clippy: less aggressive needless_borrows_for_generic_args

[pacak]

Current implementation looks for significant drops, that can change the
behavior, but that's not enough - value might not have a Drop itself but
one of its children might have it.

A good example is passing a reference to PathBuf to std::fs::File::open.
There's no benefits to pass PathBuf by value, but since clippy can't
see Drop on Vec several layers down it complains forcing pass by
value and making it impossible to use the same name later.

New implementation only looks at variables and only those that are copy,
so existing variable will never be moved but things that take a string
reference created and value is created inplace &"".to_owned() will
make it to suggest to use "".to_owned() still.

Fixes rust-lang/rust-clippy#12454

[rustbot]

r? @jieyouxu

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[rustbot]

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in run-make tests.

cc @jieyouxu

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[pacak]

Oops, included a commit from other pull request by accident. Sorry for the noise, gone now.

[oli-obk]

Since this only touches clippy, you open a PR on the clippy repo. Or is there a specific reason this is opened here?

[pacak]

Since this only touches clippy, you open a PR on the clippy repo. Or is there a specific reason this is opened here?

I wasn't sure where to open it :) Will reopen it against clippy.

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

FAILED TEST: tests/ui/needless_borrows_for_generic_args.rs
command: "checking tests/ui/needless_borrows_for_generic_args.fixed"

error: actual output differed from expected
Execute `cargo uibless` to update `tests/ui/needless_borrows_for_generic_args.fixed` to the actual output
--- tests/ui/needless_borrows_for_generic_args.fixed
+++ <fixed output>
 #![warn(clippy::needless_borrows_for_generic_args)]
... 7 lines skipped ...
 use std::ffi::OsStr;
 use std::ffi::OsStr;
 use std::fmt::{Debug, Display};
-use std::path::{Path, PathBuf};
+use std::path::Path;
 
... 144 lines skipped ...
 
         let x = X;
         let x = X;
-        f(&x); // Don't lint. Has significant drop
+        f(&x); // Don't lint, not copy, passed by a reference to a variable
     {
... 159 lines skipped ...
 
 
         let y = Y(X);
-        f(&y); // Don't lint. Has significant drop
+        f(&y); // Don't lint. Not copy, passed by a reference to value
 }


full stderr: