Rollup merge of #77758 - Stupremee:turbofish-help-for-const, r=varkor

suggest turbofish syntax for uninferred const arguments

When not providing a const generic value, and it can not be inferred, the following suggestion is suggested:
![image](https://user-images.githubusercontent.com/39732259/95616180-af127b80-0a69-11eb-8877-551c815f9627.png)

Resolves #76737

r? ``@varkor``

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -124,6 +124,11 @@ fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
                 return;
             }
         }
+
+        // FIXME(const_generics): Currently, any uninferred `const` generics arguments
+        // are handled specially, but instead they should be handled in `annotate_method_call`,
+        // which currently doesn't work because this evaluates to `false` for const arguments.
+        // See https://github.com/rust-lang/rust/pull/77758 for more details.
         if self.node_ty_contains_target(expr.hir_id).is_some() {
             match expr.kind {
                 ExprKind::Closure(..) => self.found_closure = Some(&expr),
@@ -345,11 +350,6 @@ pub fn emit_inference_failure_err(
     ) -> DiagnosticBuilder<'tcx> {
         let arg = self.resolve_vars_if_possible(arg);
         let arg_data = self.extract_inference_diagnostics_data(arg, None);
-        let kind_str = match arg.unpack() {
-            GenericArgKind::Type(_) => "type",
-            GenericArgKind::Const(_) => "the value",
-            GenericArgKind::Lifetime(_) => bug!("unexpected lifetime"),
-        };
 
         let mut local_visitor = FindHirNodeVisitor::new(&self, arg, span);
         let ty_to_string = |ty: Ty<'tcx>| -> String {
@@ -618,6 +618,28 @@ pub fn emit_inference_failure_err(
             .any(|span_label| span_label.label.is_some() && span_label.span == span)
             && local_visitor.found_arg_pattern.is_none()
         {
+            let (kind_str, const_value) = match arg.unpack() {
+                GenericArgKind::Type(_) => ("type", None),
+                GenericArgKind::Const(_) => ("the value", Some(())),
+                GenericArgKind::Lifetime(_) => bug!("unexpected lifetime"),
+            };
+
+            // FIXME(const_generics): we would like to handle const arguments
+            // as part of the normal diagnostics flow below, but there appear to
+            // be subtleties in doing so, so for now we special-case const args
+            // here.
+            if let Some(suggestion) = const_value
+                .and_then(|_| arg_data.parent_name.as_ref())
+                .map(|parent| format!("{}::<{}>", parent, arg_data.name))
+            {
+                err.span_suggestion_verbose(
+                    span,
+                    "consider specifying the const argument",
+                    suggestion,
+                    Applicability::MaybeIncorrect,
+                );
+            }
+
             // Avoid multiple labels pointing at `span`.
             err.span_label(
                 span,

src/test/ui/const-generics/infer/cannot-infer-const-args.full.stderr

@@ -3,6 +3,11 @@ error[E0282]: type annotations needed
    |
 LL |     foo();
    |     ^^^ cannot infer the value of const parameter `X` declared on the function `foo`
+   |
+help: consider specifying the const argument
+   |
+LL |     foo::<X>();
+   |     ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/const-generics/infer/cannot-infer-const-args.min.stderr

@@ -3,6 +3,11 @@ error[E0282]: type annotations needed
    |
 LL |     foo();
    |     ^^^ cannot infer the value of const parameter `X` declared on the function `foo`
+   |
+help: consider specifying the const argument
+   |
+LL |     foo::<X>();
+   |     ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/const-generics/infer/issue-77092.stderr

@@ -3,6 +3,11 @@ error[E0282]: type annotations needed
    |
 LL |         println!("{:?}", take_array_from_mut(&mut arr, i));
    |                          ^^^^^^^^^^^^^^^^^^^ cannot infer the value of const parameter `N` declared on the function `take_array_from_mut`
+   |
+help: consider specifying the const argument
+   |
+LL |         println!("{:?}", take_array_from_mut::<N>(&mut arr, i));
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/const-generics/infer/method-chain.full.stderr

