suggest removing unnecessary . to use a floating point literal

compiler/rustc_hir_typeck/src/demand.rs

@@ -43,7 +43,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_block_to_brackets_peeling_refs(err, expr, expr_ty, expected)
             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
             || self.suggest_into(err, expr, expr_ty, expected)
-            || self.suggest_option_to_bool(err, expr, expr_ty, expected);
+            || self.suggest_option_to_bool(err, expr, expr_ty, expected)
+            || self.suggest_floating_point_literal(err, expr, expected);
 
         self.note_type_is_not_clone(err, expected, expr_ty, expr);
         self.note_need_for_fn_pointer(err, expected, expr_ty);

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1204,6 +1204,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    #[instrument(skip(self, err))]
+    pub(crate) fn suggest_floating_point_literal(
+        &self,
+        err: &mut Diagnostic,
+        expr: &hir::Expr<'_>,
+        expected_ty: Ty<'tcx>,
+    ) -> bool {
+        if let ExprKind::Struct(QPath::LangItem(LangItem::Range, ..), [start, _], _) = expr.kind
+            && expected_ty.is_floating_point()
+        {
+            err.span_suggestion_verbose(
+                self.tcx.sess.source_map().next_point(start.span),
+                "remove the unnecessary `.` operator to to use a floating point literal",
+                "",
+                Applicability::MachineApplicable,
+            );
+            return true;
+        }
+        false
+    }
+
     fn is_loop(&self, id: hir::HirId) -> bool {
         let node = self.tcx.hir().get(id);
         matches!(node, Node::Expr(Expr { kind: ExprKind::Loop(..), .. }))

src/test/ui/suggestions/unnecessary_dot_for_floating_point_literal.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let _: f64 = 0..10; //~ ERROR mismatched types
+    let _: f64 = 0..; //~ ERROR mismatched types
+    let _: f64 = ..10; //~ ERROR mismatched types
+    let _: f64 = std::ops::Range { start: 0, end: 1 }; //~ ERROR mismatched types
+}

src/test/ui/suggestions/unnecessary_dot_for_floating_point_literal.stderr

@@ -0,0 +1,52 @@
+error[E0308]: mismatched types
+  --> $DIR/unnecessary_dot_for_floating_point_literal.rs:2:18
+   |
+LL |     let _: f64 = 0..10;
+   |            ---   ^^^^^ expected `f64`, found struct `std::ops::Range`
+   |            |
+   |            expected due to this
+   |
+   = note: expected type `f64`
+            found struct `std::ops::Range<{integer}>`
+help: remove the unnecessary `.` operator to to use a floating point literal
+   |
+LL -     let _: f64 = 0..10;
+LL +     let _: f64 = 0.10;
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/unnecessary_dot_for_floating_point_literal.rs:3:18
+   |
+LL |     let _: f64 = 0..;
+   |            ---   ^^^ expected `f64`, found struct `RangeFrom`
+   |            |
+   |            expected due to this
+   |
+   = note: expected type `f64`
+            found struct `RangeFrom<{integer}>`
+
+error[E0308]: mismatched types
+  --> $DIR/unnecessary_dot_for_floating_point_literal.rs:4:18
+   |
+LL |     let _: f64 = ..10;
+   |            ---   ^^^^ expected `f64`, found struct `RangeTo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected type `f64`
+            found struct `RangeTo<{integer}>`
+
+error[E0308]: mismatched types
+  --> $DIR/unnecessary_dot_for_floating_point_literal.rs:5:18
+   |
+LL |     let _: f64 = std::ops::Range { start: 0, end: 1 };
+   |            ---   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `f64`, found struct `std::ops::Range`
+   |            |
+   |            expected due to this
+   |
+   = note: expected type `f64`
+            found struct `std::ops::Range<{integer}>`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.