Blame user type in pat type error.

src/librustc/infer/error_reporting/mod.rs

@@ -581,8 +581,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
     ) {
         match cause.code {
-            ObligationCauseCode::Pattern { span, ty } => {
-                let ty = self.resolve_vars_if_possible(&ty);
+            ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
+                let ty = self.resolve_vars_if_possible(&root_ty);
                 if ty.is_suggestable() {
                     // don't show type `_`
                     err.span_label(span, format!("this expression has type `{}`", ty));
@@ -600,6 +600,9 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     }
                 }
             }
+            ObligationCauseCode::Pattern { origin_expr: false, span: Some(span), .. } => {
+                err.span_label(span, "expected due to this");
+            }
             ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
                 source,
                 ref prior_arms,

src/librustc/traits/mod.rs

@@ -251,8 +251,12 @@ pub enum ObligationCauseCode<'tcx> {
 
     /// Type error arising from type checking a pattern against an expected type.
     Pattern {
-        span: Span,
-        ty: Ty<'tcx>,
+        /// The span of the scrutinee or type expression which caused the `root_ty` type.
+        span: Option<Span>,
+        /// The root expected type induced by a scrutinee or type expression.
+        root_ty: Ty<'tcx>,
+        /// Whether the `Span` came from an expression or a type expression.
+        origin_expr: bool,
     },
 
     /// Constants in patterns must have `Structural` type.

src/librustc/traits/structural_impls.rs

@@ -521,7 +521,9 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
                     discrim_hir_id,
                 })
             }),
-            super::Pattern { span, ty } => tcx.lift(&ty).map(|ty| super::Pattern { span, ty }),
+            super::Pattern { span, root_ty, origin_expr } => {
+                tcx.lift(&root_ty).map(|root_ty| super::Pattern { span, root_ty, origin_expr })
+            }
             super::IfExpression(box super::IfExpressionCause { then, outer, semicolon }) => {
                 Some(super::IfExpression(box super::IfExpressionCause { then, outer, semicolon }))
             }

src/librustc_typeck/check/_match.rs

@@ -61,7 +61,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             .map(|arm| {
                 let mut all_pats_diverge = Diverges::WarnedAlways;
                 self.diverges.set(Diverges::Maybe);
-                self.check_pat_top(&arm.pat, discrim_ty, Some(discrim.span));
+                self.check_pat_top(&arm.pat, discrim_ty, Some(discrim.span), true);
                 all_pats_diverge &= self.diverges.get();
 
                 // As discussed with @eddyb, this is for disabling unreachable_code

src/librustc_typeck/check/mod.rs

@@ -1293,9 +1293,11 @@ fn check_fn<'a, 'tcx>(
     };
 
     // Add formal parameters.
-    for (param_ty, param) in fn_sig.inputs().iter().copied().chain(maybe_va_list).zip(body.params) {
+    let inputs_hir = hir.fn_decl_by_hir_id(fn_id).map(|decl| &decl.inputs);
+    let inputs_fn = fn_sig.inputs().iter().copied();
+    for (idx, (param_ty, param)) in inputs_fn.chain(maybe_va_list).zip(body.params).enumerate() {
         // Check the pattern.
-        fcx.check_pat_top(&param.pat, param_ty, None);
+        fcx.check_pat_top(&param.pat, param_ty, try { inputs_hir?.get(idx)?.span }, false);
 
         // Check that argument is Sized.
         // The check for a non-trivial pattern is a hack to avoid duplicate warnings
@@ -4276,16 +4278,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    /// Type check a `let` statement.
     pub fn check_decl_local(&self, local: &'tcx hir::Local<'tcx>) {
+        // Determine and write the type which we'll check the pattern against.
         let ty = self.local_ty(local.span, local.hir_id).decl_ty;
         self.write_ty(local.hir_id, ty);
 
+        // Type check the initializer.
         if let Some(ref init) = local.init {
             let init_ty = self.check_decl_initializer(local, &init);
             self.overwrite_local_ty_if_err(local, ty, init_ty);
         }
 