@@ -3,6 +3,11 @@ error[E0282]: type annotations needed
    |
 LL |     Foo.bar().bar().bar().bar().baz();
    |                                 ^^^ cannot infer the value of const parameter `N` declared on the associated function `baz`
+   |
+help: consider specifying the const argument
+   |
+LL |     Foo.bar().bar().bar().bar().baz::<N>();
+   |                                 ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/const-generics/infer/method-chain.min.stderr

@@ -3,6 +3,11 @@ error[E0282]: type annotations needed
    |
 LL |     Foo.bar().bar().bar().bar().baz();
    |                                 ^^^ cannot infer the value of const parameter `N` declared on the associated function `baz`
+   |
+help: consider specifying the const argument
+   |
+LL |     Foo.bar().bar().bar().bar().baz::<N>();
+   |                                 ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/const-generics/infer/one-param-uninferred.full.stderr

@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed
+  --> $DIR/one-param-uninferred.rs:15:23
+   |
+LL |     let _: [u8; 17] = foo();
+   |                       ^^^ cannot infer the value of const parameter `M` declared on the function `foo`
+   |
+help: consider specifying the const argument
+   |
+LL |     let _: [u8; 17] = foo::<M>();
+   |                       ^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/const-generics/infer/one-param-uninferred.min.stderr

@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed
+  --> $DIR/one-param-uninferred.rs:15:23
+   |
+LL |     let _: [u8; 17] = foo();
+   |                       ^^^ cannot infer the value of const parameter `M` declared on the function `foo`
+   |
+help: consider specifying the const argument
+   |
+LL |     let _: [u8; 17] = foo::<M>();
+   |                       ^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/const-generics/infer/one-param-uninferred.rs

@@ -0,0 +1,17 @@
+// Test that we emit an error if we cannot properly infer a constant.
+// revisions: full min
+
+#![cfg_attr(full, feature(const_generics))]
+#![cfg_attr(full, allow(incomplete_features))]
+#![cfg_attr(min, feature(min_const_generics))]
+
+fn foo<const N: usize, const M: usize>() -> [u8; N] {
+    todo!()
+}
+
+fn main() {
+    // FIXME(const_generics): Currently this only suggests one const parameter,
+    // but instead it should suggest to provide all parameters.
+    let _: [u8; 17] = foo();
+                  //~^ ERROR type annotations needed
+}

src/test/ui/const-generics/infer/uninferred-consts.full.stderr

@@ -2,7 +2,12 @@ error[E0282]: type annotations needed
   --> $DIR/uninferred-consts.rs:14:9
    |
 LL |     Foo.foo();
-   |         ^^^ cannot infer the value of const parameter `N` declared on the associated function `foo`
+   |         ^^^ cannot infer the value of const parameter `A` declared on the associated function `foo`
+   |
+help: consider specifying the const argument
+   |
+LL |     Foo.foo::<A>();
+   |         ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/const-generics/infer/uninferred-consts.min.stderr

@@ -2,7 +2,12 @@ error[E0282]: type annotations needed
   --> $DIR/uninferred-consts.rs:14:9
    |
 LL |     Foo.foo();
-   |         ^^^ cannot infer the value of const parameter `N` declared on the associated function `foo`
+   |         ^^^ cannot infer the value of const parameter `A` declared on the associated function `foo`
+   |
+help: consider specifying the const argument
+   |
+LL |     Foo.foo::<A>();
+   |         ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/const-generics/infer/uninferred-consts.rs

@@ -8,7 +8,7 @@
 // taken from https://github.com/rust-lang/rust/issues/70507#issuecomment-615268893
 struct Foo;
 impl Foo {
-    fn foo<const N: usize>(self) {}
+    fn foo<const A: usize, const B: usize>(self) {}
 }
 fn main() {
     Foo.foo();