Suggest collecting into Vec<_>

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

@@ -20,7 +20,7 @@
 use rustc_middle::ty::{self, DefIdTree, InferConst};
 use rustc_middle::ty::{GenericArg, GenericArgKind, SubstsRef};
 use rustc_middle::ty::{IsSuggestable, Ty, TyCtxt, TypeckResults};
-use rustc_span::symbol::{kw, Ident};
+use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{BytePos, Span};
 use std::borrow::Cow;
 use std::iter;
@@ -78,12 +78,12 @@ fn can_add_more_info(&self) -> bool {
     }
 
     fn where_x_is_kind(&self, in_type: Ty<'_>) -> &'static str {
-        if in_type.is_ty_infer() {
-            "empty"
-        } else if self.name == "_" {
+        if self.name == "_" {
             // FIXME: Consider specializing this message if there is a single `_`
             // in the type.
             "underscore"
+        } else if in_type.is_ty_infer() {
+            "empty"
         } else {
             "has_name"
         }
@@ -368,6 +368,7 @@ fn bad_inference_failure_err(
 }
 
 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
+    #[instrument(level = "debug", skip(self, error_code))]
     pub fn emit_inference_failure_err(
         &self,
         body_id: Option<hir::BodyId>,
@@ -406,16 +407,20 @@ pub fn emit_inference_failure_err(
         let mut infer_subdiags = Vec::new();
         let mut multi_suggestions = Vec::new();
         match kind {
-            InferSourceKind::LetBinding { insert_span, pattern_name, ty } => {
+            InferSourceKind::LetBinding { insert_span, pattern_name, ty, is_collect } => {
                 infer_subdiags.push(SourceKindSubdiag::LetLike {
                     span: insert_span,
                     name: pattern_name.map(|name| name.to_string()).unwrap_or_else(String::new),
-                    x_kind: arg_data.where_x_is_kind(ty),
+                    x_kind: if is_collect { "empty" } else { arg_data.where_x_is_kind(ty) },
                     prefix_kind: arg_data.kind.clone(),
                     prefix: arg_data.kind.try_get_prefix().unwrap_or_default(),
                     arg_name: arg_data.name,
                     kind: if pattern_name.is_some() { "with_pattern" } else { "other" },
-                    type_name: ty_to_string(self, ty),
+                    type_name: if is_collect {
+                        "Vec<_>".to_string()
+                    } else {
+                        ty_to_string(self, ty)
+                    },
                 });
             }
             InferSourceKind::ClosureArg { insert_span, ty } => {
@@ -608,6 +613,7 @@ enum InferSourceKind<'tcx> {
         insert_span: Span,
         pattern_name: Option<Ident>,
         ty: Ty<'tcx>,
+        is_collect: bool,
     },
     ClosureArg {
         insert_span: Span,
@@ -788,10 +794,19 @@ fn ty_cost(self, ty: Ty<'tcx>) -> usize {
     /// Uses `fn source_cost` to determine whether this inference source is preferable to
     /// previous sources. We generally prefer earlier sources.
     #[instrument(level = "debug", skip(self))]
-    fn update_infer_source(&mut self, new_source: InferSource<'tcx>) {
+    fn update_infer_source(&mut self, mut new_source: InferSource<'tcx>) {
         let cost = self.source_cost(&new_source) + self.attempt;
         debug!(?cost);
         self.attempt += 1;
+        if let Some(InferSource { kind: InferSourceKind::GenericArg { def_id, ..}, .. }) = self.infer_source
+            && self.infcx.tcx.get_diagnostic_item(sym::iterator_collect_fn) == Some(def_id)
+            && let InferSourceKind::LetBinding { ref ty, ref mut is_collect, ..} = new_source.kind
+            && ty.is_ty_infer()
+        {
+            // Customize the output so we talk about `let x: Vec<_> = iter.collect();` instead of
+            // `let x: _ = iter.collect();`, as this is a very common case.
+            *is_collect = true;
+        }
         if cost < self.infer_source_cost {
             self.infer_source_cost = cost;
             self.infer_source = Some(new_source);
@@ -1089,6 +1104,7 @@ fn visit_local(&mut self, local: &'tcx Local<'tcx>) {
                                 insert_span: local.pat.span.shrink_to_hi(),
                                 pattern_name: local.pat.simple_ident(),
                                 ty,
+                                is_collect: false,
                             },
                         })
                     }

compiler/rustc_span/src/symbol.rs

@@ -827,6 +827,7 @@
         item_like_imports,
         iter,
         iter_repeat,
+        iterator_collect_fn,
         kcfi,
         keyword,
         kind,

library/core/src/iter/traits/iterator.rs

@@ -1829,6 +1829,7 @@ fn by_ref(&mut self) -> &mut Self
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
+    #[cfg_attr(not(test), rustc_diagnostic_item = "iterator_collect_fn")]
     fn collect<B: FromIterator<Self::Item>>(self) -> B
     where
         Self: Sized,

src/test/ui/array-slice-vec/infer_array_len.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |     let [_, _] = a.into();
    |         ^^^^^^
    |
-help: consider giving this pattern a type
+help: consider giving this pattern a type, where the placeholders `_` are specified
    |
 LL |     let [_, _]: _ = a.into();
    |               +++

src/test/ui/closures/issue-52437.stderr

@@ -10,7 +10,7 @@ error[E0282]: type annotations needed
 LL |     [(); &(&'static: loop { |x| {}; }) as *const _ as usize]
    |                              ^
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |     [(); &(&'static: loop { |x: _| {}; }) as *const _ as usize]
    |                               +++

