diff --git a/src/shims/intrinsics.rs b/src/shims/intrinsics.rs
index 0979cd7f06..1f90042219 100644
--- a/src/shims/intrinsics.rs
+++ b/src/shims/intrinsics.rs
@@ -1,9 +1,10 @@
 use std::iter;
 use std::convert::TryFrom;
 
+use rustc_ast::ast::FloatTy;
 use rustc_middle::{mir, ty};
-use rustc_apfloat::Float;
-use rustc_target::abi::{Align, LayoutOf};
+use rustc_apfloat::{Float, Round};
+use rustc_target::abi::{Align, LayoutOf, Size};
 
 use crate::*;
 
@@ -279,6 +280,22 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.write_scalar(Scalar::from_u64(f.powi(i).to_bits()), dest)?;
             }
 
+            "float_to_int_unchecked" => {
+                let val = this.read_immediate(args[0])?;
+
+                let res = match val.layout.ty.kind {
+                    ty::Float(FloatTy::F32) => {
+                        this.float_to_int_unchecked(val.to_scalar()?.to_f32()?, dest.layout.ty)?
+                    }
+                    ty::Float(FloatTy::F64) => {
+                        this.float_to_int_unchecked(val.to_scalar()?.to_f64()?, dest.layout.ty)?
+                    }
+                    _ => bug!("`float_to_int_unchecked` called with non-float input type {:?}", val.layout.ty),
+                };
+
+                this.write_scalar(res, dest)?;
+            }
+
             // Atomic operations
             #[rustfmt::skip]
             | "atomic_load"
@@ -493,4 +510,55 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         this.go_to_block(ret);
         Ok(())
     }
+
+    fn float_to_int_unchecked<F>(
+        &self,
+        f: F,
+        dest_ty: ty::Ty<'tcx>,
+    ) -> InterpResult<'tcx, Scalar<Tag>>
+    where
+        F: Float + Into<Scalar<Tag>>
+    {
+        let this = self.eval_context_ref();
+
+        // Step 1: cut off the fractional part of `f`. The result of this is
+        // guaranteed to be precisely representable in IEEE floats.
+        let f = f.round_to_integral(Round::TowardZero).value;
+
+        // Step 2: Cast the truncated float to the target integer type and see if we lose any information in this step.
+        Ok(match dest_ty.kind {
+            // Unsigned
+            ty::Uint(t) => {
+                let width = t.bit_width().unwrap_or_else(|| this.pointer_size().bits());
+                let res = f.to_u128(usize::try_from(width).unwrap());
+                if res.status.is_empty() {
+                    // No status flags means there was no further rounding or other loss of precision.
+                    Scalar::from_uint(res.value, Size::from_bits(width))
+                } else {
+                    // `f` was not representable in this integer type.
+                    throw_ub_format!(
+                        "`float_to_int_unchecked` intrinsic called on {} which cannot be represented in target type `{:?}`",
+                        f, dest_ty,
+                    );
+                }
+            }
+            // Signed
+            ty::Int(t) => {
+                let width = t.bit_width().unwrap_or_else(|| this.pointer_size().bits());
+                let res = f.to_i128(usize::try_from(width).unwrap());
+                if res.status.is_empty() {
+                    // No status flags means there was no further rounding or other loss of precision.
+                    Scalar::from_int(res.value, Size::from_bits(width))
+                } else {
+                    // `f` was not representable in this integer type.
+                    throw_ub_format!(
+                        "`float_to_int_unchecked` intrinsic called on {} which cannot be represented in target type `{:?}`",
+                        f, dest_ty,
+                    );
+                }
+            }
+            // Nothing else
+            _ => bug!("`float_to_int_unchecked` called with non-int output type {:?}", dest_ty),
+        })
+    }
 }
diff --git a/tests/compile-fail/intrinsics/copy_overlapping.rs b/tests/compile-fail/intrinsics/copy_overlapping.rs
index 76e1e20d21..8d3c681393 100644
--- a/tests/compile-fail/intrinsics/copy_overlapping.rs
+++ b/tests/compile-fail/intrinsics/copy_overlapping.rs
@@ -1,4 +1,3 @@
-//error-pattern: copy_nonoverlapping called on overlapping ranges
 #![feature(intrinsics)]
 
