mark From<f16> for f32 as an unstable instance

folkertdev · folkertdev · commit 48aefdcdc7db · 2026-03-18T01:00:31.000+01:00
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
@@ -5636,7 +5636,7 @@ declare_lint! {
     /// ### Explanation
     ///
     /// Rust allows traits that are only implemented for a single floating point type to guide type
-    /// inferrence for floating point literals. This used to apply in the case of `f32: From<T>`
+    /// inference for floating point literals. This used to apply in the case of `f32: From<T>`
     /// (where `T` was inferred to be the same type as a floating point literal), as the only
     /// floating point type impl was `f32: From<f32>`. However, as Rust is in the process of adding
     /// support for `f16`, there are now two implementations for floating point types:
@@ -5657,7 +5657,7 @@ declare_lint! {
     Warn,
     "detects unsuffixed floating point literals whose type fallback to `f32`",
     @future_incompatible = FutureIncompatibleInfo {
-        reason: fcw!(FutureReleaseError #0),
+        reason: fcw!(FutureReleaseError #154024),
         report_in_deps: false,
     };
 }
diff --git a/library/core/src/convert/num.rs b/library/core/src/convert/num.rs
@@ -70,8 +70,8 @@ impl_from_bool!(i8 i16 i32 i64 i128 isize);
 
 /// Implement `From<$small>` for `$large`
 macro_rules! impl_from {
-    ($small:ty => $large:ty, #[$attr:meta]) => {
-        #[$attr]
+    ($small:ty => $large:ty, $(#[$attrs:meta]),+) => {
+        $(#[$attrs])+
         #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
         impl const From<$small> for $large {
             #[doc = concat!("Converts from [`", stringify!($small), "`] to [`", stringify!($large), "`] losslessly.")]
@@ -174,9 +174,14 @@ impl_from!(u32 => f128, #[stable(feature = "lossless_float_conv", since = "1.6.0
 // impl_from!(u64 => f128, #[stable(feature = "lossless_float_conv", since = "1.6.0")]);
 
 // float -> float
-// FIXME(f16,f128): adding additional `From<{float}>` impls to `f32` breaks inference. See
-// <https://github.com/rust-lang/rust/issues/123831>
-impl_from!(f16 => f32, #[stable(feature = "lossless_float_conv", since = "1.6.0")]);
+
+// FIXME(f16): adding the additional `From<{float}>` impl to `f32` would break inference in cases
+// like `f32::from(1.0)`. The type checker has a custom workaround to keep that and similar code
+// compiling even with the second `From<16> for f32` instance. We keep this instance unstable for
+// now so that we can later remove the workaround.
+//
+// See also <https://github.com/rust-lang/rust/issues/123831>.
+impl_from!(f16 => f32, #[unstable(feature = "f32_from_f16", issue = "154005")], #[unstable_feature_bound(f32_from_f16)]);
 impl_from!(f16 => f64, #[stable(feature = "lossless_float_conv", since = "1.6.0")]);
 impl_from!(f16 => f128, #[stable(feature = "lossless_float_conv", since = "1.6.0")]);
 impl_from!(f32 => f64, #[stable(feature = "lossless_float_conv", since = "1.6.0")]);
diff --git a/tests/ui/feature-gates/feature-gate-f16.e2015.stderr b/tests/ui/feature-gates/feature-gate-f16.e2015.stderr
@@ -19,7 +19,7 @@ LL |     let a: f16 = 100.0;
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: the type `f16` is unstable
-  --> $DIR/feature-gate-f16.rs:16:11
+  --> $DIR/feature-gate-f16.rs:19:11
    |
 LL | fn foo(a: f16) {}
    |           ^^^
@@ -29,7 +29,7 @@ LL | fn foo(a: f16) {}
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: the type `f16` is unstable
-  --> $DIR/feature-gate-f16.rs:19:8
+  --> $DIR/feature-gate-f16.rs:22:8
    |
 LL |     a: f16,
    |        ^^^
@@ -58,6 +58,27 @@ LL |     let c = 0f16;
    = help: add `#![feature(f16)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 6 previous errors
+error[E0658]: the type `f16` is unstable
+  --> $DIR/feature-gate-f16.rs:13:18
+   |
+LL |     let d: f32 = 1.0f16.into();
+   |                  ^^^^^^
+   |
+   = note: see issue #116909 <https://github.com/rust-lang/rust/issues/116909> for more information
+   = help: add `#![feature(f16)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature `f32_from_f16`
+  --> $DIR/feature-gate-f16.rs:13:25
+   |
+LL |     let d: f32 = 1.0f16.into();
+   |                         ^^^^
+   |
+   = help: add `#![feature(f32_from_f16)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: required for `f32` to implement `From<f16>`
+   = note: required for `f16` to implement `Into<f32>`
+
+error: aborting due to 8 previous errors
 
