Detect negative literal inferred to unsigned integer

```
error[E0277]: the trait bound `usize: Neg` is not satisfied
  --> $DIR/negative-literal-infered-to-unsigned.rs:2:14
   |
LL |     for x in -5..5 {
   |              ^^ the trait `Neg` is not implemented for `usize`
   |
help: consider specifying an integer type that can be negative
   |
LL |     for x in -5isize..5 {
   |                +++++
```

Author: estebank

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -3,7 +3,8 @@ use std::borrow::Cow;
 use std::path::PathBuf;
 
 use rustc_abi::ExternAbi;
-use rustc_ast::TraitObjectSyntax;
+use rustc_ast::ast::LitKind;
+use rustc_ast::{LitIntType, TraitObjectSyntax};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::codes::*;
@@ -280,6 +281,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             (suggested, noted_missing_impl) = self.try_conversion_context(&obligation, main_trait_predicate, &mut err);
                         }
 
+                        suggested |= self.detect_negative_literal(
+                            &obligation,
+                            main_trait_predicate,
+                            &mut err,
+                        );
+
                         if let Some(ret_span) = self.return_type_span(&obligation) {
                             if is_try_conversion {
                                 let ty = self.tcx.short_string(
@@ -950,6 +957,38 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         Ok(())
     }
 
+    fn detect_negative_literal(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+        err: &mut Diag<'_>,
+    ) -> bool {
+        if let ObligationCauseCode::BinOp { lhs_hir_id, .. } = obligation.cause.code()
+            && let hir::Node::Expr(expr) = self.tcx.hir_node(*lhs_hir_id)
+            && let hir::ExprKind::Unary(hir::UnOp::Neg, inner) = expr.kind
+            && let hir::ExprKind::Lit(lit) = inner.kind
+            && let LitKind::Int(_, LitIntType::Unsuffixed) = lit.node
+        {
+            err.span_suggestion_verbose(
+                lit.span.shrink_to_hi(),
+                "consider specifying an integer type that can be negative",
+                match trait_pred.skip_binder().self_ty().kind() {
+                    ty::Uint(ty::UintTy::Usize) => "isize",
+                    ty::Uint(ty::UintTy::U8) => "i8",
+                    ty::Uint(ty::UintTy::U16) => "i16",
+                    ty::Uint(ty::UintTy::U32) => "i32",
+                    ty::Uint(ty::UintTy::U64) => "i64",
+                    ty::Uint(ty::UintTy::U128) => "i128",
+                    _ => "i64",
+                }
+                .to_string(),
+                Applicability::MaybeIncorrect,
+            );
+            return true;
+        }
+        false
+    }
+
     /// When the `E` of the resulting `Result<T, E>` in an expression `foo().bar().baz()?`,
     /// identify those method chain sub-expressions that could or could not have been annotated
     /// with `?`.

tests/ui/suggestions/negative-literal-infered-to-unsigned.rs

@@ -0,0 +1,13 @@
+fn main() {
+    for x in -5..5 {
+        //~^ ERROR: the trait bound `usize: Neg` is not satisfied
+        //~| HELP: consider specifying an integer type that can be negative
+        do_something(x);
+    }
+    let x = -5;
+    //~^ ERROR: the trait bound `usize: Neg` is not satisfied
+    //~| HELP: consider specifying an integer type that can be negative
+    do_something(x);
+}
+
+fn do_something(_val: usize) {}

tests/ui/suggestions/negative-literal-infered-to-unsigned.stderr

@@ -0,0 +1,25 @@
+error[E0277]: the trait bound `usize: Neg` is not satisfied
+  --> $DIR/negative-literal-infered-to-unsigned.rs:2:14
+   |
+LL |     for x in -5..5 {
+   |              ^^ the trait `Neg` is not implemented for `usize`
+   |
+help: consider specifying an integer type that can be negative
+   |
+LL |     for x in -5isize..5 {
+   |                +++++
+
+error[E0277]: the trait bound `usize: Neg` is not satisfied
+  --> $DIR/negative-literal-infered-to-unsigned.rs:7:13
+   |
+LL |     let x = -5;
+   |             ^^ the trait `Neg` is not implemented for `usize`
+   |
+help: consider specifying an integer type that can be negative
+   |
+LL |     let x = -5isize;
+   |               +++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.