 // Directly call intrinsic to avoid debug assertions in libstd
@@ -11,6 +10,6 @@ fn main() {
     unsafe {
         let a = data.as_mut_ptr();
         let b = a.wrapping_offset(1) as *mut _;
-        copy_nonoverlapping(a, b, 2);
+        copy_nonoverlapping(a, b, 2); //~ ERROR copy_nonoverlapping called on overlapping ranges
     }
 }
diff --git a/tests/compile-fail/intrinsics/copy_unaligned.rs b/tests/compile-fail/intrinsics/copy_unaligned.rs
index a2a4762239..84f4de9346 100644
--- a/tests/compile-fail/intrinsics/copy_unaligned.rs
+++ b/tests/compile-fail/intrinsics/copy_unaligned.rs
@@ -1,4 +1,3 @@
-//error-pattern: accessing memory with alignment 1, but alignment 2 is required
 #![feature(intrinsics)]
 
 // Directly call intrinsic to avoid debug assertions in libstd
@@ -10,5 +9,5 @@ fn main() {
     let mut data = [0u16; 8];
     let ptr = (&mut data[0] as *mut u16 as *mut u8).wrapping_add(1) as *mut u16;
     // Even copying 0 elements to something unaligned should error
-    unsafe { copy_nonoverlapping(&data[5], ptr, 0); }
+    unsafe { copy_nonoverlapping(&data[5], ptr, 0); } //~ ERROR accessing memory with alignment 1, but alignment 2 is required
 }
diff --git a/tests/compile-fail/intrinsics/float_to_int_32_inf1.rs b/tests/compile-fail/intrinsics/float_to_int_32_inf1.rs
new file mode 100644
index 0000000000..a56f4aefad
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_32_inf1.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, i32>(f32::INFINITY); } //~ ERROR: cannot be represented in target type `i32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_32_infneg1.rs b/tests/compile-fail/intrinsics/float_to_int_32_infneg1.rs
new file mode 100644
index 0000000000..d18f75fcca
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_32_infneg1.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, i32>(f32::NEG_INFINITY); } //~ ERROR: cannot be represented in target type `i32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_32_nan.rs b/tests/compile-fail/intrinsics/float_to_int_32_nan.rs
new file mode 100644
index 0000000000..e1fe8c7cf2
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_32_nan.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, u32>(f32::NAN); } //~ ERROR: cannot be represented in target type `u32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_32_nanneg.rs b/tests/compile-fail/intrinsics/float_to_int_32_nanneg.rs
new file mode 100644
index 0000000000..38899045c9
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_32_nanneg.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, u32>(-f32::NAN); } //~ ERROR: cannot be represented in target type `u32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_32_neg.rs b/tests/compile-fail/intrinsics/float_to_int_32_neg.rs
new file mode 100644
index 0000000000..f15cf9a9cd
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_32_neg.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, u32>(-1.000000001f32); } //~ ERROR: cannot be represented in target type `u32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_32_too_big1.rs b/tests/compile-fail/intrinsics/float_to_int_32_too_big1.rs
new file mode 100644
index 0000000000..ccbf917c8e
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_32_too_big1.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, i32>(2147483648.0f32); } //~ ERROR: cannot be represented in target type `i32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_32_too_big2.rs b/tests/compile-fail/intrinsics/float_to_int_32_too_big2.rs
new file mode 100644
index 0000000000..6598fd36e0
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_32_too_big2.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, u32>((u32::MAX-127) as f32); } //~ ERROR: cannot be represented in target type `u32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_32_too_small1.rs b/tests/compile-fail/intrinsics/float_to_int_32_too_small1.rs
new file mode 100644
index 0000000000..89f09e1e3f
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_32_too_small1.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, i32>(-2147483904.0f32); } //~ ERROR: cannot be represented in target type `i32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_inf1.rs b/tests/compile-fail/intrinsics/float_to_int_64_inf1.rs
new file mode 100644
index 0000000000..e1a7b818d8
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_inf1.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(f64::INFINITY); } //~ ERROR: cannot be represented in target type `u128`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_infneg1.rs b/tests/compile-fail/intrinsics/float_to_int_64_infneg1.rs
new file mode 100644
index 0000000000..a1d757b151
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_infneg1.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(f64::NEG_INFINITY); } //~ ERROR: cannot be represented in target type `u128`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_infneg2.rs b/tests/compile-fail/intrinsics/float_to_int_64_infneg2.rs
new file mode 100644
index 0000000000..e48d19f1a6
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_infneg2.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i128>(f64::NEG_INFINITY); } //~ ERROR: cannot be represented in target type `i128`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_nan.rs b/tests/compile-fail/intrinsics/float_to_int_64_nan.rs
new file mode 100644
index 0000000000..03f378f5bc
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_nan.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u32>(f64::NAN); } //~ ERROR: cannot be represented in target type `u32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_neg.rs b/tests/compile-fail/intrinsics/float_to_int_64_neg.rs
new file mode 100644
index 0000000000..d0b5a3e21c
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_neg.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(-1.0000000000001f64); } //~ ERROR: cannot be represented in target type `u128`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_big1.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_big1.rs
new file mode 100644
index 0000000000..f928f16187
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_big1.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i32>(2147483648.0f64); } //~ ERROR: cannot be represented in target type `i32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_big2.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_big2.rs
new file mode 100644
index 0000000000..feb24c362d
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_big2.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i64>(9223372036854775808.0f64); } //~ ERROR: cannot be represented in target type `i64`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_big3.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_big3.rs
new file mode 100644
index 0000000000..cd491bfed7
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_big3.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u64>(18446744073709551616.0f64); } //~ ERROR: cannot be represented in target type `u64`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_big4.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_big4.rs
new file mode 100644
index 0000000000..e9623dba94
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_big4.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(u128::MAX as f64); } //~ ERROR: cannot be represented in target type `u128`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_big5.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_big5.rs
new file mode 100644
index 0000000000..9c31c690b4
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_big5.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i128>(240282366920938463463374607431768211455.0f64); } //~ ERROR: cannot be represented in target type `i128`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_small1.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_small1.rs
new file mode 100644
index 0000000000..08f2f9e3fd
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_small1.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i32>(-2147483649.0f64); } //~ ERROR: cannot be represented in target type `i32`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_small2.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_small2.rs
new file mode 100644
index 0000000000..f7b205de53
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_small2.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i64>(-9223372036854777856.0f64); } //~ ERROR: cannot be represented in target type `i64`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_small3.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_small3.rs
new file mode 100644
index 0000000000..779441f744
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_small3.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i128>(-240282366920938463463374607431768211455.0f64); } //~ ERROR: cannot be represented in target type `i128`
+}
diff --git a/tests/run-pass/float.rs b/tests/run-pass/float.rs
index 738df2f6c5..364388571f 100644
--- a/tests/run-pass/float.rs
+++ b/tests/run-pass/float.rs
@@ -10,6 +10,71 @@ fn assert_eq<T: PartialEq + Debug>(x: T, y: T) {
     assert_eq!(x, y);
 }
 