 For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/feature-gates/feature-gate-f16.e2018.stderr b/tests/ui/feature-gates/feature-gate-f16.e2018.stderr
@@ -19,7 +19,7 @@ LL |     let a: f16 = 100.0;
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: the type `f16` is unstable
-  --> $DIR/feature-gate-f16.rs:16:11
+  --> $DIR/feature-gate-f16.rs:19:11
    |
 LL | fn foo(a: f16) {}
    |           ^^^
@@ -29,7 +29,7 @@ LL | fn foo(a: f16) {}
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: the type `f16` is unstable
-  --> $DIR/feature-gate-f16.rs:19:8
+  --> $DIR/feature-gate-f16.rs:22:8
    |
 LL |     a: f16,
    |        ^^^
@@ -58,6 +58,27 @@ LL |     let c = 0f16;
    = help: add `#![feature(f16)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 6 previous errors
+error[E0658]: the type `f16` is unstable
+  --> $DIR/feature-gate-f16.rs:13:18
+   |
+LL |     let d: f32 = 1.0f16.into();
+   |                  ^^^^^^
+   |
+   = note: see issue #116909 <https://github.com/rust-lang/rust/issues/116909> for more information
+   = help: add `#![feature(f16)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature `f32_from_f16`
+  --> $DIR/feature-gate-f16.rs:13:25
+   |
+LL |     let d: f32 = 1.0f16.into();
+   |                         ^^^^
+   |
+   = help: add `#![feature(f32_from_f16)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: required for `f32` to implement `From<f16>`
+   = note: required for `f16` to implement `Into<f32>`
+
+error: aborting due to 8 previous errors
 
 For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/feature-gates/feature-gate-f16.rs b/tests/ui/feature-gates/feature-gate-f16.rs
@@ -10,6 +10,9 @@ pub fn main() {
     let a: f16 = 100.0; //~ ERROR the type `f16` is unstable
     let b = 0.0f16; //~ ERROR the type `f16` is unstable
     let c = 0f16; //~ ERROR the type `f16` is unstable
+    let d: f32 = 1.0f16.into();
+    //~^ ERROR the type `f16` is unstable
+    //~| ERROR use of unstable library feature `f32_from_f16`
     foo(1.23);
 }
 
diff --git a/tests/ui/float/f32-into-f32.next-solver.stderr b/tests/ui/float/f32-into-f32.next-solver.stderr
@@ -5,7 +5,7 @@ LL |     foo(1.0);
    |         ^^^ help: explicitly specify the type as `f32`: `1.0_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #0 <https://github.com/rust-lang/rust/issues/0>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
    = note: `#[warn(float_literal_f32_fallback)]` on by default
 
 warning: falling back to `f32` as the trait bound `f32: From<f64>` is not satisfied
@@ -15,7 +15,7 @@ LL |     foo(-(2.5));
    |           ^^^ help: explicitly specify the type as `f32`: `2.5_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #0 <https://github.com/rust-lang/rust/issues/0>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
 
 warning: falling back to `f32` as the trait bound `f32: From<f64>` is not satisfied
   --> $DIR/f32-into-f32.rs:15:9
@@ -24,7 +24,7 @@ LL |     foo(1e5);
    |         ^^^ help: explicitly specify the type as `f32`: `1e5_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #0 <https://github.com/rust-lang/rust/issues/0>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
 
 warning: falling back to `f32` as the trait bound `f32: From<f64>` is not satisfied
   --> $DIR/f32-into-f32.rs:19:14
@@ -33,7 +33,7 @@ LL |     let x = -4.0;
    |              ^^^ help: explicitly specify the type as `f32`: `4.0_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #0 <https://github.com/rust-lang/rust/issues/0>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
 
