make matching on NaN a hard error, and remove the rest of illegal_floating_point_literal_pattern

compiler/rustc_lint/src/lib.rs

@@ -516,6 +516,10 @@ fn register_builtins(store: &mut LintStore) {
         "converted into hard error, see PR #118649 \
          <https://github.com/rust-lang/rust/pull/118649> for more information",
     );
+    store.register_removed(
+        "illegal_floating_point_literal_pattern",
+        "no longer a warning, float patterns behave the same as `==`",
+    );
 }
 
 fn register_internals(store: &mut LintStore) {

compiler/rustc_lint_defs/src/builtin.rs

@@ -45,7 +45,6 @@ declare_lint_pass! {
         FUZZY_PROVENANCE_CASTS,
         HIDDEN_GLOB_REEXPORTS,
         ILL_FORMED_ATTRIBUTE_INPUT,
-        ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
         INCOMPLETE_INCLUDE,
         INDIRECT_STRUCTURAL_MATCH,
         INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
@@ -1873,55 +1872,6 @@ declare_lint! {
      };
 }
 
-declare_lint! {
-    /// The `illegal_floating_point_literal_pattern` lint detects
-    /// floating-point literals used in patterns.
-    ///
-    /// ### Example
-    ///
-    /// ```rust
-    /// let x = 42.0;
-    ///
-    /// match x {
-    ///     5.0 => {}
-    ///     _ => {}
-    /// }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// Previous versions of the compiler accepted floating-point literals in
-    /// patterns, but it was later determined this was a mistake. The
-    /// semantics of comparing floating-point values may not be clear in a
-    /// pattern when contrasted with "structural equality". Typically you can
-    /// work around this by using a [match guard], such as:
-    ///
-    /// ```rust
-    /// # let x = 42.0;
-    ///
-    /// match x {
-    ///     y if y == 5.0 => {}
-    ///     _ => {}
-    /// }
-    /// ```
-    ///
-    /// This is a [future-incompatible] lint to transition this to a hard
-    /// error in the future. See [issue #41620] for more details.
-    ///
-    /// [issue #41620]: https://github.com/rust-lang/rust/issues/41620
-    /// [match guard]: https://doc.rust-lang.org/reference/expressions/match-expr.html#match-guards
-    /// [future-incompatible]: ../index.md#future-incompatible-lints
-    pub ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
-    Warn,
-    "floating-point literals cannot be used in patterns",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps,
-        reference: "issue #41620 <https://github.com/rust-lang/rust/issues/41620>",
-    };
-}
-
 declare_lint! {
     /// The `unstable_name_collisions` lint detects that you have used a name
     /// that the standard library plans to add in the future.

compiler/rustc_middle/src/mir/interpret/value.rs

@@ -418,8 +418,8 @@ impl<'tcx, Prov: Provenance> Scalar<Prov> {
 
     #[inline]
     pub fn to_float<F: Float>(self) -> InterpResult<'tcx, F> {
-        // Going through `to_uint` to check size and truncation.
-        Ok(F::from_bits(self.to_uint(Size::from_bits(F::BITS))?))
+        // Going through `to_bits` to check size and truncation.
+        Ok(F::from_bits(self.to_bits(Size::from_bits(F::BITS))?))
     }
 
     #[inline]

compiler/rustc_middle/src/ty/consts/int.rs

@@ -249,11 +249,6 @@ impl ScalarInt {
         }
     }
 
-    #[inline]
-    pub fn try_to_target_usize(&self, tcx: TyCtxt<'_>) -> Result<u64, Size> {
-        Ok(self.to_bits(tcx.data_layout.pointer_size)? as u64)
-    }
-
     /// Tries to convert the `ScalarInt` to an unsigned integer of the given size.
     /// Fails if the size of the `ScalarInt` is not equal to `size` and returns the
     /// `ScalarInt`s size in that case.
@@ -262,56 +257,61 @@ impl ScalarInt {
         self.to_bits(size)
     }
 