+trait FloatToInt<Int>: Copy {
+    fn cast(self) -> Int;
+    unsafe fn cast_unchecked(self) -> Int;
+}
+
+impl FloatToInt<i8> for f32 {
+    fn cast(self) -> i8 { self as _ }
+    unsafe fn cast_unchecked(self) -> i8 { self.to_int_unchecked() }
+}
+impl FloatToInt<i32> for f32 {
+    fn cast(self) -> i32 { self as _ }
+    unsafe fn cast_unchecked(self) -> i32 { self.to_int_unchecked() }
+}
+impl FloatToInt<u32> for f32 {
+    fn cast(self) -> u32 { self as _ }
+    unsafe fn cast_unchecked(self) -> u32 { self.to_int_unchecked() }
+}
+impl FloatToInt<i64> for f32 {
+    fn cast(self) -> i64 { self as _ }
+    unsafe fn cast_unchecked(self) -> i64 { self.to_int_unchecked() }
+}
+impl FloatToInt<u64> for f32 {
+    fn cast(self) -> u64 { self as _ }
+    unsafe fn cast_unchecked(self) -> u64 { self.to_int_unchecked() }
+}
+
+impl FloatToInt<i8> for f64 {
+    fn cast(self) -> i8 { self as _ }
+    unsafe fn cast_unchecked(self) -> i8 { self.to_int_unchecked() }
+}
+impl FloatToInt<i32> for f64 {
+    fn cast(self) -> i32 { self as _ }
+    unsafe fn cast_unchecked(self) -> i32 { self.to_int_unchecked() }
+}
+impl FloatToInt<u32> for f64 {
+    fn cast(self) -> u32 { self as _ }
+    unsafe fn cast_unchecked(self) -> u32 { self.to_int_unchecked() }
+}
+impl FloatToInt<i64> for f64 {
+    fn cast(self) -> i64 { self as _ }
+    unsafe fn cast_unchecked(self) -> i64 { self.to_int_unchecked() }
+}
+impl FloatToInt<u64> for f64 {
+    fn cast(self) -> u64 { self as _ }
+    unsafe fn cast_unchecked(self) -> u64 { self.to_int_unchecked() }
+}
+impl FloatToInt<i128> for f64 {
+    fn cast(self) -> i128 { self as _ }
+    unsafe fn cast_unchecked(self) -> i128 { self.to_int_unchecked() }
+}
+impl FloatToInt<u128> for f64 {
+    fn cast(self) -> u128 { self as _ }
+    unsafe fn cast_unchecked(self) -> u128 { self.to_int_unchecked() }
+}
+
+/// Test this cast both via `as` and via `approx_unchecked` (i.e., it must not saturate).
+#[track_caller]
+#[inline(never)]
+fn test_cast<F, I>(x: F, y: I)
+    where F: FloatToInt<I>, I: PartialEq + Debug
+{
+    assert_eq!(x.cast(), y);
+    assert_eq!(unsafe { x.cast_unchecked() }, y);
+}
+
 fn main() {
     basic();
     casts();
@@ -50,19 +115,23 @@ fn basic() {
 }
 
