32::from(x0);
-    f64::from(x0);
+    let _ = f32::from(x0);
+    let _ = f64::from(x0);
     let x1 = 1u8;
-    f32::from(x1);
-    f64::from(x1);
+    let _ = f32::from(x1);
+    let _ = f64::from(x1);
     let x2 = 1i16;
-    f32::from(x2);
-    f64::from(x2);
+    let _ = f32::from(x2);
+    let _ = f64::from(x2);
     let x3 = 1u16;
-    f32::from(x3);
-    f64::from(x3);
+    let _ = f32::from(x3);
+    let _ = f64::from(x3);
     let x4 = 1i32;
-    f64::from(x4);
+    let _ = f64::from(x4);
     let x5 = 1u32;
-    f64::from(x5);
+    let _ = f64::from(x5);
 
     // Test with casts from floating-point types
-    f64::from(1.0f32);
+    let _ = f64::from(1.0f32);
 }
 
 // The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,

src/tools/clippy/tests/ui/cast_lossless_float.rs

@@ -6,24 +6,24 @@
 fn main() {
     // Test clippy::cast_lossless with casts to floating-point types
     let x0 = 1i8;
-    x0 as f32;
-    x0 as f64;
+    let _ = x0 as f32;
+    let _ = x0 as f64;
     let x1 = 1u8;
-    x1 as f32;
-    x1 as f64;
+    let _ = x1 as f32;
+    let _ = x1 as f64;
     let x2 = 1i16;
-    x2 as f32;
-    x2 as f64;
+    let _ = x2 as f32;
+    let _ = x2 as f64;
     let x3 = 1u16;
-    x3 as f32;
-    x3 as f64;
+    let _ = x3 as f32;
+    let _ = x3 as f64;
     let x4 = 1i32;
-    x4 as f64;
+    let _ = x4 as f64;
     let x5 = 1u32;
-    x5 as f64;
+    let _ = x5 as f64;
 
     // Test with casts from floating-point types
-    1.0f32 as f64;
+    let _ = 1.0f32 as f64;
 }
 
 // The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,

src/tools/clippy/tests/ui/cast_lossless_float.stderr

@@ -1,70 +1,70 @@
 error: casting `i8` to `f32` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:9:5
+  --> $DIR/cast_lossless_float.rs:9:13
    |
-LL |     x0 as f32;
-   |     ^^^^^^^^^ help: try: `f32::from(x0)`
+LL |     let _ = x0 as f32;
+   |             ^^^^^^^^^ help: try: `f32::from(x0)`
    |
    = note: `-D clippy::cast-lossless` implied by `-D warnings`
 
 error: casting `i8` to `f64` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:10:5
+  --> $DIR/cast_lossless_float.rs:10:13
    |
-LL |     x0 as f64;
-   |     ^^^^^^^^^ help: try: `f64::from(x0)`
+LL |     let _ = x0 as f64;
+   |             ^^^^^^^^^ help: try: `f64::from(x0)`
 
 error: casting `u8` to `f32` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:12:5
+  --> $DIR/cast_lossless_float.rs:12:13
    |
-LL |     x1 as f32;
-   |     ^^^^^^^^^ help: try: `f32::from(x1)`
+LL |     let _ = x1 as f32;
+   |             ^^^^^^^^^ help: try: `f32::from(x1)`
 
 error: casting `u8` to `f64` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:13:5
+  --> $DIR/cast_lossless_float.rs:13:13
    |
-LL |     x1 as f64;
-   |     ^^^^^^^^^ help: try: `f64::from(x1)`
+LL |     let _ = x1 as f64;
+   |             ^^^^^^^^^ help: try: `f64::from(x1)`
 
 error: casting `i16` to `f32` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:15:5
+  --> $DIR/cast_lossless_float.rs:15:13
    |
-LL |     x2 as f32;
-   |     ^^^^^^^^^ help: try: `f32::from(x2)`
+LL |     let _ = x2 as f32;
+   |             ^^^^^^^^^ help: try: `f32::from(x2)`
 
 error: casting `i16` to `f64` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:16:5
+  --> $DIR/cast_lossless_float.rs:16:13
    |
-LL |     x2 as f64;
-   |     ^^^^^^^^^ help: try: `f64::from(x2)`
+LL |     let _ = x2 as f64;
+   |             ^^^^^^^^^ help: try: `f64::from(x2)`
 
 error: casting `u16` to `f32` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:18:5
+  --> $DIR/cast_lossless_float.rs:18:13
    |
-LL |     x3 as f32;
-   |     ^^^^^^^^^ help: try: `f32::from(x3)`
+LL |     let _ = x3 as f32;
+   |             ^^^^^^^^^ help: try: `f32::from(x3)`
 