src/test/ui/error-codes/E0282.stderr

@@ -6,8 +6,8 @@ LL |     let x = "hello".chars().rev().collect();
    |
 help: consider giving `x` an explicit type
    |
-LL |     let x: _ = "hello".chars().rev().collect();
-   |          +++
+LL |     let x: Vec<_> = "hello".chars().rev().collect();
+   |          ++++++++
 
 error: aborting due to previous error
 

src/test/ui/impl-trait/issues/issue-86719.stderr

@@ -18,7 +18,7 @@ error[E0282]: type annotations needed
 LL |         |_| true
    |          ^
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |         |_: _| true
    |           +++

src/test/ui/inference/issue-72690.stderr

@@ -30,7 +30,7 @@ error[E0282]: type annotations needed
 LL |     |x| String::from("x".as_ref());
    |      ^
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |     |x: _| String::from("x".as_ref());
    |       +++

src/test/ui/issues/issue-18159.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |     let x;
    |         ^
    |
-help: consider giving `x` an explicit type
+help: consider giving `x` an explicit type, where the placeholders `_` are specified
    |
 LL |     let x: _;
    |          +++

src/test/ui/issues/issue-2151.stderr

@@ -6,7 +6,7 @@ LL |     let x = panic!();
 LL |     x.clone();
    |     - type must be known at this point
    |
-help: consider giving `x` an explicit type
+help: consider giving `x` an explicit type, where the placeholders `_` are specified
    |
 LL |     let x: _ = panic!();
    |          +++

src/test/ui/issues/issue-24036.stderr

@@ -17,7 +17,7 @@ error[E0282]: type annotations needed
 LL |         1 => |c| c + 1,
    |               ^
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |         1 => |c: _| c + 1,
    |                +++

src/test/ui/lazy-type-alias-impl-trait/branches3.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |         |s| s.len()
    |          ^  - type must be known at this point
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |         |s: _| s.len()
    |           +++
@@ -15,7 +15,7 @@ error[E0282]: type annotations needed
 LL |         |s| s.len()
    |          ^  - type must be known at this point
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |         |s: _| s.len()
    |           +++
@@ -26,7 +26,7 @@ error[E0282]: type annotations needed
 LL |         |s| s.len()
    |          ^  - type must be known at this point
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |         |s: _| s.len()
    |           +++
@@ -37,7 +37,7 @@ error[E0282]: type annotations needed
 LL |         |s| s.len()
    |          ^  - type must be known at this point
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |         |s: _| s.len()
    |           +++

src/test/ui/match/match-unresolved-one-arm.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |     let x = match () {
    |         ^
    |
-help: consider giving `x` an explicit type
+help: consider giving `x` an explicit type, where the placeholders `_` are specified
    |
 LL |     let x: _ = match () {
    |          +++

src/test/ui/pattern/pat-tuple-bad-type.stderr

@@ -7,7 +7,7 @@ LL |     let x;
 LL |         (..) => {}
    |         ---- type must be known at this point
    |
-help: consider giving `x` an explicit type
+help: consider giving `x` an explicit type, where the placeholders `_` are specified
    |
 LL |     let x: _;
    |          +++

src/test/ui/pattern/rest-pat-semantic-disallowed.stderr

@@ -191,7 +191,7 @@ error[E0282]: type annotations needed
 LL |     let x @ ..;
    |         ^^^^^^
    |
-help: consider giving this pattern a type
+help: consider giving this pattern a type, where the placeholders `_` are specified
    |
 LL |     let x @ ..: _;
    |               +++

src/test/ui/resolve/issue-85348.stderr

@@ -19,7 +19,7 @@ error[E0282]: type annotations needed
 LL |         let mut N;
    |             ^^^^^
    |
-help: consider giving `N` an explicit type
+help: consider giving `N` an explicit type, where the placeholders `_` are specified
    |
 LL |         let mut N: _;
    |                  +++

src/test/ui/span/method-and-field-eager-resolution.stderr

@@ -7,7 +7,7 @@ LL |
 LL |     x.0;
    |     - type must be known at this point
    |
-help: consider giving `x` an explicit type
+help: consider giving `x` an explicit type, where the placeholders `_` are specified
    |
 LL |     let mut x: _ = Default::default();
    |              +++
@@ -21,7 +21,7 @@ LL |
 LL |     x[0];
    |     - type must be known at this point
    |
-help: consider giving `x` an explicit type
+help: consider giving `x` an explicit type, where the placeholders `_` are specified
    |
 LL |     let mut x: _ = Default::default();
    |              +++

src/test/ui/type-alias-impl-trait/closures_in_branches.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |         |x| x.len()
    |          ^  - type must be known at this point
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |         |x: _| x.len()
    |           +++
@@ -15,7 +15,7 @@ error[E0282]: type annotations needed
 LL |         |x| x.len()
    |          ^  - type must be known at this point
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |         |x: _| x.len()
    |           +++

src/test/ui/type/type-check/unknown_type_for_closure.stderr

@@ -10,7 +10,7 @@ error[E0282]: type annotations needed
 LL |     let x = |_| {};
    |              ^
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |     let x = |_: _| {};
    |               +++

src/test/ui/type/type-path-err-node-types.stderr

@@ -28,7 +28,7 @@ error[E0282]: type annotations needed
 LL |     let _ = |a, b: _| -> _ { 0 };
    |              ^
    |
-help: consider giving this closure parameter an explicit type
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
    |
 LL |     let _ = |a: _, b: _| -> _ { 0 };
    |               +++