 fn casts() {
+    // f32 -> i8
+    test_cast::<f32, i8>(127.99, 127);
+    test_cast::<f32, i8>(-128.99, -128);
+
     // f32 -> i32
-    assert_eq::<i32>(0.0f32 as i32, 0);
-    assert_eq::<i32>(-0.0f32 as i32, 0);
-    assert_eq::<i32>(/*0x1p-149*/ f32::from_bits(0x00000001) as i32, 0);
-    assert_eq::<i32>(/*-0x1p-149*/ f32::from_bits(0x80000001) as i32, 0);
-    assert_eq::<i32>(/*0x1.19999ap+0*/ f32::from_bits(0x3f8ccccd) as i32, 1);
-    assert_eq::<i32>(/*-0x1.19999ap+0*/ f32::from_bits(0xbf8ccccd) as i32, -1);
-    assert_eq::<i32>(1.9f32 as i32, 1);
-    assert_eq::<i32>(-1.9f32 as i32, -1);
-    assert_eq::<i32>(5.0f32 as i32, 5);
-    assert_eq::<i32>(-5.0f32 as i32, -5);
-    assert_eq::<i32>(2147483520.0f32 as i32, 2147483520);
-    assert_eq::<i32>(-2147483648.0f32 as i32, -2147483648);
+    test_cast::<f32, i32>(0.0, 0);
+    test_cast::<f32, i32>(-0.0, 0);
+    test_cast::<f32, i32>(/*0x1p-149*/ f32::from_bits(0x00000001), 0);
+    test_cast::<f32, i32>(/*-0x1p-149*/ f32::from_bits(0x80000001), 0);
+    test_cast::<f32, i32>(/*0x1.19999ap+0*/ f32::from_bits(0x3f8ccccd), 1);
+    test_cast::<f32, i32>(/*-0x1.19999ap+0*/ f32::from_bits(0xbf8ccccd), -1);
+    test_cast::<f32, i32>(1.9, 1);
+    test_cast::<f32, i32>(-1.9, -1);
+    test_cast::<f32, i32>(5.0, 5);
+    test_cast::<f32, i32>(-5.0, -5);
+    test_cast::<f32, i32>(2147483520.0, 2147483520);
+    test_cast::<f32, i32>(-2147483648.0, -2147483648);
     // unrepresentable casts
     assert_eq::<i32>(2147483648.0f32 as i32, i32::MAX);
     assert_eq::<i32>(-2147483904.0f32 as i32, i32::MIN);
@@ -74,20 +143,21 @@ fn casts() {
     assert_eq::<i32>((-f32::NAN) as i32, 0);
 