-        self.check_pat_top(&local.pat, ty, local.init.map(|init| init.span));
+        // Does the expected pattern type originate from an expression and what is the span?
+        let (origin_expr, ty_span) = match (local.ty, local.init) {
+            (Some(ty), _) => (false, Some(ty.span)), // Bias towards the explicit user type.
+            (_, Some(init)) => (true, Some(init.span)), // No explicit type; so use the scrutinee.
+            _ => (false, None), // We have `let $pat;`, so the expected type is unconstrained.
+        };
+
+        // Type check the pattern. Override if necessary to avoid knock-on errors.
+        self.check_pat_top(&local.pat, ty, ty_span, origin_expr);
         let pat_ty = self.node_ty(local.pat.hir_id);
         self.overwrite_local_ty_if_err(local, ty, pat_ty);
     }
@@ -4297,7 +4310,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         ty: Ty<'tcx>,
     ) {
         if ty.references_error() {
-            // Override the types everywhere with `types.err` to avoid knock down errors.
+            // Override the types everywhere with `types.err` to avoid knock on errors.
             self.write_ty(local.hir_id, ty);
             self.write_ty(local.pat.hir_id, ty);
             let local_ty = LocalTy { decl_ty, revealed_ty: ty };

src/librustc_typeck/check/pat.rs

@@ -37,9 +37,13 @@ https://doc.rust-lang.org/reference/types.html#trait-objects";
 struct TopInfo<'tcx> {
     /// The `expected` type at the top level of type checking a pattern.
     expected: Ty<'tcx>,
+    /// Was the origin of the `span` from a scrutinee expression?
+    ///
+    /// Otherwise there is no scrutinee and it could be e.g. from the type of a formal parameter.
+    origin_expr: bool,
     /// The span giving rise to the `expected` type, if one could be provided.
     ///
-    /// This is the span of the scrutinee as in:
+    /// If `origin_expr` is `true`, then this is the span of the scrutinee as in:
     ///
     /// - `match scrutinee { ... }`
     /// - `let _ = scrutinee;`
@@ -70,11 +74,8 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         actual: Ty<'tcx>,
         ti: TopInfo<'tcx>,
     ) -> Option<DiagnosticBuilder<'tcx>> {
-        let cause = if let Some(span) = ti.span {
-            self.cause(cause_span, Pattern { span, ty: ti.expected })
-        } else {
-            self.misc(cause_span)
-        };
+        let code = Pattern { span: ti.span, root_ty: ti.expected, origin_expr: ti.origin_expr };
+        let cause = self.cause(cause_span, code);
         self.demand_eqtype_with_origin(&cause, expected, actual)
     }
 
@@ -92,11 +93,21 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Type check the given top level pattern against the `expected` type.
     ///
-    /// If a `Some(span)` is provided, then the `span` represents the scrutinee's span.
+    /// If a `Some(span)` is provided and `origin_expr` holds,
+    /// then the `span` represents the scrutinee's span.
     /// The scrutinee is found in e.g. `match scrutinee { ... }` and `let pat = scrutinee;`.
-    pub fn check_pat_top(&self, pat: &'tcx Pat<'tcx>, expected: Ty<'tcx>, span: Option<Span>) {
+    ///
+    /// Otherwise, `Some(span)` represents the span of a type expression
+    /// which originated the `expected` type.
+    pub fn check_pat_top(
+        &self,
+        pat: &'tcx Pat<'tcx>,
+        expected: Ty<'tcx>,
+        span: Option<Span>,
+        origin_expr: bool,
+    ) {
         let def_bm = BindingMode::BindByValue(hir::Mutability::Not);
-        self.check_pat(pat, expected, def_bm, TopInfo { expected, span });
+        self.check_pat(pat, expected, def_bm, TopInfo { expected, origin_expr, span });
     }
 
