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);
+}