Fallback {float} to f32 when f32: From<{float}> and add impl From<f16> for f32

compiler/rustc_hir/src/lang_items.rs

@@ -439,6 +439,9 @@ language_item_table! {
     DefaultTrait3,           sym::default_trait3,      default_trait3_trait,       Target::Trait,          GenericRequirement::None;
     DefaultTrait2,           sym::default_trait2,      default_trait2_trait,       Target::Trait,          GenericRequirement::None;
     DefaultTrait1,           sym::default_trait1,      default_trait1_trait,       Target::Trait,          GenericRequirement::None;
+
+    // Used to fallback `{float}` to `f32` when `f32: From<{float}>`
+    From,                    sym::From,                from_trait,                 Target::Trait,          GenericRequirement::Exact(1);
 }
 
 /// The requirement imposed on the generics of a lang item

compiler/rustc_hir_typeck/messages.ftl

@@ -92,6 +92,10 @@ hir_typeck_field_multiply_specified_in_initializer =
     .label = used more than once
     .previous_use_label = first use of `{$ident}`
 
+hir_typeck_float_literal_f32_fallback =
+    falling back to `f32` as the trait bound `f32: From<f64>` is not satisfied
+    .suggestion = explicitly specify the type as `f32`
+
 hir_typeck_fn_item_to_variadic_function = can't pass a function item to a variadic function
     .suggestion = use a function pointer instead
     .help = a function item is zero-sized and needs to be cast into a function pointer to be used in FFI

compiler/rustc_hir_typeck/src/demand.rs

@@ -342,7 +342,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     match infer {
                         ty::TyVar(_) => self.next_ty_var(DUMMY_SP),
                         ty::IntVar(_) => self.next_int_var(),
-                        ty::FloatVar(_) => self.next_float_var(),
+                        ty::FloatVar(_) => self.next_float_var(DUMMY_SP),
                         ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {
                             bug!("unexpected fresh ty outside of the trait solver")
                         }

compiler/rustc_hir_typeck/src/errors.rs

@@ -979,3 +979,11 @@ pub(crate) enum SupertraitItemShadowee {
         traits: DiagSymbolList,
     },
 }
+
+#[derive(LintDiagnostic)]
+#[diag(hir_typeck_float_literal_f32_fallback)]
+pub(crate) struct FloatLiteralF32Fallback {
+    pub literal: String,
+    #[suggestion(code = "{literal}_f32", applicability = "machine-applicable")]
+    pub span: Option<Span>,
+}

compiler/rustc_hir_typeck/src/fallback.rs

@@ -6,12 +6,15 @@ use rustc_data_structures::graph::iterate::DepthFirstSearch;
 use rustc_data_structures::graph::vec_graph::VecGraph;
 use rustc_data_structures::graph::{self};
 use rustc_data_structures::unord::{UnordBag, UnordMap, UnordSet};
-use rustc_hir as hir;
-use rustc_hir::HirId;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::{InferKind, Visitor};
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable};
+use rustc_hir::{self as hir, CRATE_HIR_ID, HirId};
+use rustc_lint::builtin::FLOAT_LITERAL_F32_FALLBACK;
+use rustc_middle::ty::{
+    self, ClauseKind, FloatVid, PredicatePolarity, TraitPredicate, Ty, TyCtxt, TypeSuperVisitable,
+    TypeVisitable,
+};
 use rustc_session::lint;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::{DUMMY_SP, Span};
@@ -92,14 +95,16 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
 
         let diverging_fallback = self
             .calculate_diverging_fallback(&unresolved_variables, self.diverging_fallback_behavior);
+        let fallback_to_f32 = self.calculate_fallback_to_f32(&unresolved_variables);
 
         // We do fallback in two passes, to try to generate
         // better error messages.
         // The first time, we do *not* replace opaque types.
         let mut fallback_occurred = false;
         for ty in unresolved_variables {
             debug!("unsolved_variable = {:?}", ty);
-            fallback_occurred |= self.fallback_if_possible(ty, &diverging_fallback);
+            fallback_occurred |=
+                self.fallback_if_possible(ty, &diverging_fallback, &fallback_to_f32);
         }
 