-    // Tries to convert the `ScalarInt` to `bool`. Fails if the `size` of the `ScalarInt`
-    // in not equal to `Size { raw: 1 }` or if the value is not 0 or 1 and returns the `size`
-    // value of the `ScalarInt` in that case.
-    #[inline]
-    pub fn try_to_bool(self) -> Result<bool, Size> {
-        match self.try_to_u8()? {
-            0 => Ok(false),
-            1 => Ok(true),
-            _ => Err(self.size()),
-        }
-    }
-
     // Tries to convert the `ScalarInt` to `u8`. Fails if the `size` of the `ScalarInt`
     // in not equal to `Size { raw: 1 }` and returns the `size` value of the `ScalarInt` in
     // that case.
     #[inline]
     pub fn try_to_u8(self) -> Result<u8, Size> {
-        self.to_bits(Size::from_bits(8)).map(|v| u8::try_from(v).unwrap())
+        self.try_to_uint(Size::from_bits(8)).map(|v| u8::try_from(v).unwrap())
     }
 
     /// Tries to convert the `ScalarInt` to `u16`. Fails if the size of the `ScalarInt`
     /// in not equal to `Size { raw: 2 }` and returns the `size` value of the `ScalarInt` in
     /// that case.
     #[inline]
     pub fn try_to_u16(self) -> Result<u16, Size> {
-        self.to_bits(Size::from_bits(16)).map(|v| u16::try_from(v).unwrap())
+        self.try_to_uint(Size::from_bits(16)).map(|v| u16::try_from(v).unwrap())
     }
 
     /// Tries to convert the `ScalarInt` to `u32`. Fails if the `size` of the `ScalarInt`
     /// in not equal to `Size { raw: 4 }` and returns the `size` value of the `ScalarInt` in
     /// that case.
     #[inline]
     pub fn try_to_u32(self) -> Result<u32, Size> {
-        self.to_bits(Size::from_bits(32)).map(|v| u32::try_from(v).unwrap())
+        self.try_to_uint(Size::from_bits(32)).map(|v| u32::try_from(v).unwrap())
     }
 
     /// Tries to convert the `ScalarInt` to `u64`. Fails if the `size` of the `ScalarInt`
     /// in not equal to `Size { raw: 8 }` and returns the `size` value of the `ScalarInt` in
     /// that case.
     #[inline]
     pub fn try_to_u64(self) -> Result<u64, Size> {
-        self.to_bits(Size::from_bits(64)).map(|v| u64::try_from(v).unwrap())
+        self.try_to_uint(Size::from_bits(64)).map(|v| u64::try_from(v).unwrap())
     }
 
     /// Tries to convert the `ScalarInt` to `u128`. Fails if the `size` of the `ScalarInt`
     /// in not equal to `Size { raw: 16 }` and returns the `size` value of the `ScalarInt` in
     /// that case.
     #[inline]
     pub fn try_to_u128(self) -> Result<u128, Size> {
-        self.to_bits(Size::from_bits(128))
+        self.try_to_uint(Size::from_bits(128))
+    }
+
+    #[inline]
+    pub fn try_to_target_usize(&self, tcx: TyCtxt<'_>) -> Result<u64, Size> {
+        self.try_to_uint(tcx.data_layout.pointer_size).map(|v| u64::try_from(v).unwrap())
+    }
+
+    // Tries to convert the `ScalarInt` to `bool`. Fails if the `size` of the `ScalarInt`
+    // in not equal to `Size { raw: 1 }` or if the value is not 0 or 1 and returns the `size`
+    // value of the `ScalarInt` in that case.
+    #[inline]
+    pub fn try_to_bool(self) -> Result<bool, Size> {
+        match self.try_to_u8()? {
+            0 => Ok(false),
+            1 => Ok(true),
+            _ => Err(self.size()),
+        }
     }
 
