Further improve impl Trait/dyn Trait suggestions

src/librustc/traits/error_reporting/suggestions.rs

@@ -600,17 +600,20 @@ 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 (last_ty, all_returns_have_same_type) =
-            ret_types.clone().fold((None, true), |(last_ty, mut same), returned_ty| {
-                same &= last_ty.map_or(true, |ty| ty == returned_ty);
-                (Some(returned_ty), same)
-            });
+        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| {
+                same &= last_ty.map_or(true, |last_ty| last_ty == ty) && ty.kind != ty::Error;
+                (Some(ty), same)
+            },
+        );
         let all_returns_conform_to_trait =
             if let Some(ty_ret_ty) = tables.node_type_opt(ret_ty.hir_id) {
                 match ty_ret_ty.kind {
@@ -625,7 +628,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             })
                         })
                     }
-                    _ => true,
+                    _ => false,
                 }
             } else {
                 true
@@ -675,7 +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| {
                         (
@@ -735,10 +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));
@@ -1689,7 +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>;
@@ -1699,20 +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/librustc_errors/emitter.rs

@@ -456,9 +456,14 @@ impl Emitter for SilentEmitter {
     fn emit_diagnostic(&mut self, _: &Diagnostic) {}
 }
 
-/// maximum number of lines we will print for each error; arbitrary.
+/// Maximum number of lines we will print for each error; arbitrary.
 pub const MAX_HIGHLIGHT_LINES: usize = 6;
-/// maximum number of suggestions to be shown
+/// Maximum number of lines we will print for a multiline suggestion; arbitrary.
+///
+/// This should be replaced with a more involved mechanism to output multiline suggestions that
+/// more closely mimmics the regular diagnostic output, where irrelevant code lines are ellided.
+pub const MAX_SUGGESTION_HIGHLIGHT_LINES: usize = 20;
+/// Maximum number of suggestions to be shown
 ///
 /// Arbitrary, but taken from trait import suggestion limit
 pub const MAX_SUGGESTIONS: usize = 4;
@@ -1521,7 +1526,7 @@ impl EmitterWriter {
             draw_col_separator_no_space(&mut buffer, 1, max_line_num_len + 1);
             let mut line_pos = 0;
             let mut lines = complete.lines();
-            for line in lines.by_ref().take(MAX_HIGHLIGHT_LINES) {
+            for line in lines.by_ref().take(MAX_SUGGESTION_HIGHLIGHT_LINES) {
                 // Print the span column to avoid confusion
                 buffer.puts(
                     row_num,

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

@@ -22,13 +22,53 @@ 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
+    }
+    42 //~ ERROR mismatched types
+}
+fn baq() -> Box<dyn Trait> {
+    if true {
+        return 0; //~ ERROR mismatched types
+    }
+    42 //~ ERROR mismatched types
+}
+fn baz() -> Box<dyn Trait> {
+    if true {
+        Struct //~ ERROR mismatched types
+    } else {
+        42 //~ ERROR mismatched types
+    }
+}
+fn baw() -> Box<dyn Trait> {
+    if true {
+        0 //~ ERROR mismatched types
+    } else {
+        42 //~ ERROR mismatched types
+    }
+}
 
 // Suggest using `impl Trait`
 fn bat() -> dyn Trait { //~ ERROR E0746
     if true {
-        return 0;
+        return 0u32;
+    }
+    42u32
+}
+fn bay() -> dyn Trait { //~ ERROR E0746
+    if true {
+        0u32
+    } else {
+        42u32
     }
-    42
 }
 
 fn main() {}