     /// Type check the given `pat` against the `expected` type

src/librustc_typeck/lib.rs

@@ -65,6 +65,7 @@ This API is completely unstable and subject to change.
 #![feature(in_band_lifetimes)]
 #![feature(nll)]
 #![feature(slice_patterns)]
+#![feature(try_blocks)]
 #![feature(never_type)]
 #![recursion_limit = "256"]
 

src/test/ui/or-patterns/inconsistent-modes.stderr

@@ -60,7 +60,7 @@ error[E0308]: mismatched types
   --> $DIR/inconsistent-modes.rs:13:25
    |
 LL |     let Ok(ref a) | Err(ref mut a): Result<&u8, &mut u8> = Ok(&0);
-   |                         ^^^^^^^^^                          ------ this expression has type `std::result::Result<&u8, &mut u8>`
+   |                         ^^^^^^^^^   -------------------- expected due to this
    |                         |
    |                         types differ in mutability
    |

src/test/ui/pattern/pat-type-err-formal-param.rs

@@ -0,0 +1,8 @@
+// Test the `.span_label(..)` to the type when there's a
+// type error in a pattern due to a the formal parameter.
+
+fn main() {}
+
+struct Tuple(u8);
+
+fn foo(Tuple(_): String) {} //~ ERROR mismatched types

src/test/ui/pattern/pat-type-err-formal-param.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/pat-type-err-formal-param.rs:8:8
+   |
+LL | fn foo(Tuple(_): String) {}
+   |        ^^^^^^^^  ------ expected due to this
+   |        |
+   |        expected struct `std::string::String`, found struct `Tuple`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/pattern/pat-type-err-let-stmt.rs

@@ -0,0 +1,16 @@
+// Test the `.span_label` to the type / scrutinee
+// when there's a type error in checking a pattern.
+
+fn main() {
+    // We want to point at the `Option<u8>`.
+    let Ok(0): Option<u8> = 42u8;
+    //~^ ERROR mismatched types
+    //~| ERROR mismatched types
+
+    // We want to point at the `Option<u8>`.
+    let Ok(0): Option<u8>;
+    //~^ ERROR mismatched types
+
+    // We want to point at the scrutinee.
+    let Ok(0) = 42u8; //~ ERROR mismatched types
+}

src/test/ui/pattern/pat-type-err-let-stmt.stderr

@@ -0,0 +1,49 @@
+error[E0308]: mismatched types
+  --> $DIR/pat-type-err-let-stmt.rs:6:29
+   |
+LL |     let Ok(0): Option<u8> = 42u8;
+   |                ----------   ^^^^
+   |                |            |
+   |                |            expected enum `std::option::Option`, found `u8`
+   |                |            help: try using a variant of the expected enum: `Some(42u8)`
+   |                expected due to this
+   |
+   = note: expected enum `std::option::Option<u8>`
+              found type `u8`
+
+error[E0308]: mismatched types
+  --> $DIR/pat-type-err-let-stmt.rs:6:9
+   |
+LL |     let Ok(0): Option<u8> = 42u8;
+   |         ^^^^^  ---------- expected due to this
+   |         |
+   |         expected enum `std::option::Option`, found enum `std::result::Result`
+   |
+   = note: expected enum `std::option::Option<u8>`
+              found enum `std::result::Result<_, _>`
+
+error[E0308]: mismatched types
+  --> $DIR/pat-type-err-let-stmt.rs:11:9
+   |
+LL |     let Ok(0): Option<u8>;
+   |         ^^^^^  ---------- expected due to this
+   |         |
+   |         expected enum `std::option::Option`, found enum `std::result::Result`
+   |
+   = note: expected enum `std::option::Option<u8>`
+              found enum `std::result::Result<_, _>`
+
+error[E0308]: mismatched types
+  --> $DIR/pat-type-err-let-stmt.rs:15:9
+   |
+LL |     let Ok(0) = 42u8;
+   |         ^^^^^   ---- this expression has type `u8`
+   |         |
+   |         expected `u8`, found enum `std::result::Result`
+   |
+   = note: expected type `u8`
+              found enum `std::result::Result<_, _>`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.