     // f32 -> u32
-    assert_eq::<u32>(0.0f32 as u32, 0);
-    assert_eq::<u32>(-0.0f32 as u32, 0);
-    assert_eq::<u32>(/*0x1p-149*/ f32::from_bits(0x1) as u32, 0);
-    assert_eq::<u32>(/*-0x1p-149*/ f32::from_bits(0x80000001) as u32, 0);
-    assert_eq::<u32>(/*0x1.19999ap+0*/ f32::from_bits(0x3f8ccccd) as u32, 1);
-    assert_eq::<u32>(1.9f32 as u32, 1);
-    assert_eq::<u32>(5.0f32 as u32, 5);
-    assert_eq::<u32>(2147483648.0f32 as u32, 0x8000_0000);
-    assert_eq::<u32>(4294967040.0f32 as u32, 0u32.wrapping_sub(256));
-    assert_eq::<u32>(/*-0x1.ccccccp-1*/ f32::from_bits(0xbf666666) as u32, 0);
-    assert_eq::<u32>(/*-0x1.fffffep-1*/ f32::from_bits(0xbf7fffff) as u32, 0);
-    assert_eq::<u32>((u32::MAX-127) as f32 as u32, u32::MAX); // rounding loss
-    assert_eq::<u32>((u32::MAX-128) as f32 as u32, u32::MAX-255); // rounding loss
+    test_cast::<f32, u32>(0.0, 0);
+    test_cast::<f32, u32>(-0.0, 0);
+    test_cast::<f32, u32>(-0.9999999, 0);
+    test_cast::<f32, u32>(/*0x1p-149*/ f32::from_bits(0x1), 0);
+    test_cast::<f32, u32>(/*-0x1p-149*/ f32::from_bits(0x80000001), 0);
+    test_cast::<f32, u32>(/*0x1.19999ap+0*/ f32::from_bits(0x3f8ccccd), 1);
+    test_cast::<f32, u32>(1.9, 1);
+    test_cast::<f32, u32>(5.0, 5);
+    test_cast::<f32, u32>(2147483648.0, 0x8000_0000);
+    test_cast::<f32, u32>(4294967040.0, 0u32.wrapping_sub(256));
+    test_cast::<f32, u32>(/*-0x1.ccccccp-1*/ f32::from_bits(0xbf666666), 0);
+    test_cast::<f32, u32>(/*-0x1.fffffep-1*/ f32::from_bits(0xbf7fffff), 0);
+    test_cast::<f32, u32>((u32::MAX-128) as f32, u32::MAX-255); // rounding loss
     // unrepresentable casts
+    assert_eq::<u32>((u32::MAX-127) as f32 as u32, u32::MAX); // rounds up and then becomes unrepresentable
     assert_eq::<u32>(4294967296.0f32 as u32, u32::MAX);
     assert_eq::<u32>(-5.0f32 as u32, 0);
     assert_eq::<u32>(f32::MAX as u32, u32::MAX);
@@ -98,40 +168,44 @@ fn casts() {
     assert_eq::<u32>((-f32::NAN) as u32, 0);
 
     // f32 -> i64
-    assert_eq::<i64>(4294967296.0f32 as i64, 4294967296);
-    assert_eq::<i64>(-4294967296.0f32 as i64, -4294967296);
-    assert_eq::<i64>(9223371487098961920.0f32 as i64, 9223371487098961920);
-    assert_eq::<i64>(-9223372036854775808.0f32 as i64, -9223372036854775808);
+    test_cast::<f32, i64>(4294967296.0, 4294967296);
+    test_cast::<f32, i64>(-4294967296.0, -4294967296);
+    test_cast::<f32, i64>(9223371487098961920.0, 9223371487098961920);
+    test_cast::<f32, i64>(-9223372036854775808.0, -9223372036854775808);
+
+    // f64 -> i8
+    test_cast::<f64, i8>(127.99, 127);
+    test_cast::<f64, i8>(-128.99, -128);
 
