Use more accurate return path spans

No longer suggest `Box::new(if foo { Type1 } else { Type2 })`, instead
suggesting `if foo { Box::new(Type1) } else { Box::new(Type2) }`.

Author: estebank

src/librustc/traits/error_reporting/suggestions.rs

@@ -600,12 +600,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
         // Visit to make sure there's a single `return` type to suggest `impl Trait`,
         // otherwise suggest using `Box<dyn Trait>` or an enum.
-        let mut visitor = ReturnsVisitor(vec![]);
+        let mut visitor = ReturnsVisitor::new();
         visitor.visit_body(&body);
 
         let tables = self.in_progress_tables.map(|t| t.borrow()).unwrap();
 
-        let mut ret_types = visitor.0.iter().filter_map(|expr| tables.node_type_opt(expr.hir_id));
+        let mut ret_types =
+            visitor.returns.iter().filter_map(|expr| tables.node_type_opt(expr.hir_id));
         let (last_ty, all_returns_have_same_type) = ret_types.clone().fold(
             (None, true),
             |(last_ty, mut same): (std::option::Option<Ty<'_>>, bool), ty| {
@@ -677,7 +678,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 // Suggest `-> Box<dyn Trait>` and `Box::new(returned_value)`.
                 // Get all the return values and collect their span and suggestion.
                 let mut suggestions = visitor
-                    .0
+                    .returns
                     .iter()
                     .map(|expr| {
                         (
@@ -737,10 +738,10 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         {
             let body = hir.body(*body_id);
             // Point at all the `return`s in the function as they have failed trait bounds.
-            let mut visitor = ReturnsVisitor(vec![]);
+            let mut visitor = ReturnsVisitor::new();
             visitor.visit_body(&body);
             let tables = self.in_progress_tables.map(|t| t.borrow()).unwrap();
-            for expr in &visitor.0 {
+            for expr in &visitor.returns {
                 if let Some(returned_ty) = tables.node_type_opt(expr.hir_id) {
                     let ty = self.resolve_vars_if_possible(&returned_ty);
                     err.span_label(expr.span, &format!("this returned value is of type `{}`", ty));
@@ -1691,7 +1692,16 @@ pub fn suggest_constraining_type_param(
 
 /// Collect all the returned expressions within the input expression.
 /// Used to point at the return spans when we want to suggest some change to them.
-struct ReturnsVisitor<'v>(Vec<&'v hir::Expr<'v>>);
+struct ReturnsVisitor<'v> {
+    returns: Vec<&'v hir::Expr<'v>>,
+    in_block_tail: bool,
+}
+
+impl ReturnsVisitor<'_> {
+    fn new() -> Self {
+        ReturnsVisitor { returns: vec![], in_block_tail: false }
+    }
+}
 
 impl<'v> Visitor<'v> for ReturnsVisitor<'v> {
     type Map = rustc::hir::map::Map<'v>;
@@ -1701,20 +1711,41 @@ impl<'v> Visitor<'v> for ReturnsVisitor<'v> {
     }
 
     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
-        if let hir::ExprKind::Ret(Some(ex)) = ex.kind {
-            self.0.push(ex);
+        match ex.kind {
+            hir::ExprKind::Ret(Some(ex)) => {
+                self.returns.push(ex);
+            }
+            hir::ExprKind::Block(block, _) if self.in_block_tail => {
+                self.in_block_tail = false;
+                for stmt in block.stmts {
+                    hir::intravisit::walk_stmt(self, stmt);
+                }
+                self.in_block_tail = true;
+                if let Some(expr) = block.expr {
+                    self.visit_expr(expr);
+                }
+            }
+            hir::ExprKind::Match(_, arms, _) if self.in_block_tail => {
+                for arm in arms {
+                    self.visit_expr(arm.body);
+                }
+            }
+            // We need to walk to find `return`s in the entire body.
+            _ if !self.in_block_tail => hir::intravisit::walk_expr(self, ex),
+            _ => self.returns.push(ex),
         }
-        hir::intravisit::walk_expr(self, ex);
     }
 
     fn visit_body(&mut self, body: &'v hir::Body<'v>) {
+        let prev = self.in_block_tail;
         if body.generator_kind().is_none() {
             if let hir::ExprKind::Block(block, None) = body.value.kind {
-                if let Some(expr) = block.expr {
-                    self.0.push(expr);
+                if block.expr.is_some() {
+                    self.in_block_tail = true;
                 }
             }
         }
         hir::intravisit::walk_body(self, body);
+        self.in_block_tail = prev;
     }
 }

src/test/ui/error-codes/E0746.fixed

@@ -10,9 +10,9 @@ fn foo() -> impl Trait { Struct }
 
 fn bar() -> impl Trait { //~ ERROR E0746
     if true {
-        return 0;
+        return 0u32;
     }
-    42
+    42u32
 }
 
 fn main() {}

src/test/ui/error-codes/E0746.rs

@@ -10,9 +10,9 @@ fn foo() -> dyn Trait { Struct }
 
 fn bar() -> dyn Trait { //~ ERROR E0746
     if true {
-        return 0;
+        return 0u32;
     }
-    42
+    42u32
 }
 