 warning: 4 warnings emitted
 
diff --git a/tests/ui/float/f32-into-f32.old-solver.stderr b/tests/ui/float/f32-into-f32.old-solver.stderr
@@ -5,7 +5,7 @@ LL |     foo(1.0);
    |         ^^^ help: explicitly specify the type as `f32`: `1.0_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #0 <https://github.com/rust-lang/rust/issues/0>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
    = note: `#[warn(float_literal_f32_fallback)]` on by default
 
 warning: falling back to `f32` as the trait bound `f32: From<f64>` is not satisfied
@@ -15,7 +15,7 @@ LL |     foo(-(2.5));
    |           ^^^ help: explicitly specify the type as `f32`: `2.5_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #0 <https://github.com/rust-lang/rust/issues/0>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
 
 warning: falling back to `f32` as the trait bound `f32: From<f64>` is not satisfied
   --> $DIR/f32-into-f32.rs:15:9
@@ -24,7 +24,7 @@ LL |     foo(1e5);
    |         ^^^ help: explicitly specify the type as `f32`: `1e5_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #0 <https://github.com/rust-lang/rust/issues/0>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
 
 warning: falling back to `f32` as the trait bound `f32: From<f64>` is not satisfied
   --> $DIR/f32-into-f32.rs:19:14
@@ -33,7 +33,7 @@ LL |     let x = -4.0;
    |              ^^^ help: explicitly specify the type as `f32`: `4.0_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #0 <https://github.com/rust-lang/rust/issues/0>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
 
 warning: 4 warnings emitted
 
diff --git a/tests/ui/float/trait-f16-or-f32.stderr b/tests/ui/float/trait-f16-or-f32.stderr
@@ -6,9 +6,13 @@ LL |     foo(1.0);
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the following other types implement trait `Trait`:
-             f16
-             f32
+help: the following other types implement trait `Trait`
+  --> $DIR/trait-f16-or-f32.rs:6:1
+   |
+LL | impl Trait for f16 {}
+   | ^^^^^^^^^^^^^^^^^^ `f16`
+LL | impl Trait for f32 {}
+   | ^^^^^^^^^^^^^^^^^^ `f32`
 note: required by a bound in `foo`
   --> $DIR/trait-f16-or-f32.rs:9:16
    |
diff --git a/tests/ui/inference/untyped-primitives.stderr b/tests/ui/inference/untyped-primitives.stderr
@@ -5,7 +5,7 @@ LL |     let x = f32::from(3.14);
    |                       ^^^^ help: explicitly specify the type as `f32`: `3.14_f32`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #FIXME <https://github.com/rust-lang/rust/issues/FIXME>
+   = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>
    = note: `#[warn(float_literal_f32_fallback)]` on by default
 
 warning: 1 warning emitted

Original file line number	Diff line number	Diff line change
`@@ -10,6 +10,9 @@ pub fn main() {`
`10`	`10`	let a: f16 = 100.0; //~ ERROR the type `f16` is unstable
`11`	`11`	let b = 0.0f16; //~ ERROR the type `f16` is unstable
`12`	`12`	let c = 0f16; //~ ERROR the type `f16` is unstable
	`13`	`+ let d: f32 = 1.0f16.into();`
	`14`	+ //~^ ERROR the type `f16` is unstable
	`15`	+ //~\| ERROR use of unstable library feature `f32_from_f16`
`13`	`16`	`foo(1.23);`
`14`	`17`	`}`
`15`	`18`
Original file line number	Diff line number	Diff line change
`@@ -5,7 +5,7 @@ LL \| let x = f32::from(3.14);`
`5`	`5`	\| ^^^^ help: explicitly specify the type as `f32`: `3.14_f32`
`6`	`6`	`\|`
`7`	`7`	`= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!`
`8`		`- = note: for more information, see issue #FIXME <https://github.com/rust-lang/rust/issues/FIXME>`
	`8`	`+ = note: for more information, see issue #154024 <https://github.com/rust-lang/rust/issues/154024>`
`9`	`9`	= note: `#[warn(float_literal_f32_fallback)]` on by default
`10`	`10`
`11`	`11`	`warning: 1 warning emitted`