Rollup merge of rust-lang#78063 - camelid:improve-cannot-multiply-error, r=estebank

Improve wording of "cannot multiply" type error

For example, if you had this code:

    fn foo(x: i32, y: f32) -> f32 {
        x * y
    }

You would get this error:

    error[E0277]: cannot multiply `f32` to `i32`
     --> src/lib.rs:2:7
      |
    2 |     x * y
      |       ^ no implementation for `i32 * f32`
      |
      = help: the trait `Mul<f32>` is not implemented for `i32`

However, that's not usually how people describe multiplication. People
usually describe multiplication like how the division error words it:

    error[E0277]: cannot divide `i32` by `f32`
     --> src/lib.rs:2:7
      |
    2 |     x / y
      |       ^ no implementation for `i32 / f32`
      |
      = help: the trait `Div<f32>` is not implemented for `i32`

So that's what this change does. It changes this:

    error[E0277]: cannot multiply `f32` to `i32`
     --> src/lib.rs:2:7
      |
    2 |     x * y
      |       ^ no implementation for `i32 * f32`
      |
      = help: the trait `Mul<f32>` is not implemented for `i32`

To this:

    error[E0277]: cannot multiply `i32` by `f32`
     --> src/lib.rs:2:7
      |
    2 |     x * y
      |       ^ no implementation for `i32 * f32`
      |
      = help: the trait `Mul<f32>` is not implemented for `i32`

Author: JohnTitor

compiler/rustc_typeck/src/check/op.rs

@@ -302,7 +302,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 true,
                             ),
                             hir::BinOpKind::Mul => (
-                                format!("cannot multiply `{}` to `{}`", rhs_ty, lhs_ty),
+                                format!("cannot multiply `{}` by `{}`", lhs_ty, rhs_ty),
                                 Some("std::ops::Mul"),
                                 true,
                             ),

library/core/src/ops/arith.rs

@@ -302,7 +302,7 @@ sub_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 #[lang = "mul"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_on_unimplemented(
-    message = "cannot multiply `{Rhs}` to `{Self}`",
+    message = "cannot multiply `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} * {Rhs}`"
 )]
 #[doc(alias = "*")]
@@ -826,7 +826,7 @@ sub_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 #[lang = "mul_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
 #[rustc_on_unimplemented(
-    message = "cannot multiply-assign `{Rhs}` to `{Self}`",
+    message = "cannot multiply-assign `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} *= {Rhs}`"
 )]
 #[doc(alias = "*")]

src/test/ui/binop/binop-mul-bool.rs

@@ -1,3 +1,3 @@
-// error-pattern:cannot multiply `bool` to `bool`
+// error-pattern:cannot multiply `bool` by `bool`
 
 fn main() { let x = true * false; }

src/test/ui/binop/binop-mul-bool.stderr

@@ -1,4 +1,4 @@
-error[E0369]: cannot multiply `bool` to `bool`
+error[E0369]: cannot multiply `bool` by `bool`
   --> $DIR/binop-mul-bool.rs:3:26
    |
 LL | fn main() { let x = true * false; }

src/test/ui/binop/binop-mul-i32-f32.rs

@@ -0,0 +1,5 @@
+fn foo(x: i32, y: f32) -> f32 {
+    x * y //~ ERROR cannot multiply `i32` by `f32`
+}
+
+fn main() {}

src/test/ui/binop/binop-mul-i32-f32.stderr

@@ -0,0 +1,11 @@
+error[E0277]: cannot multiply `i32` by `f32`
+  --> $DIR/binop-mul-i32-f32.rs:2:7
+   |
+LL |     x * y
+   |       ^ no implementation for `i32 * f32`
+   |
+   = help: the trait `Mul<f32>` is not implemented for `i32`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/issues/issue-28837.rs

@@ -7,7 +7,7 @@ fn main() {
 
     a - a; //~ ERROR cannot subtract `A` from `A`
 
-    a * a; //~ ERROR cannot multiply `A` to `A`
+    a * a; //~ ERROR cannot multiply `A` by `A`
 
     a / a; //~ ERROR cannot divide `A` by `A`
 

src/test/ui/issues/issue-28837.stderr

@@ -18,7 +18,7 @@ LL |     a - a;
    |
    = note: an implementation of `std::ops::Sub` might be missing for `A`
 
-error[E0369]: cannot multiply `A` to `A`
+error[E0369]: cannot multiply `A` by `A`
   --> $DIR/issue-28837.rs:10:7
    |
 LL |     a * a;

src/test/ui/issues/issue-35668.rs

@@ -1,6 +1,6 @@
 fn func<'a, T>(a: &'a [T]) -> impl Iterator<Item=&'a T> {
     a.iter().map(|a| a*a)
-    //~^ ERROR cannot multiply `&T` to `&T`
+    //~^ ERROR cannot multiply `&T` by `&T`
 }
 
 fn main() {

src/test/ui/issues/issue-35668.stderr

@@ -1,4 +1,4 @@
-error[E0369]: cannot multiply `&T` to `&T`
+error[E0369]: cannot multiply `&T` by `&T`
   --> $DIR/issue-35668.rs:2:23
    |
 LL |     a.iter().map(|a| a*a)