         fallback_occurred
@@ -109,7 +114,8 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
     //
     // - Unconstrained ints are replaced with `i32`.
     //
-    // - Unconstrained floats are replaced with `f64`.
+    // - Unconstrained floats are replaced with `f64`, except when there is a trait predicate
+    //   `f32: From<{float}>`, in which case `f32` is used as the fallback instead.
     //
     // - Non-numerics may get replaced with `()` or `!`, depending on
     //   how they were categorized by `calculate_diverging_fallback`
@@ -124,6 +130,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         &self,
         ty: Ty<'tcx>,
         diverging_fallback: &UnordMap<Ty<'tcx>, Ty<'tcx>>,
+        fallback_to_f32: &UnordSet<FloatVid>,
     ) -> bool {
         // Careful: we do NOT shallow-resolve `ty`. We know that `ty`
         // is an unsolved variable, and we determine its fallback
@@ -146,6 +153,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         let fallback = match ty.kind() {
             _ if let Some(e) = self.tainted_by_errors() => Ty::new_error(self.tcx, e),
             ty::Infer(ty::IntVar(_)) => self.tcx.types.i32,
+            ty::Infer(ty::FloatVar(vid)) if fallback_to_f32.contains(vid) => self.tcx.types.f32,
             ty::Infer(ty::FloatVar(_)) => self.tcx.types.f64,
             _ => match diverging_fallback.get(&ty) {
                 Some(&fallback_ty) => fallback_ty,
@@ -160,6 +168,78 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         true
     }
 
+    /// Existing code relies on `f32: From<T>` (usually written as `T: Into<f32>`) resolving `T` to
+    /// `f32` when the type of `T` is inferred from an unsuffixed float literal. Using the default
+    /// fallback of `f64`, this would break when adding `impl From<f16> for f32`, as there are now
+    /// two float type which could be `T`, meaning that the fallback of `f64` would be used and
+    /// compilation error would occur as `f32` does not implement `From<f64>`. To avoid breaking
+    /// existing code, we instead fallback `T` to `f32` when there is a trait predicate
+    /// `f32: From<T>`. This means code like the following will continue to compile:
+    ///
+    /// ```rust
+    /// fn foo<T: Into<f32>>(_: T) {}
+    ///
+    /// foo(1.0);
+    /// ```
+    fn calculate_fallback_to_f32(&self, unresolved_variables: &[Ty<'tcx>]) -> UnordSet<FloatVid> {
+        let Some(from_trait) = self.tcx.lang_items().from_trait() else {
+            return UnordSet::new();
+        };
+        let pending_obligations = self.fulfillment_cx.borrow_mut().pending_obligations();
+        debug!("calculate_fallback_to_f32: pending_obligations={:?}", pending_obligations);
+        let roots: UnordSet<ty::FloatVid> = pending_obligations
+            .into_iter()
+            .filter_map(|obligation| {
+                // The predicates we are looking for look like
+                // `TraitPredicate(<f32 as std::convert::From<{float}>>, polarity:Positive)`.
+                // They will have no bound variables.
+                obligation.predicate.kind().no_bound_vars()
+            })
+            .filter_map(|predicate| match predicate {
+                ty::PredicateKind::Clause(ClauseKind::Trait(TraitPredicate {
+                    polarity: PredicatePolarity::Positive,
+                    trait_ref,
+                })) if trait_ref.def_id == from_trait
+                    && self.shallow_resolve(trait_ref.self_ty()).kind()
+                        == &ty::Float(ty::FloatTy::F32) =>
+                {
+                    self.root_float_vid(trait_ref.args.type_at(1))
+                }
+                _ => None,
+            })
+            .collect();
+        debug!("calculate_fallback_to_f32: roots={:?}", roots);
+        if roots.is_empty() {
+            // Most functions have no `f32: From<{float}>` predicates, so short-circuit and return
+            // an empty set when this is the case.
+            return UnordSet::new();
+        }
+        // Calculate all the unresolved variables that need to fallback to `f32` here. This ensures
+        // we don't need to find root variables in `fallback_if_possible`: see the comment at the
+        // top of that function for details.
+        let fallback_to_f32 = unresolved_variables
+            .iter()
+            .flat_map(|ty| ty.float_vid())
+            .filter(|vid| roots.contains(&self.root_float_var(*vid)))
+            .inspect(|vid| {
+                let span = self.float_var_origin(*vid);
+                // Show the entire literal in the suggestion to make it clearer.
+                let literal = self.tcx.sess.source_map().span_to_snippet(span).ok();
+                self.tcx.emit_node_span_lint(
+                    FLOAT_LITERAL_F32_FALLBACK,
+                    CRATE_HIR_ID,
+                    span,
+                    errors::FloatLiteralF32Fallback {
+                        span: literal.as_ref().map(|_| span),
+                        literal: literal.unwrap_or_default(),
+                    },
+                );
+            })
+            .collect();
+        debug!("calculate_fallback_to_f32: fallback_to_f32={:?}", fallback_to_f32);
+        fallback_to_f32
+    }
+
     /// The "diverging fallback" system is rather complicated. This is
     /// a result of our need to balance 'do the right thing' with
     /// backwards compatibility.
@@ -565,6 +645,11 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         Some(self.root_var(self.shallow_resolve(ty).ty_vid()?))
     }
 
