Note numeric literals that can never fit in an expected type

re https://github.com/rust-lang/rust/pull/72380#discussion_r438289385

Given the toy code

```rust
fn is_positive(n: usize) {
  n > -1_isize;
}
```

We currently get a type mismatch error like the following:

```
error[E0308]: mismatched types
 --> src/main.rs:2:9
  |
2 |     n > -1_isize;
  |         ^^^^^^^^ expected `usize`, found `isize`
  |
help: you can convert an `isize` to `usize` and panic if the converted value wouldn't fit
  |
2 |     n > (-1_isize).try_into().unwrap();
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

But clearly, `-1` can never fit into a `usize`, so the suggestion will
always panic. A more useful message would tell the user that the value
can never fit in the expected type:

```
error[E0308]: mismatched types
 --> test.rs:2:9
  |
2 |     n > -1_isize;
  |         ^^^^^^^^ expected `usize`, found `isize`
  |
note: `-1_isize` can never fit into `usize`
 --> test.rs:2:9
  |
2 |     n > -1_isize;
  |         ^^^^^^^^
```

Which is what this commit implements.

I only added this check for negative literals because

- Currently we can only perform such a check for literals (constant
  value propagation is outside the scope of the typechecker at this
  point)
- A lint error for out-of-range numeric literals is already emitted

IMO it makes more sense to put this check in librustc_lint, but as far
as I can tell the typecheck pass happens before the lint pass, so I've
added it here.

r? @estebank

src/librustc_typeck/check/demand.rs

@@ -783,6 +783,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
         };
+        let is_negative_int =
+            |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::UnNeg, ..));
+        let is_uint = |ty: Ty<'_>| matches!(ty.kind, ty::Uint(..));
 
         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
 
@@ -807,7 +810,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         "you can convert `{}` from `{}` to `{}`, matching the type of `{}`",
                         lhs_src, expected_ty, checked_ty, src
                     );
-                    let suggestion = format!("{}::from({})", checked_ty, lhs_src,);
+                    let suggestion = format!("{}::from({})", checked_ty, lhs_src);
                     (lhs_expr.span, msg, suggestion)
                 } else {
                     let msg = format!("{} and panic if the converted value wouldn't fit", msg);
@@ -822,8 +825,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             |err: &mut DiagnosticBuilder<'_>,
              found_to_exp_is_fallible: bool,
              exp_to_found_is_fallible: bool| {
+                let always_fallible = found_to_exp_is_fallible
+                    && (exp_to_found_is_fallible || expected_ty_expr.is_none());
                 let msg = if literal_is_ty_suffixed(expr) {
                     &lit_msg
+                } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
+                    // We now know that converting either the lhs or rhs is fallible. Before we
+                    // suggest a fallible conversion, check if the value can never fit in the
+                    // expected type.
+                    let msg = format!("`{}` can never fit into `{}`", src, expected_ty);
+                    err.span_note(expr.span, &msg);
+                    return;
                 } else if in_const_context {
                     // Do not recommend `into` or `try_into` in const contexts.
                     return;

src/test/ui/numeric/numeric-cast-no-fix.rs

@@ -0,0 +1,22 @@
+#[allow(unused_must_use)]
+fn main() {
+    let x_usize: usize = 1;
+    let x_u128: u128 = 2;
+    let x_u64: u64 = 3;
+    let x_u32: u32 = 4;
+    let x_u16: u16 = 5;
+    let x_u8: u8 = 6;
+
+    x_usize > -1_isize;
+    //~^ ERROR mismatched types
+    x_u128 > -1_isize;
+    //~^ ERROR mismatched types
+    x_u64 > -1_isize;
+    //~^ ERROR mismatched types
+    x_u32 > -1_isize;
+    //~^ ERROR mismatched types
+    x_u16 > -1_isize;
+    //~^ ERROR mismatched types
+    x_u8 > -1_isize;
+    //~^ ERROR mismatched types
+}

src/test/ui/numeric/numeric-cast-no-fix.stderr

@@ -0,0 +1,74 @@
+error[E0308]: mismatched types
+  --> $DIR/numeric-cast-no-fix.rs:10:15
+   |
+LL |     x_usize > -1_isize;
+   |               ^^^^^^^^ expected `usize`, found `isize`
+   |
+note: `-1_isize` can never fit into `usize`
+  --> $DIR/numeric-cast-no-fix.rs:10:15
+   |
+LL |     x_usize > -1_isize;
+   |               ^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/numeric-cast-no-fix.rs:12:14
+   |
+LL |     x_u128 > -1_isize;
+   |              ^^^^^^^^ expected `u128`, found `isize`
+   |
+note: `-1_isize` can never fit into `u128`
+  --> $DIR/numeric-cast-no-fix.rs:12:14
+   |
+LL |     x_u128 > -1_isize;
+   |              ^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/numeric-cast-no-fix.rs:14:13
+   |
+LL |     x_u64 > -1_isize;
+   |             ^^^^^^^^ expected `u64`, found `isize`
+   |
+note: `-1_isize` can never fit into `u64`
+  --> $DIR/numeric-cast-no-fix.rs:14:13
+   |
+LL |     x_u64 > -1_isize;
+   |             ^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/numeric-cast-no-fix.rs:16:13
+   |
+LL |     x_u32 > -1_isize;
+   |             ^^^^^^^^ expected `u32`, found `isize`
+   |
+note: `-1_isize` can never fit into `u32`
+  --> $DIR/numeric-cast-no-fix.rs:16:13
+   |
+LL |     x_u32 > -1_isize;
+   |             ^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/numeric-cast-no-fix.rs:18:13
+   |
+LL |     x_u16 > -1_isize;
+   |             ^^^^^^^^ expected `u16`, found `isize`
+   |
+note: `-1_isize` can never fit into `u16`
+  --> $DIR/numeric-cast-no-fix.rs:18:13
+   |
+LL |     x_u16 > -1_isize;
+   |             ^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/numeric-cast-no-fix.rs:20:12
+   |
+LL |     x_u8 > -1_isize;
+   |            ^^^^^^^^ expected `u8`, found `isize`
+   |
+help: you can convert `x_u8` from `u8` to `isize`, matching the type of `-1_isize`
+   |
+LL |     isize::from(x_u8) > -1_isize;
+   |     ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/repeat_count.stderr

@@ -39,12 +39,24 @@ error[E0308]: mismatched types
    |
 LL |     let f = [0; -4_isize];
    |                 ^^^^^^^^ expected `usize`, found `isize`
+   |
+note: `-4_isize` can never fit into `usize`
+  --> $DIR/repeat_count.rs:19:17
+   |
+LL |     let f = [0; -4_isize];
+   |                 ^^^^^^^^
 
 error[E0308]: mismatched types
   --> $DIR/repeat_count.rs:22:23
    |
 LL |     let f = [0_usize; -1_isize];
    |                       ^^^^^^^^ expected `usize`, found `isize`
+   |
+note: `-1_isize` can never fit into `usize`
+  --> $DIR/repeat_count.rs:22:23
+   |
+LL |     let f = [0_usize; -1_isize];
+   |                       ^^^^^^^^
 
 error[E0308]: mismatched types
   --> $DIR/repeat_count.rs:25:17