Rollup merge of #83018 - oli-obk:float_check, r=davidtwco

Reintroduce accidentally deleted assertions.

These were removed in https://github.com/rust-lang/rust/pull/50198

compiler/rustc_mir_build/src/thir/constant.rs

@@ -1,3 +1,4 @@
+use rustc_apfloat::Float;
 use rustc_ast as ast;
 use rustc_middle::mir::interpret::{
     Allocation, ConstValue, LitToConstError, LitToConstInput, Scalar,
@@ -61,20 +62,40 @@ fn parse_float<'tcx>(num: Symbol, fty: ty::FloatTy, neg: bool) -> Result<ConstVa
     use rustc_apfloat::ieee::{Double, Single};
     let scalar = match fty {
         ty::FloatTy::F32 => {
-            num.parse::<f32>().map_err(|_| ())?;
+            let rust_f = num.parse::<f32>().map_err(|_| ())?;
             let mut f = num.parse::<Single>().unwrap_or_else(|e| {
                 panic!("apfloat::ieee::Single failed to parse `{}`: {:?}", num, e)
             });
+            assert!(
+                u128::from(rust_f.to_bits()) == f.to_bits(),
+                "apfloat::ieee::Single gave different result for `{}`: \
+                 {}({:#x}) vs Rust's {}({:#x})",
+                rust_f,
+                f,
+                f.to_bits(),
+                Single::from_bits(rust_f.to_bits().into()),
+                rust_f.to_bits()
+            );
             if neg {
                 f = -f;
             }
             Scalar::from_f32(f)
         }
         ty::FloatTy::F64 => {
-            num.parse::<f64>().map_err(|_| ())?;
+            let rust_f = num.parse::<f64>().map_err(|_| ())?;
             let mut f = num.parse::<Double>().unwrap_or_else(|e| {
                 panic!("apfloat::ieee::Double failed to parse `{}`: {:?}", num, e)
             });
+            assert!(
+                u128::from(rust_f.to_bits()) == f.to_bits(),
+                "apfloat::ieee::Double gave different result for `{}`: \
+                 {}({:#x}) vs Rust's {}({:#x})",
+                rust_f,
+                f,
+                f.to_bits(),
+                Double::from_bits(rust_f.to_bits().into()),
+                rust_f.to_bits()
+            );
             if neg {
                 f = -f;
             }