     /// Tries to convert the `ScalarInt` to a signed integer of the given size.
@@ -357,6 +357,27 @@ impl ScalarInt {
     pub fn try_to_i128(self) -> Result<i128, Size> {
         self.try_to_int(Size::from_bits(128))
     }
+
+    #[inline]
+    pub fn try_to_target_isize(&self, tcx: TyCtxt<'_>) -> Result<i64, Size> {
+        self.try_to_int(tcx.data_layout.pointer_size).map(|v| i64::try_from(v).unwrap())
+    }
+
+    #[inline]
+    pub fn try_to_float<F: Float>(self) -> Result<F, Size> {
+        // Going through `to_uint` to check size and truncation.
+        Ok(F::from_bits(self.to_bits(Size::from_bits(F::BITS))?))
+    }
+
+    #[inline]
+    pub fn try_to_f32(self) -> Result<Single, Size> {
+        self.try_to_float()
+    }
+
+    #[inline]
+    pub fn try_to_f64(self) -> Result<Double, Size> {
+        self.try_to_float()
+    }
 }
 
 macro_rules! from {
@@ -399,11 +420,7 @@ impl TryFrom<ScalarInt> for bool {
     type Error = Size;
     #[inline]
     fn try_from(int: ScalarInt) -> Result<Self, Size> {
-        int.to_bits(Size::from_bytes(1)).and_then(|u| match u {
-            0 => Ok(false),
-            1 => Ok(true),
-            _ => Err(Size::from_bytes(1)),
-        })
+        int.try_to_bool()
     }
 }
 

compiler/rustc_mir_build/messages.ftl

@@ -107,8 +107,6 @@ mir_build_extern_static_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
     .note = extern statics are not controlled by the Rust type system: invalid data, aliasing violations or data races will cause undefined behavior
     .label = use of extern static
 
-mir_build_float_pattern = floating-point types cannot be used in patterns
-
 mir_build_indirect_structural_match =
     to use a constant of type `{$non_sm_ty}` in a pattern, `{$non_sm_ty}` must be annotated with `#[derive(PartialEq)]`
 
@@ -232,6 +230,10 @@ mir_build_mutation_of_layout_constrained_field_requires_unsafe_unsafe_op_in_unsa
     .note = mutating layout constrained fields cannot statically be checked for valid values
     .label = mutation of layout constrained field
 
+mir_build_nan_pattern = cannot use NaN in patterns
+    .note = NaNs compare inequal to everything, even themselves, so this pattern would never match
+    .help = try using the `is_nan` method instead
+
 mir_build_non_const_path = runtime values cannot be referenced in patterns
 
 mir_build_non_empty_never_pattern =

compiler/rustc_mir_build/src/errors.rs

@@ -780,9 +780,14 @@ pub struct UnsizedPattern<'tcx> {
     pub non_sm_ty: Ty<'tcx>,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(mir_build_float_pattern)]
-pub struct FloatPattern;
+#[derive(Diagnostic)]
+#[diag(mir_build_nan_pattern)]
+#[note]
+#[help]
+pub struct NaNPattern {
+    #[primary_span]
+    pub span: Span,
+}
 
 #[derive(LintDiagnostic)]
 #[diag(mir_build_pointer_pattern)]

compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

@@ -1,3 +1,4 @@
+use rustc_apfloat::Float;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_index::Idx;
@@ -16,7 +17,7 @@ use std::cell::Cell;
 
 use super::PatCtxt;
 use crate::errors::{
-    FloatPattern, IndirectStructuralMatch, InvalidPattern, NonPartialEqMatch,
+    IndirectStructuralMatch, InvalidPattern, NaNPattern, NonPartialEqMatch,
     NontrivialStructuralMatch, PointerPattern, TypeNotStructural, UnionPattern, UnsizedPattern,
 };
 
@@ -317,16 +318,6 @@ impl<'tcx> ConstToPat<'tcx> {
         let param_env = self.param_env;
 
