When not finding assoc fn on type, look for builder fn

When we have a resolution error when looking at a fully qualified path
on a type, look for all associated functions on inherent impls that
return `Self` and mention them to the user.

Fix rust-lang#69512.

Author: estebank

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -409,6 +409,87 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             err.downgrade_to_delayed_bug();
         }
 
+        if let (ty::Adt(adt_def, _), SelfSource::QPath(_)) = (rcvr_ty.kind(), source) {
+            // Look at all the associated functions without receivers in the type's inherent impls
+            // to look for builders that return `Self`, `Option<Self>` or `Result<Self, _>`.
+            let rcvr_ty = self.tcx.erase_regions(rcvr_ty);
+            let mut items = self
+                .tcx
+                .inherent_impls(adt_def.did())
+                .iter()
+                .flat_map(|i| self.tcx.associated_items(i).in_definition_order())
+                .filter(|item| {
+                    // Only assoc fn with no receivers.
+                    matches!(item.kind, ty::AssocKind::Fn) && !item.fn_has_self_parameter
+                })
+                .filter_map(|item| {
+                    // Only assoc fns that return `Self`, `Option<Self>` or `Result<Self, _>`.
+                    let ret_ty = self.tcx.fn_sig(item.def_id).skip_binder().output().skip_binder();
+                    let ret_ty = self.tcx.erase_regions(ret_ty);
+                    let ty::Adt(def, args) = ret_ty.kind() else {
+                        return None;
+                    };
+                    // Check for `-> Self`
+                    if self.can_eq(self.param_env, ret_ty, rcvr_ty) {
+                        return Some((item.def_id, ret_ty));
+                    }
+                    // Check for `-> Option<Self>` or `-> Result<Self, _>`
+                    if ![
+                        self.tcx.lang_items().option_type(),
+                        self.tcx.get_diagnostic_item(sym::Result),
+                    ]
+                    .contains(&Some(def.did()))
+                    {
+                        return None;
+                    }
+                    let arg = self.tcx.erase_regions(args.get(0)?.expect_ty());
+                    if self.can_eq(self.param_env, rcvr_ty, arg) {
+                        Some((item.def_id, ret_ty))
+                    } else {
+                        None
+                    }
+                })
+                .collect::<Vec<_>>();
+            let post = if items.len() > 5 {
+                let items_len = items.len();
+                items.truncate(4);
+                format!("\nand {} others", items_len - 4)
+            } else {
+                String::new()
+            };
+            match &items[..] {
+                [] => {}
+                [(def_id, ret_ty)] => {
+                    err.span_note(
+                        self.tcx.def_span(def_id),
+                        format!(
+                            "if you're trying to build a new `{rcvr_ty}`, consider using `{}` \
+                             which returns `{ret_ty}`",
+                            self.tcx.def_path_str(def_id),
+                        ),
+                    );
+                }
+                _ => {
+                    let span: MultiSpan = items
+                        .iter()
+                        .map(|(def_id, _)| self.tcx.def_span(def_id))
+                        .collect::<Vec<Span>>()
+                        .into();
+                    err.span_note(
+                        span,
+                        format!(
+                            "if you're trying to build a new `{rcvr_ty}` consider using one of the \
+                             following associated functions:\n{}{post}",
+                            items
+                                .iter()
+                                .map(|(def_id, _ret_ty)| self.tcx.def_path_str(def_id))
+                                .collect::<Vec<String>>()
+                                .join("\n")
+                        ),
+                    );
+                }
+            }
+        };
         if tcx.ty_is_opaque_future(rcvr_ty) && item_name.name == sym::poll {
             err.help(format!(
                 "method `poll` found on `Pin<&mut {ty_str}>`, \

tests/ui/issues/issue-42880.stderr

@@ -3,6 +3,14 @@ error[E0599]: no associated item named `String` found for struct `String` in the
    |
 LL |     let f = |&Value::String(_)| ();
    |                      ^^^^^^ associated item not found in `String`
+   |
+note: if you're trying to build a new `String` consider using one of the following associated functions:
+      String::new
+      String::with_capacity
+      String::from_utf8
+      String::from_utf16
+      and 7 others
+  --> $SRC_DIR/alloc/src/string.rs:LL:COL
 
 error: aborting due to previous error
 

tests/ui/parser/emoji-identifiers.stderr

@@ -75,6 +75,12 @@ LL |     👀::full_of✨()
    |         |
    |         function or associated item not found in `👀`
    |         help: there is an associated function with a similar name: `full_of_✨`
+   |
+note: if you're trying to build a new `👀`, consider using `👀::full_of_✨` which returns `👀`
+  --> $DIR/emoji-identifiers.rs:4:5
+   |
+LL |     fn full_of_✨() -> 👀 {
+   |     ^^^^^^^^^^^^^^^^^^^^^
 
 error[E0425]: cannot find function `i_like_to_😄_a_lot` in this scope
   --> $DIR/emoji-identifiers.rs:13:13

tests/ui/resolve/fn-new-doesnt-exist.rs

@@ -0,0 +1,5 @@
+use std::net::TcpStream;
+
+fn main() {
+   let stream = TcpStream::new(); //~ ERROR no function or associated item named `new` found
+}

tests/ui/resolve/fn-new-doesnt-exist.stderr

@@ -0,0 +1,14 @@
+error[E0599]: no function or associated item named `new` found for struct `TcpStream` in the current scope
+  --> $DIR/fn-new-doesnt-exist.rs:4:28
+   |
+LL |    let stream = TcpStream::new();
+   |                            ^^^ function or associated item not found in `TcpStream`
+   |
+note: if you're trying to build a new `TcpStream` consider using one of the following associated functions:
+      TcpStream::connect
+      TcpStream::connect_timeout
+  --> $SRC_DIR/std/src/net/tcp.rs:LL:COL
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0599`.

tests/ui/resolve/issue-82865.stderr

@@ -15,6 +15,13 @@ LL |     Box::z
 LL |     mac!();
    |     ------ in this macro invocation
    |
+note: if you're trying to build a new `Box<_, _>` consider using one of the following associated functions:
+      Box::<T>::new
+      Box::<T>::new_uninit
+      Box::<T>::new_zeroed
+      Box::<T>::try_new
+      and 18 others
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
    = note: this error originates in the macro `mac` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors

tests/ui/suggestions/deref-path-method.stderr

@@ -4,6 +4,13 @@ error[E0599]: no function or associated item named `contains` found for struct `
 LL |     Vec::contains(&vec, &0);
    |          ^^^^^^^^ function or associated item not found in `Vec<_, _>`
    |
+note: if you're trying to build a new `Vec<_, _>` consider using one of the following associated functions:
+      Vec::<T>::new
+      Vec::<T>::with_capacity
+      Vec::<T>::from_raw_parts
+      Vec::<T, A>::new_in
+      and 2 others
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 help: the function `contains` is implemented on `[_]`
    |
 LL |     <[_]>::contains(&vec, &0);

tests/ui/suggestions/issue-109291.stderr

@@ -6,6 +6,13 @@ LL |     println!("Custom backtrace: {}", std::backtrace::Backtrace::forced_capt
    |                                                                 |
    |                                                                 function or associated item not found in `Backtrace`
    |                                                                 help: there is an associated function with a similar name: `force_capture`
+   |
+note: if you're trying to build a new `Backtrace` consider using one of the following associated functions:
+      Backtrace::capture
+      Backtrace::force_capture
+      Backtrace::disabled
+      Backtrace::create
+  --> $SRC_DIR/std/src/backtrace.rs:LL:COL
 
 error: aborting due to previous error