Point at type in let assignment on type errors

Author: estebank

src/librustc_typeck/check/demand.rs

@@ -102,12 +102,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     // N.B., this code relies on `self.diverges` to be accurate. In
     // particular, assignments to `!` will be permitted if the
     // diverges flag is currently "always".
-    pub fn demand_coerce_diag(&self,
-                              expr: &hir::Expr,
-                              checked_ty: Ty<'tcx>,
-                              expected: Ty<'tcx>,
-                              allow_two_phase: AllowTwoPhase)
-                              -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx>>) {
+    pub fn demand_coerce_diag(
+        &self,
+        expr: &hir::Expr,
+        checked_ty: Ty<'tcx>,
+        expected: Ty<'tcx>,
+        allow_two_phase: AllowTwoPhase,
+    ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx>>) {
         let expected = self.resolve_vars_with_obligations(expected);
 
         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase) {
@@ -126,6 +127,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return (expected, None)
         }
 
+        self.annotate_expected_due_to_let_ty(&mut err, expr);
         self.suggest_compatible_variants(&mut err, expr, expected, expr_ty);
         self.suggest_ref_or_into(&mut err, expr, expected, expr_ty);
         self.suggest_boxing_when_appropriate(&mut err, expr, expected, expr_ty);
@@ -134,6 +136,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         (expected, Some(err))
     }
 
+    fn annotate_expected_due_to_let_ty(&self, err: &mut DiagnosticBuilder<'_>, expr: &hir::Expr) {
+        let parent = self.tcx.hir().get_parent_node(expr.hir_id);
+        if let Some(hir::Node::Local(hir::Local {
+            ty: Some(ty),
+            init: Some(init),
+            ..
+        })) = self.tcx.hir().find(parent) {
+            if init.hir_id == expr.hir_id {
+                // Point at `let` assignment type.
+                err.span_label(ty.span, "expected due to this");
+            }
+        }
+    }
+
     /// Returns whether the expected type is `bool` and the expression is `x = y`.
     pub fn is_assign_to_bool(&self, expr: &hir::Expr, expected: Ty<'tcx>) -> bool {
         if let hir::ExprKind::Assign(..) = expr.kind {

src/test/rustdoc-ui/failed-doctest-missing-codes.stdout

@@ -9,7 +9,9 @@ error[E0308]: mismatched types
   --> $DIR/failed-doctest-missing-codes.rs:9:13
    |
 LL | let x: () = 5i32;
-   |             ^^^^ expected `()`, found `i32`
+   |        --   ^^^^ expected `()`, found `i32`
+   |        |
+   |        expected due to this
 
 error: aborting due to previous error
 

src/test/ui/array-not-vector.stderr

@@ -2,13 +2,17 @@ error[E0308]: mismatched types
   --> $DIR/array-not-vector.rs:2:19
    |
 LL |     let _x: i32 = [1, 2, 3];
-   |                   ^^^^^^^^^ expected `i32`, found array `[{integer}; 3]`
+   |             ---   ^^^^^^^^^ expected `i32`, found array `[{integer}; 3]`
+   |             |
+   |             expected due to this
 
 error[E0308]: mismatched types
   --> $DIR/array-not-vector.rs:7:20
    |
 LL |     let _y: &i32 = x;
-   |                    ^ expected `i32`, found slice `[i32]`
+   |             ----   ^ expected `i32`, found slice `[i32]`
+   |             |
+   |             expected due to this
    |
    = note: expected reference `&i32`
               found reference `&[i32]`

src/test/ui/associated-types/associated-types-eq-3.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/associated-types-eq-3.rs:23:18
    |
 LL |     let _: Bar = x.boo();
-   |                  ^^^^^^^ expected struct `Bar`, found associated type
+   |            ---   ^^^^^^^ expected struct `Bar`, found associated type
+   |            |
+   |            expected due to this
    |
    = note:       expected struct `Bar`
            found associated type `<I as Foo>::A`