+    /// If `ty` is an unresolved float type variable, returns its root vid.
+    fn root_float_vid(&self, ty: Ty<'tcx>) -> Option<ty::FloatVid> {
+        Some(self.root_float_var(self.shallow_resolve(ty).float_vid()?))
+    }
+
     /// Given a set of diverging vids and coercions, walk the HIR to gather a
     /// set of suggestions which can be applied to preserve fallback to unit.
     fn try_to_suggest_annotations(

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1669,7 +1669,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     ty::Float(_) => Some(ty),
                     _ => None,
                 });
-                opt_ty.unwrap_or_else(|| self.next_float_var())
+                opt_ty.unwrap_or_else(|| self.next_float_var(lit.span))
             }
             ast::LitKind::Bool(_) => tcx.types.bool,
             ast::LitKind::CStr(_, _) => Ty::new_imm_ref(

compiler/rustc_infer/src/infer/canonical/mod.rs

@@ -116,7 +116,7 @@ impl<'tcx> InferCtxt<'tcx> {
 
                     CanonicalTyVarKind::Int => self.next_int_var(),
 
-                    CanonicalTyVarKind::Float => self.next_float_var(),
+                    CanonicalTyVarKind::Float => self.next_float_var(span),
                 };
                 ty.into()
             }

compiler/rustc_infer/src/infer/mod.rs

@@ -21,6 +21,7 @@ use rustc_data_structures::unify as ut;
 use rustc_errors::{DiagCtxtHandle, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_index::IndexVec;
 use rustc_macros::extension;
 pub use rustc_macros::{TypeFoldable, TypeVisitable};
 use rustc_middle::bug;
@@ -109,6 +110,10 @@ pub struct InferCtxtInner<'tcx> {
     /// Map from floating variable to the kind of float it represents.
     float_unification_storage: ut::UnificationTableStorage<ty::FloatVid>,
 
+    /// Map from floating variable to the origin span it came from. This is only used for the FCW
+    /// for the fallback to `f32`, so can be removed once the `f32` fallback is removed.
+    float_origin_span_storage: IndexVec<FloatVid, Span>,
+
     /// Tracks the set of region variables and the constraints between them.
     ///
     /// This is initially `Some(_)` but when
@@ -165,6 +170,7 @@ impl<'tcx> InferCtxtInner<'tcx> {
             const_unification_storage: Default::default(),
             int_unification_storage: Default::default(),
             float_unification_storage: Default::default(),
+            float_origin_span_storage: Default::default(),
             region_constraint_storage: Some(Default::default()),
             region_obligations: vec![],
             opaque_type_storage: Default::default(),
@@ -633,6 +639,13 @@ impl<'tcx> InferCtxt<'tcx> {
         self.inner.borrow_mut().type_variables().var_origin(vid)
     }
 
+    /// Returns the origin of the float type variable identified by `vid`.
+    ///
+    /// No attempt is made to resolve `vid` to its root variable.
+    pub fn float_var_origin(&self, vid: FloatVid) -> Span {
+        self.inner.borrow_mut().float_origin_span_storage[vid]
+    }
+
     /// Returns the origin of the const variable identified by `vid`
     // FIXME: We should store origins separately from the unification table
     // so this doesn't need to be optional.
@@ -818,9 +831,11 @@ impl<'tcx> InferCtxt<'tcx> {
         Ty::new_int_var(self.tcx, next_int_var_id)
     }
 
-    pub fn next_float_var(&self) -> Ty<'tcx> {
-        let next_float_var_id =
-            self.inner.borrow_mut().float_unification_table().new_key(ty::FloatVarValue::Unknown);
+    pub fn next_float_var(&self, span: Span) -> Ty<'tcx> {
+        let mut inner = self.inner.borrow_mut();
+        let next_float_var_id = inner.float_unification_table().new_key(ty::FloatVarValue::Unknown);
+        let span_index = inner.float_origin_span_storage.push(span);
+        debug_assert_eq!(next_float_var_id, span_index);
         Ty::new_float_var(self.tcx, next_float_var_id)
     }
 
