Add FCW for unsuffixed float literal f32 fallback

Author: beetrees

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

@@ -1035,3 +1035,11 @@ pub(crate) struct NakedFunctionsMustNakedAsm {
  #[label]
  pub span: Span,
 }
+
+#[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

@@ -6,11 +6,11 @@ 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_hir::{self as hir, CRATE_HIR_ID, HirId};
+use rustc_lint::builtin::FLOAT_LITERAL_F32_FALLBACK;
 use rustc_middle::ty::{self, FloatVid, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable};
 use rustc_session::lint;
 use rustc_span::def_id::LocalDefId;
@@ -192,6 +192,20 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
  .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

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1674,7 +1674,7 @@ 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;
@@ -110,6 +111,10 @@ 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
@@ -166,6 +171,7 @@ 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(),
@@ -625,6 +631,13 @@ 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.
@@ -797,9 +810,11 @@ 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)
  }
 

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

@@ -5,13 +5,16 @@ use rustc_middle::ty::{
  self, ConstVid, FloatVid, IntVid, RegionVid, Ty, TyCtxt, TyVid, TypeFoldable, TypeFolder,
  TypeSuperFoldable, TypeVisitableExt,
 };
+use rustc_span::Span;
 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>,
@@ -24,6 +27,14 @@ 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
  }

compiler/rustc_lint_defs/src/builtin.rs

@@ -5111,3 +5111,52 @@ declare_lint! {
  report_in_deps: true,
  };
 }
+
+declare_lint! {
+ /// The `float_literal_f32_fallback` lint detects situations where the type of an unsuffixed
+ /// float literal falls back to `f32` instead of `f64` to avoid a compilation error. This occurs
+ /// when there is a trait bound `f32: From<T>` (or equivalent, such as `T: Into<f32>`) and the
+ /// literal is inferred to have the same type as `T`.
+ ///
+ /// ### Example
+ ///
+ /// ```rust
+ /// fn foo(x: impl Into<f32>) -> f32 {
+ /// x.into()
+ /// }
+ ///
+ /// fn main() {
+ /// dbg!(foo(2.5));
+ /// }
+ /// ```
+ ///
+ /// {{produces}}
+ ///
+ /// ### 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>`
+ /// (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:
+ /// `f32: From<f16>` and `f32: From<f32>`. This means that the trait bound `f32: From<T>` can no
+ /// longer guide inferrence for the type of the floating point literal. The default fallback for
+ /// unsuffixed floating point literals is `f64`. As `f32` does not implement `From<f64>`,
+ /// falling back to `f64` would cause a compilation error; therefore, the float type fallback
+ /// has been tempoarily adjusted to fallback to `f32` in this scenario.
+ ///
+ /// The lint will automatically provide a machine-applicable suggestion to add a `_f32` suffix
+ /// to the literal, which will fix the problem.
+ ///
+ /// This is a [future-incompatible] lint to transition this to a hard error in the future. See
+ /// [issue #FIXME] for more details.
+ ///
+ /// [issue #FIXME]: https://github.com/rust-lang/rust/issues/FIXME
+ pub FLOAT_LITERAL_F32_FALLBACK,
+ Warn,
+ "detects unsuffixed floating point literals whose type fallback to `f32`",
+ @future_incompatible = FutureIncompatibleInfo {
+ reason: FutureIncompatibilityReason::FutureReleaseError,
+ reference: "issue #FIXME <https://github.com/rust-lang/rust/issues/FIXME>",
+ };
+}

tests/ui/float/f32-into-f32.next-solver.fixed

@@ -0,0 +1,23 @@
+//@ revisions: old-solver next-solver
+//@[next-solver] compile-flags: -Znext-solver
+//@ run-pass
+//@ run-rustfix
+
+fn foo(_: impl Into<f32>) {}
+
+fn main() {
+ foo(1.0_f32);
+ //~^ WARN falling back to `f32`
+ //~| WARN this was previously accepted
+ foo(-(2.5_f32));
+ //~^ WARN falling back to `f32`
+ //~| WARN this was previously accepted
+ foo(1e5_f32);
+ //~^ WARN falling back to `f32`
+ //~| WARN this was previously accepted
+ foo(4f32); // no warning
+ let x = -4.0_f32;
+ //~^ WARN falling back to `f32`
+ //~| WARN this was previously accepted
+ foo(x);
+}