 error: casting `u16` to `f64` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:19:5
+  --> $DIR/cast_lossless_float.rs:19:13
    |
-LL |     x3 as f64;
-   |     ^^^^^^^^^ help: try: `f64::from(x3)`
+LL |     let _ = x3 as f64;
+   |             ^^^^^^^^^ help: try: `f64::from(x3)`
 
 error: casting `i32` to `f64` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:21:5
+  --> $DIR/cast_lossless_float.rs:21:13
    |
-LL |     x4 as f64;
-   |     ^^^^^^^^^ help: try: `f64::from(x4)`
+LL |     let _ = x4 as f64;
+   |             ^^^^^^^^^ help: try: `f64::from(x4)`
 
 error: casting `u32` to `f64` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:23:5
+  --> $DIR/cast_lossless_float.rs:23:13
    |
-LL |     x5 as f64;
-   |     ^^^^^^^^^ help: try: `f64::from(x5)`
+LL |     let _ = x5 as f64;
+   |             ^^^^^^^^^ help: try: `f64::from(x5)`
 
 error: casting `f32` to `f64` may become silently lossy if you later change the type
-  --> $DIR/cast_lossless_float.rs:26:5
+  --> $DIR/cast_lossless_float.rs:26:13
    |
-LL |     1.0f32 as f64;
-   |     ^^^^^^^^^^^^^ help: try: `f64::from(1.0f32)`
+LL |     let _ = 1.0f32 as f64;
+   |             ^^^^^^^^^^^^^ help: try: `f64::from(1.0f32)`
 
 error: aborting due to 11 previous errors
 

src/tools/clippy/tests/ui/cast_lossless_integer.fixed

@@ -5,27 +5,27 @@
 
 fn main() {
     // Test clippy::cast_lossless with casts to integer types
-    i16::from(1i8);
-    i32::from(1i8);
-    i64::from(1i8);
-    i16::from(1u8);
-    i32::from(1u8);
-    i64::from(1u8);
-    u16::from(1u8);
-    u32::from(1u8);
-    u64::from(1u8);
-    i32::from(1i16);
-    i64::from(1i16);
-    i32::from(1u16);
-    i64::from(1u16);
-    u32::from(1u16);
-    u64::from(1u16);
-    i64::from(1i32);
-    i64::from(1u32);
-    u64::from(1u32);
+    let _ = i16::from(1i8);
+    let _ = i32::from(1i8);
+    let _ = i64::from(1i8);
+    let _ = i16::from(1u8);
+    let _ = i32::from(1u8);
+    let _ = i64::from(1u8);
+    let _ = u16::from(1u8);
+    let _ = u32::from(1u8);
+    let _ = u64::from(1u8);
+    let _ = i32::from(1i16);
+    let _ = i64::from(1i16);
+    let _ = i32::from(1u16);
+    let _ = i64::from(1u16);
+    let _ = u32::from(1u16);
+    let _ = u64::from(1u16);
+    let _ = i64::from(1i32);
+    let _ = i64::from(1u32);
+    let _ = u64::from(1u32);
 
     // Test with an expression wrapped in parens
-    u16::from(1u8 + 1u8);
+    let _ = u16::from(1u8 + 1u8);
 }
 
 // The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,

src/tools/clippy/tests/ui/cast_lossless_integer.rs

@@ -5,27 +5,27 @@
 
 fn main() {
     // Test clippy::cast_lossless with casts to integer types
-    1i8 as i16;
-    1i8 as i32;
-    1i8 as i64;
-    1u8 as i16;
-    1u8 as i32;
-    1u8 as i64;
-    1u8 as u16;
-    1u8 as u32;
-    1u8 as u64;
-    1i16 as i32;
-    1i16 as i64;
-    1u16 as i32;
-    1u16 as i64;
-    1u16 as u32;
-    1u16 as u64;
-    1i32 as i64;
-    1u32 as i64;
-    1u32 as u64;
+    let _ = 1i8 as i16;
+    let _ = 1i8 as i32;
+    let _ = 1i8 as i64;
+    let _ = 1u8 as i16;
+    let _ = 1u8 as i32;
+    let _ = 1u8 as i64;
+    let _ = 1u8 as u16;
+    let _ = 1u8 as u32;
+    let _ = 1u8 as u64;
+    let _ = 1i16 as i32;
+    let _ = 1i16 as i64;
+    let _ = 1u16 as i32;
+    let _ = 1u16 as i64;
+    let _ = 1u16 as u32;
+    let _ = 1u16 as u64;
+    let _ = 1i32 as i64;
+    let _ = 1u32 as i64;
+    let _ = 1u32 as u64;
 
     // Test with an expression wrapped in parens
-    (1u8 + 1u8) as u16;
+    let _ = (1u8 + 1u8) as u16;
 }
 
 // The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,