 fn main() {}

src/test/ui/error-codes/E0746.stderr

@@ -17,7 +17,7 @@ LL | fn bar() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-help: return `impl Trait` instead, as all return paths are of type `{integer}`, which implements `Trait`
+help: return `impl Trait` instead, as all return paths are of type `u32`, which implements `Trait`
    |
 LL | fn bar() -> impl Trait {
    |             ^^^^^^^^^^

src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs

@@ -22,6 +22,13 @@ fn bal() -> dyn Trait { //~ ERROR E0746
     }
     42
 }
+fn bax() -> dyn Trait { //~ ERROR E0746
+    if true {
+        Struct
+    } else {
+        42
+    }
+}
 fn bam() -> Box<dyn Trait> {
     if true {
         return Struct; //~ ERROR mismatched types
@@ -52,9 +59,9 @@ fn baw() -> Box<dyn Trait> {
 // Suggest using `impl Trait`
 fn bat() -> dyn Trait { //~ ERROR E0746
     if true {
-        return 0;
+        return 0u32;
     }
-    42
+    42u32
 }
 fn bay() -> dyn Trait { //~ ERROR E0746
     if true {

src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr

@@ -95,8 +95,27 @@ LL |     }
 LL |     Box::new(42)
    |
 
+error[E0746]: return type cannot have an unboxed trait object
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:13
+   |
+LL | fn bax() -> dyn Trait {
+   |             ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
+   = note: if all the returned values were of the same type you could use `impl Trait` as the return type
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: you can create a new `enum` with a variant for each returned type
+help: return a boxed trait object instead
+   |
+LL | fn bax() -> Box<dyn Trait> {
+LL |     if true {
+LL |         Box::new(Struct)
+LL |     } else {
+LL |         Box::new(42)
+   |
+
 error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:27:16
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:34:16
    |
 LL | fn bam() -> Box<dyn Trait> {
    |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
@@ -112,7 +131,7 @@ LL |         return Struct;
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
 error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:29:5
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:36:5
    |
 LL | fn bam() -> Box<dyn Trait> {
    |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
@@ -128,7 +147,7 @@ LL |     42
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
 error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:33:16
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:40:16
    |
 LL | fn baq() -> Box<dyn Trait> {
    |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
@@ -144,7 +163,7 @@ LL |         return 0;
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
 error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:35:5
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:42:5
    |
 LL | fn baq() -> Box<dyn Trait> {
    |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
@@ -160,7 +179,7 @@ LL |     42
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
 error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:39:9
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:46:9
    |
 LL | fn baz() -> Box<dyn Trait> {
    |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
@@ -176,7 +195,7 @@ LL |         Struct
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
 error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:41:9
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:48:9
    |
 LL | fn baz() -> Box<dyn Trait> {
    |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
@@ -192,7 +211,7 @@ LL |         42
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
 error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:46:9
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:53:9
    |
 LL | fn baw() -> Box<dyn Trait> {
    |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
@@ -208,7 +227,7 @@ LL |         0
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
 error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:48:9
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:55:9
    |
 LL | fn baw() -> Box<dyn Trait> {
    |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
@@ -224,38 +243,30 @@ LL |         42
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
 error[E0746]: return type cannot have an unboxed trait object
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:53:13
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:60:13
    |
 LL | fn bat() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-help: return `impl Trait` instead, as all return paths are of type `{integer}`, which implements `Trait`
+help: return `impl Trait` instead, as all return paths are of type `u32`, which implements `Trait`
    |
 LL | fn bat() -> impl Trait {
    |             ^^^^^^^^^^
 
 error[E0746]: return type cannot have an unboxed trait object
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:59:13
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:66:13
    |
 LL | fn bay() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
-   = note: if all the returned values were of the same type you could use `impl Trait` as the return type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-   = note: you can create a new `enum` with a variant for each returned type
-help: return a boxed trait object instead
-   |
-LL | fn bay() -> Box<dyn Trait> {
-LL |     Box::new(if true {
-LL |         0u32
-LL |     } else {
-LL |         42u32
-LL |     })
+help: return `impl Trait` instead, as all return paths are of type `u32`, which implements `Trait`
    |
+LL | fn bay() -> impl Trait {
+   |             ^^^^^^^^^^
 
-error: aborting due to 18 previous errors
+error: aborting due to 19 previous errors
 
 Some errors have detailed explanations: E0277, E0308, E0746.
 For more information about an error, try `rustc --explain E0277`.

src/test/ui/never_type/feature-gate-never_type_fallback.stderr

@@ -5,7 +5,7 @@ LL | fn should_ret_unit() -> impl T {
    |                         ^^^^^^ the trait `T` is not implemented for `()`
 LL |
 LL |     panic!()
-   |     -------- this returned value is of type `()`
+   |     -------- this returned value is of type `!`
    |
    = note: the return type of a function must have a statically known size
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)