         let kind = match ty.kind() {
-            ty::Float(_) => {
-                self.saw_const_match_lint.set(true);
-                tcx.emit_node_span_lint(
-                    lint::builtin::ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
-                    id,
-                    span,
-                    FloatPattern,
-                );
-                return Err(FallbackToOpaqueConst);
-            }
             // If the type is not structurally comparable, just emit the constant directly,
             // causing the pattern match code to treat it opaquely.
             // FIXME: This code doesn't emit errors itself, the caller emits the errors.
@@ -486,6 +477,22 @@ impl<'tcx> ConstToPat<'tcx> {
                     }
                 }
             },
+            ty::Float(flt) => {
+                let v = cv.unwrap_leaf();
+                let is_nan = match flt {
+                    ty::FloatTy::F32 => v.try_to_f32().unwrap().is_nan(),
+                    ty::FloatTy::F64 => v.try_to_f64().unwrap().is_nan(),
+                };
+                if is_nan {
+                    // NaNs are not ever equal to anything so they make no sense as patterns.
+                    // Also see <https://github.com/rust-lang/rfcs/pull/3535>.
+                    let e = tcx.dcx().emit_err(NaNPattern { span });
+                    self.saw_const_match_error.set(Some(e));
+                    return Err(FallbackToOpaqueConst);
+                } else {
+                    PatKind::Constant { value: mir::Const::Ty(ty::Const::new_value(tcx, cv, ty)) }
+                }
+            }
             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::RawPtr(..) => {
                 // The raw pointers we see here have been "vetted" by valtree construction to be
                 // just integers, so we simply allow them.

src/tools/clippy/tests/ui/expect_tool_lint_rfc_2383.rs

@@ -20,12 +20,8 @@ mod rustc_ok {
     pub fn rustc_lints() {
         let x = 42.0;
 
-        #[expect(illegal_floating_point_literal_pattern)]
-        match x {
-            5.0 => {}
-            6.0 => {}
-            _ => {}
-        }
+        #[expect(invalid_nan_comparisons)]
+        let _b = x == f32::NAN;
     }
 }
 
@@ -38,13 +34,9 @@ mod rustc_warn {
     pub fn rustc_lints() {
         let x = 42;
 
-        #[expect(illegal_floating_point_literal_pattern)]
+        #[expect(invalid_nan_comparisons)]
         //~^ ERROR: this lint expectation is unfulfilled
-        match x {
-            5 => {}
-            6 => {}
-            _ => {}
-        }
+        let _b = x == 5;
     }
 }
 

src/tools/clippy/tests/ui/expect_tool_lint_rfc_2383.stderr

@@ -1,5 +1,5 @@
 error: this lint expectation is unfulfilled
-  --> $DIR/expect_tool_lint_rfc_2383.rs:35:14
+  --> $DIR/expect_tool_lint_rfc_2383.rs:31:14
    |
 LL |     #[expect(dead_code)]
    |              ^^^^^^^^^
@@ -8,31 +8,31 @@ LL |     #[expect(dead_code)]
    = help: to override `-D warnings` add `#[allow(unfulfilled_lint_expectations)]`
 
 error: this lint expectation is unfulfilled
-  --> $DIR/expect_tool_lint_rfc_2383.rs:41:18
+  --> $DIR/expect_tool_lint_rfc_2383.rs:37:18
    |
-LL |         #[expect(illegal_floating_point_literal_pattern)]
-   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |         #[expect(invalid_nan_comparisons)]
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this lint expectation is unfulfilled
-  --> $DIR/expect_tool_lint_rfc_2383.rs:116:14
+  --> $DIR/expect_tool_lint_rfc_2383.rs:108:14
    |
 LL |     #[expect(clippy::almost_swapped)]
    |              ^^^^^^^^^^^^^^^^^^^^^^
 
 error: this lint expectation is unfulfilled
-  --> $DIR/expect_tool_lint_rfc_2383.rs:124:14
+  --> $DIR/expect_tool_lint_rfc_2383.rs:116:14
    |
 LL |     #[expect(clippy::bytes_nth)]
    |              ^^^^^^^^^^^^^^^^^
 
 error: this lint expectation is unfulfilled
-  --> $DIR/expect_tool_lint_rfc_2383.rs:130:14
+  --> $DIR/expect_tool_lint_rfc_2383.rs:122:14
    |
 LL |     #[expect(clippy::if_same_then_else)]
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this lint expectation is unfulfilled
-  --> $DIR/expect_tool_lint_rfc_2383.rs:136:14
+  --> $DIR/expect_tool_lint_rfc_2383.rs:128:14
    |
 LL |     #[expect(clippy::overly_complex_bool_expr)]
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^