     // f64 -> i32
-    assert_eq::<i32>(0.0f64 as i32, 0);
-    assert_eq::<i32>(-0.0f64 as i32, 0);
-    assert_eq::<i32>(/*0x1.199999999999ap+0*/ f64::from_bits(0x3ff199999999999a) as i32, 1);
-    assert_eq::<i32>(/*-0x1.199999999999ap+0*/ f64::from_bits(0xbff199999999999a) as i32, -1);
-    assert_eq::<i32>(1.9f64 as i32, 1);
-    assert_eq::<i32>(-1.9f64 as i32, -1);
-    assert_eq::<i32>(1e8f64 as i32, 100_000_000);
-    assert_eq::<i32>(2147483647.0f64 as i32, 2147483647);
-    assert_eq::<i32>(-2147483648.0f64 as i32, -2147483648);
+    test_cast::<f64, i32>(0.0, 0);
+    test_cast::<f64, i32>(-0.0, 0);
+    test_cast::<f64, i32>(/*0x1.199999999999ap+0*/ f64::from_bits(0x3ff199999999999a), 1);
+    test_cast::<f64, i32>(/*-0x1.199999999999ap+0*/ f64::from_bits(0xbff199999999999a), -1);
+    test_cast::<f64, i32>(1.9, 1);
+    test_cast::<f64, i32>(-1.9, -1);
+    test_cast::<f64, i32>(1e8, 100_000_000);
+    test_cast::<f64, i32>(2147483647.0, 2147483647);
+    test_cast::<f64, i32>(-2147483648.0, -2147483648);
     // unrepresentable casts
     assert_eq::<i32>(2147483648.0f64 as i32, i32::MAX);
     assert_eq::<i32>(-2147483649.0f64 as i32, i32::MIN);
 
     // f64 -> i64
-    assert_eq::<i64>(0.0f64 as i64, 0);
-    assert_eq::<i64>(-0.0f64 as i64, 0);
-    assert_eq::<i64>(/*0x0.0000000000001p-1022*/ f64::from_bits(0x1) as i64, 0);
-    assert_eq::<i64>(/*-0x0.0000000000001p-1022*/ f64::from_bits(0x8000000000000001) as i64, 0);
-    assert_eq::<i64>(/*0x1.199999999999ap+0*/ f64::from_bits(0x3ff199999999999a) as i64, 1);
-    assert_eq::<i64>(/*-0x1.199999999999ap+0*/ f64::from_bits(0xbff199999999999a) as i64, -1);
-    assert_eq::<i64>(5.0f64 as i64, 5);
-    assert_eq::<i64>(5.9f64 as i64, 5);
-    assert_eq::<i64>(-5.0f64 as i64, -5);
-    assert_eq::<i64>(-5.9f64 as i64, -5);
-    assert_eq::<i64>(4294967296.0f64 as i64, 4294967296);
-    assert_eq::<i64>(-4294967296.0f64 as i64, -4294967296);
-    assert_eq::<i64>(9223372036854774784.0f64 as i64, 9223372036854774784);
-    assert_eq::<i64>(-9223372036854775808.0f64 as i64, -9223372036854775808);
+    test_cast::<f64, i64>(0.0, 0);
+    test_cast::<f64, i64>(-0.0, 0);
+    test_cast::<f64, i64>(/*0x0.0000000000001p-1022*/ f64::from_bits(0x1), 0);
+    test_cast::<f64, i64>(/*-0x0.0000000000001p-1022*/ f64::from_bits(0x8000000000000001), 0);
+    test_cast::<f64, i64>(/*0x1.199999999999ap+0*/ f64::from_bits(0x3ff199999999999a), 1);
+    test_cast::<f64, i64>(/*-0x1.199999999999ap+0*/ f64::from_bits(0xbff199999999999a), -1);
+    test_cast::<f64, i64>(5.0, 5);
+    test_cast::<f64, i64>(5.9, 5);
+    test_cast::<f64, i64>(-5.0, -5);
+    test_cast::<f64, i64>(-5.9, -5);
+    test_cast::<f64, i64>(4294967296.0, 4294967296);
+    test_cast::<f64, i64>(-4294967296.0, -4294967296);
+    test_cast::<f64, i64>(9223372036854774784.0, 9223372036854774784);
+    test_cast::<f64, i64>(-9223372036854775808.0, -9223372036854775808);
     // unrepresentable casts
     assert_eq::<i64>(9223372036854775808.0f64 as i64, i64::MAX);
     assert_eq::<i64>(-9223372036854777856.0f64 as i64, i64::MIN);
@@ -143,15 +217,16 @@ fn casts() {
     assert_eq::<i64>((-f64::NAN) as i64, 0);
 
