Suggest appropriate path when calling associated item on bare types

When looking at the documentation for `std::f32` or `std::str`, for
example, it is easy to get confused and assume `std::f32` and `f32`
are the same thing. Because of this, it is not uncommon to attempt
writing `f32::consts::PI` instead of the correct
`std::f32::consts::PI`. When encountering the former, which results
in an access error due to it being an inexistent path, try to access
the same path under `std`. If this succeeds, this information is
stored for later tweaking of the final E0599 to provide an
appropriate suggestion.

This suggestion applies to both E0233 and E0599 and is only checked
when the first ident of a path corresponds to a primitive type.

src/librustc/session/mod.rs

@@ -165,6 +165,10 @@ pub struct Session {
 
     /// `Span`s of trait methods that weren't found to avoid emitting object safety errors
     pub trait_methods_not_found: Lock<FxHashSet<Span>>,
+
+    /// Mapping from ident span to path span for paths that don't exist as written, but that
+    /// exist under `std`. For example, wrote `str::from_utf8` instead of `std::str::from_utf8`.
+    pub confused_type_with_std_module: Lock<FxHashMap<Span, Span>>,
 }
 
 pub struct PerfStats {
@@ -1248,6 +1252,7 @@ fn build_session_(
         has_panic_handler: Once::new(),
         driver_lint_caps,
         trait_methods_not_found: Lock::new(Default::default()),
+        confused_type_with_std_module: Lock::new(Default::default()),
     };
 
     validate_commandline_args_with_session_available(&sess);