src/test/ui/associated-types/associated-types-path-2.stderr

@@ -43,7 +43,9 @@ error[E0308]: mismatched types
   --> $DIR/associated-types-path-2.rs:41:18
    |
 LL |     let _: i32 = f2(2i32);
-   |                  ^^^^^^^^ expected `i32`, found `u32`
+   |            ---   ^^^^^^^^ expected `i32`, found `u32`
+   |            |
+   |            expected due to this
    |
 help: you can convert an `u32` to `i32` and panic if the converted value wouldn't fit
    |

src/test/ui/c-variadic/variadic-ffi-1.stderr

@@ -26,7 +26,9 @@ error[E0308]: mismatched types
   --> $DIR/variadic-ffi-1.rs:19:56
    |
 LL |         let x: unsafe extern "C" fn(f: isize, x: u8) = foo;
-   |                                                        ^^^ expected non-variadic fn, found variadic function
+   |                -------------------------------------   ^^^ expected non-variadic fn, found variadic function
+   |                |
+   |                expected due to this
    |
    = note: expected fn pointer `unsafe extern "C" fn(isize, u8)`
                  found fn item `unsafe extern "C" fn(isize, u8, ...) {foo}`
@@ -35,7 +37,9 @@ error[E0308]: mismatched types
   --> $DIR/variadic-ffi-1.rs:20:54
    |
 LL |         let y: extern "C" fn(f: isize, x: u8, ...) = bar;
-   |                                                      ^^^ expected variadic fn, found non-variadic function
+   |                -----------------------------------   ^^^ expected variadic fn, found non-variadic function
+   |                |
+   |                expected due to this
    |
    = note: expected fn pointer `extern "C" fn(isize, u8, ...)`
                  found fn item `extern "C" fn(isize, u8) {bar}`

src/test/ui/closures/closure-no-fn-1.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/closure-no-fn-1.rs:6:29
    |
 LL |     let foo: fn(u8) -> u8 = |v: u8| { a += v; a };
-   |                             ^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found closure
+   |              ------------   ^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found closure
+   |              |
+   |              expected due to this
    |
    = note: expected fn pointer `fn(u8) -> u8`
                  found closure `[closure@$DIR/closure-no-fn-1.rs:6:29: 6:50 a:_]`

src/test/ui/closures/closure-no-fn-2.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/closure-no-fn-2.rs:6:27
    |
 LL |     let bar: fn() -> u8 = || { b };
-   |                           ^^^^^^^^ expected fn pointer, found closure
+   |              ----------   ^^^^^^^^ expected fn pointer, found closure
+   |              |
+   |              expected due to this
    |
    = note: expected fn pointer `fn() -> u8`
                  found closure `[closure@$DIR/closure-no-fn-2.rs:6:27: 6:35 b:_]`

src/test/ui/coercion/coerce-to-bang.stderr

@@ -47,7 +47,9 @@ error[E0308]: mismatched types
   --> $DIR/coerce-to-bang.rs:48:21
    |
 LL |     let x: [!; 2] = [return, 22];
-   |                     ^^^^^^^^^^^^ expected `!`, found integer
+   |            ------   ^^^^^^^^^^^^ expected `!`, found integer
+   |            |
+   |            expected due to this
    |
    = note: expected array `[!; 2]`
               found array `[{integer}; 2]`

src/test/ui/coercion/coercion-slice.stderr

@@ -2,10 +2,11 @@ error[E0308]: mismatched types
   --> $DIR/coercion-slice.rs:4:21
    |
 LL |     let _: &[i32] = [0];
-   |                     ^^^
-   |                     |
-   |                     expected `&[i32]`, found array `[{integer}; 1]`
-   |                     help: consider borrowing here: `&[0]`
+   |            ------   ^^^
+   |            |        |
+   |            |        expected `&[i32]`, found array `[{integer}; 1]`
+   |            |        help: consider borrowing here: `&[0]`
+   |            expected due to this
 
 error: aborting due to previous error