From ced11a83cb6cae998df514eb08d1f3dc4bd990c8 Mon Sep 17 00:00:00 2001
From: Andy Russell <arussell123@gmail.com>
Date: Mon, 5 Oct 2020 12:23:36 -0400
Subject: [PATCH] suggest `MAX` constant if -1 is assigned to unsigned type

---
 compiler/rustc_typeck/src/check/op.rs         | 25 +++++++++++++
 .../feature-gate-negate-unsigned.stderr       |  5 ++-
 src/test/ui/unsigned-literal-negation.rs      |  5 +++
 src/test/ui/unsigned-literal-negation.stderr  | 36 +++++++++++++++++++
 4 files changed, 70 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/unsigned-literal-negation.rs
 create mode 100644 src/test/ui/unsigned-literal-negation.stderr

diff --git a/compiler/rustc_typeck/src/check/op.rs b/compiler/rustc_typeck/src/check/op.rs
index 66975f32a1f..2f3fb49717b 100644
--- a/compiler/rustc_typeck/src/check/op.rs
+++ b/compiler/rustc_typeck/src/check/op.rs
@@ -2,6 +2,7 @@
 
 use super::method::MethodCallee;
 use super::FnCtxt;
+use rustc_ast as ast;
 use rustc_errors::{self, struct_span_err, Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
@@ -13,6 +14,7 @@ use rustc_middle::ty::TyKind::{Adt, Array, Char, FnDef, Never, Ref, Str, Tuple,
 use rustc_middle::ty::{
     self, suggest_constraining_type_param, Ty, TyCtxt, TypeFoldable, TypeVisitor,
 };
+use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
 use rustc_trait_selection::infer::InferCtxtExt;
@@ -673,6 +675,29 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     match actual.kind() {
                         Uint(_) if op == hir::UnOp::UnNeg => {
                             err.note("unsigned values cannot be negated");
+
+                            if let hir::ExprKind::Unary(
+                                _,
+                                hir::Expr {
+                                    kind:
+                                        hir::ExprKind::Lit(Spanned {
+                                            node: ast::LitKind::Int(1, _),
+                                            ..
+                                        }),
+                                    ..
+                                },
+                            ) = ex.kind
+                            {
+                                err.span_suggestion(
+                                    ex.span,
+                                    &format!(
+                                        "you may have meant the maximum value of `{}`",
+                                        actual
+                                    ),
+                                    format!("{}::MAX", actual),
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
                         }
                         Str | Never | Char | Tuple(_) | Array(_, _) => {}
                         Ref(_, ref lty, _) if *lty.kind() == Str => {}
diff --git a/src/test/ui/feature-gates/feature-gate-negate-unsigned.stderr b/src/test/ui/feature-gates/feature-gate-negate-unsigned.stderr
index aa84c27e40f..d1f4ed5cb04 100644
--- a/src/test/ui/feature-gates/feature-gate-negate-unsigned.stderr
+++ b/src/test/ui/feature-gates/feature-gate-negate-unsigned.stderr
@@ -2,7 +2,10 @@ error[E0600]: cannot apply unary operator `-` to type `usize`
   --> $DIR/feature-gate-negate-unsigned.rs:10:23
    |
 LL |     let _max: usize = -1;
-   |                       ^^ cannot apply unary operator `-`
+   |                       ^^
+   |                       |
+   |                       cannot apply unary operator `-`
+   |                       help: you may have meant the maximum value of `usize`: `usize::MAX`
    |
    = note: unsigned values cannot be negated
 
diff --git a/src/test/ui/unsigned-literal-negation.rs b/src/test/ui/unsigned-literal-negation.rs
new file mode 100644
index 00000000000..943c7f79742
--- /dev/null
+++ b/src/test/ui/unsigned-literal-negation.rs
@@ -0,0 +1,5 @@
+fn main() {
+    let x = -1 as usize; //~ ERROR: cannot apply unary operator `-`
+    let x = (-1) as usize; //~ ERROR: cannot apply unary operator `-`
+    let x: u32 = -1; //~ ERROR: cannot apply unary operator `-`
+}
diff --git a/src/test/ui/unsigned-literal-negation.stderr b/src/test/ui/unsigned-literal-negation.stderr
new file mode 100644
index 00000000000..0aaa8c3b72f
--- /dev/null
+++ b/src/test/ui/unsigned-literal-negation.stderr
@@ -0,0 +1,36 @@
+error[E0600]: cannot apply unary operator `-` to type `usize`
+  --> $DIR/unsigned-literal-negation.rs:2:13
+   |
+LL |     let x = -1 as usize;
+   |             ^^
+   |             |
+   |             cannot apply unary operator `-`
+   |             help: you may have meant the maximum value of `usize`: `usize::MAX`
+   |
+   = note: unsigned values cannot be negated
+
+error[E0600]: cannot apply unary operator `-` to type `usize`
+  --> $DIR/unsigned-literal-negation.rs:3:13
+   |
+LL |     let x = (-1) as usize;
+   |             ^^^^
+   |             |
+   |             cannot apply unary operator `-`
+   |             help: you may have meant the maximum value of `usize`: `usize::MAX`
+   |
+   = note: unsigned values cannot be negated
+
+error[E0600]: cannot apply unary operator `-` to type `u32`
+  --> $DIR/unsigned-literal-negation.rs:4:18
+   |
+LL |     let x: u32 = -1;
+   |                  ^^
+   |                  |
+   |                  cannot apply unary operator `-`
+   |                  help: you may have meant the maximum value of `u32`: `u32::MAX`
+   |
+   = note: unsigned values cannot be negated
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0600`.