     // f64 -> u64
-    assert_eq::<u64>(0.0f64 as u64, 0);
-    assert_eq::<u64>(-0.0f64 as u64, 0);
-    assert_eq::<u64>(5.0f64 as u64, 5);
-    assert_eq::<u64>(-5.0f64 as u64, 0);
-    assert_eq::<u64>(1e16f64 as u64, 10000000000000000);
-    assert_eq::<u64>((u64::MAX-1023) as f64 as u64, u64::MAX); // rounding loss
-    assert_eq::<u64>((u64::MAX-1024) as f64 as u64, u64::MAX-2047); // rounding loss
-    assert_eq::<u64>(9223372036854775808.0f64 as u64, 9223372036854775808);
+    test_cast::<f64, u64>(0.0, 0);
+    test_cast::<f64, u64>(-0.0, 0);
+    test_cast::<f64, u64>(-0.99999999999, 0);
+    test_cast::<f64, u64>(5.0, 5);
+    test_cast::<f64, u64>(1e16, 10000000000000000);
+    test_cast::<f64, u64>((u64::MAX-1024) as f64, u64::MAX-2047); // rounding loss
+    test_cast::<f64, u64>(9223372036854775808.0, 9223372036854775808);
     // unrepresentable casts
+    assert_eq::<u64>(-5.0f64 as u64, 0);
+    assert_eq::<u64>((u64::MAX-1023) as f64 as u64, u64::MAX); // rounds up and then becomes unrepresentable
     assert_eq::<u64>(18446744073709551616.0f64 as u64, u64::MAX);
     assert_eq::<u64>(f64::MAX as u64, u64::MAX);
     assert_eq::<u64>(f64::MIN as u64, 0);
@@ -160,6 +235,14 @@ fn casts() {
     assert_eq::<u64>(f64::NAN as u64, 0);
     assert_eq::<u64>((-f64::NAN) as u64, 0);
 
+    // f64 -> i128
+    assert_eq::<i128>(f64::MAX as i128, i128::MAX);
+    assert_eq::<i128>(f64::MIN as i128, i128::MIN);
+
+    // f64 -> u128
+    assert_eq::<u128>(f64::MAX as u128, u128::MAX);
+    assert_eq::<u128>(f64::MIN as u128, 0);
+
     // int -> f32
     assert_eq::<f32>(127i8 as f32, 127.0);
     assert_eq::<f32>(2147483647i32 as f32, 2147483648.0);
@@ -210,10 +293,8 @@ fn casts() {
     assert_eq::<f32>(5.0f64 as f32, 5.0f32);
     assert_eq::<f32>(/*0x0.0000000000001p-1022*/ f64::from_bits(0x1) as f32, 0.0);
     assert_eq::<f32>(/*-0x0.0000000000001p-1022*/ (-f64::from_bits(0x1)) as f32, -0.0);
-
     assert_eq::<f32>(/*0x1.fffffe0000000p-127*/ f64::from_bits(0x380fffffe0000000) as f32, /*0x1p-149*/ f32::from_bits(0x800000));
     assert_eq::<f32>(/*0x1.4eae4f7024c7p+108*/ f64::from_bits(0x46b4eae4f7024c70) as f32, /*0x1.4eae5p+108*/ f32::from_bits(0x75a75728));
-
     assert_eq::<f32>(f64::MAX as f32, f32::INFINITY);
     assert_eq::<f32>(f64::MIN as f32, f32::NEG_INFINITY);
     assert_eq::<f32>(f64::INFINITY as f32, f32::INFINITY);