@@ -1061,6 +1076,10 @@ impl<'tcx> InferCtxt<'tcx> {
         self.inner.borrow_mut().type_variables().root_var(var)
     }
 
+    pub fn root_float_var(&self, var: ty::FloatVid) -> ty::FloatVid {
+        self.inner.borrow_mut().float_unification_table().find(var)
+    }
+
     pub fn root_const_var(&self, var: ty::ConstVid) -> ty::ConstVid {
         self.inner.borrow_mut().const_unification_table().find(var).vid
     }

compiler/rustc_infer/src/infer/snapshot/fudge.rs

@@ -5,14 +5,17 @@ use rustc_middle::ty::{
     self, ConstVid, FloatVid, IntVid, RegionVid, Ty, TyCtxt, TyVid, TypeFoldable, TypeFolder,
     TypeSuperFoldable,
 };
+use rustc_span::Span;
 use rustc_type_ir::TypeVisitableExt;
 use tracing::instrument;
 use ut::UnifyKey;
 
 use super::VariableLengths;
 use crate::infer::type_variable::TypeVariableOrigin;
 use crate::infer::unify_key::{ConstVariableValue, ConstVidKey};
-use crate::infer::{ConstVariableOrigin, InferCtxt, RegionVariableOrigin, UnificationTable};
+use crate::infer::{
+    ConstVariableOrigin, InferCtxt, InferCtxtInner, RegionVariableOrigin, UnificationTable,
+};
 
 fn vars_since_snapshot<'tcx, T>(
     table: &UnificationTable<'_, 'tcx, T>,
@@ -25,6 +28,14 @@ where
     T::from_index(snapshot_var_len as u32)..T::from_index(table.len() as u32)
 }
 
+fn float_vars_since_snapshot(
+    inner: &mut InferCtxtInner<'_>,
+    snapshot_var_len: usize,
+) -> (Range<FloatVid>, Vec<Span>) {
+    let range = vars_since_snapshot(&inner.float_unification_table(), snapshot_var_len);
+    (range.clone(), range.map(|index| inner.float_origin_span_storage[index]).collect())
+}
+
 fn const_vars_since_snapshot<'tcx>(
     table: &mut UnificationTable<'_, 'tcx, ConstVidKey<'tcx>>,
     snapshot_var_len: usize,
@@ -128,7 +139,7 @@ struct SnapshotVarData {
     region_vars: (Range<RegionVid>, Vec<RegionVariableOrigin>),
     type_vars: (Range<TyVid>, Vec<TypeVariableOrigin>),
     int_vars: Range<IntVid>,
-    float_vars: Range<FloatVid>,
+    float_vars: (Range<FloatVid>, Vec<Span>),
     const_vars: (Range<ConstVid>, Vec<ConstVariableOrigin>),
 }
 
@@ -141,8 +152,7 @@ impl SnapshotVarData {
         let type_vars = inner.type_variables().vars_since_snapshot(vars_pre_snapshot.type_var_len);
         let int_vars =
             vars_since_snapshot(&inner.int_unification_table(), vars_pre_snapshot.int_var_len);
-        let float_vars =
-            vars_since_snapshot(&inner.float_unification_table(), vars_pre_snapshot.float_var_len);
+        let float_vars = float_vars_since_snapshot(&mut inner, vars_pre_snapshot.float_var_len);
 
         let const_vars = const_vars_since_snapshot(
             &mut inner.const_unification_table(),
@@ -156,7 +166,7 @@ impl SnapshotVarData {
         region_vars.0.is_empty()
             && type_vars.0.is_empty()
             && int_vars.is_empty()
-            && float_vars.is_empty()
+            && float_vars.0.is_empty()
             && const_vars.0.is_empty()
     }
 }
@@ -201,8 +211,10 @@ impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for InferenceFudger<'a, 'tcx> {
                     }
                 }
                 ty::FloatVar(vid) => {
-                    if self.snapshot_vars.float_vars.contains(&vid) {
-                        self.infcx.next_float_var()
+                    if self.snapshot_vars.float_vars.0.contains(&vid) {
+                        let idx = vid.as_usize() - self.snapshot_vars.float_vars.0.start.as_usize();
+                        let span = self.snapshot_vars.float_vars.1[idx];
+                        self.infcx.next_float_var(span)
                     } else {
                         ty
                     }