Fix `read_zero_byte_vec` suggests wrongly inside `let` stmt (#15582)

Closes rust-lang/rust-clippy#15575

changelog: [`read_zero_byte_vec`] fix wrong suggestions inside `let`
stmt

2 files changed, 89 insertions, 3 deletions

diff --git a/tests/ui/read_zero_byte_vec.rs b/tests/ui/read_zero_byte_vec.rs
index 938d61b6860..720276cb554 100644
--- a/tests/ui/read_zero_byte_vec.rs
+++ b/tests/ui/read_zero_byte_vec.rs
@@ -120,3 +120,30 @@ fn allow_works<F: std::io::Read>(mut f: F) {
 }
 
 fn main() {}
+
+fn issue15575() -> usize {
+    use std::io::Read;
+    use std::net::TcpListener;
+
+    let listener = TcpListener::bind("127.0.0.1:9010").unwrap();
+    let mut stream_and_addr = listener.accept().unwrap();
+    let mut buf = Vec::with_capacity(32);
+    let num_bytes_received = stream_and_addr.0.read(&mut buf).unwrap();
+    //~^ read_zero_byte_vec
+
+    let cap = 1000;
+    let mut buf = Vec::with_capacity(cap);
+    let num_bytes_received = stream_and_addr.0.read(&mut buf).unwrap();
+    //~^ read_zero_byte_vec
+
+    let cap = 1000;
+    let mut buf = Vec::with_capacity(cap);
+    let num_bytes_received = { stream_and_addr.0.read(&mut buf) }.unwrap();
+    //~^ read_zero_byte_vec
+
+    use std::fs::File;
+    let mut f = File::open("foo.txt").unwrap();
+    let mut data = Vec::with_capacity(100);
+    f.read(&mut data).unwrap()
+    //~^ read_zero_byte_vec
+}
diff --git a/tests/ui/read_zero_byte_vec.stderr b/tests/ui/read_zero_byte_vec.stderr
index 8f255bc87ab..8dd74592e4c 100644
--- a/tests/ui/read_zero_byte_vec.stderr
+++ b/tests/ui/read_zero_byte_vec.stderr
@@ -2,16 +2,27 @@ error: reading zero byte data to `Vec`
   --> tests/ui/read_zero_byte_vec.rs:22:5
    |
 LL |     f.read_exact(&mut data).unwrap();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `data.resize(20, 0); f.read_exact(&mut data)`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::read-zero-byte-vec` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::read_zero_byte_vec)]`
+help: try
+   |
+LL ~     data.resize(20, 0);
+LL ~     f.read_exact(&mut data).unwrap();
+   |
 
 error: reading zero byte data to `Vec`
   --> tests/ui/read_zero_byte_vec.rs:28:5
    |
 LL |     f.read_exact(&mut data2)?;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `data2.resize(cap, 0); f.read_exact(&mut data2)`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL ~     data2.resize(cap, 0);
+LL ~     f.read_exact(&mut data2)?;
+   |
 
 error: reading zero byte data to `Vec`
   --> tests/ui/read_zero_byte_vec.rs:33:5
@@ -67,5 +78,53 @@ error: reading zero byte data to `Vec`
 LL |     r.read_exact(&mut data2).await.unwrap();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 11 previous errors
+error: reading zero byte data to `Vec`
+  --> tests/ui/read_zero_byte_vec.rs:131:30
+   |
+LL |     let num_bytes_received = stream_and_addr.0.read(&mut buf).unwrap();
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL ~     buf.resize(32, 0);
+LL ~     let num_bytes_received = stream_and_addr.0.read(&mut buf).unwrap();
+   |
+
+error: reading zero byte data to `Vec`
+  --> tests/ui/read_zero_byte_vec.rs:136:30
+   |
+LL |     let num_bytes_received = stream_and_addr.0.read(&mut buf).unwrap();
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL ~     buf.resize(cap, 0);
+LL ~     let num_bytes_received = stream_and_addr.0.read(&mut buf).unwrap();
+   |
+
+error: reading zero byte data to `Vec`
+  --> tests/ui/read_zero_byte_vec.rs:141:32
+   |
+LL |     let num_bytes_received = { stream_and_addr.0.read(&mut buf) }.unwrap();
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL ~     buf.resize(cap, 0);
+LL ~     let num_bytes_received = { stream_and_addr.0.read(&mut buf) }.unwrap();
+   |
+
+error: reading zero byte data to `Vec`
+  --> tests/ui/read_zero_byte_vec.rs:147:5
+   |
+LL |     f.read(&mut data).unwrap()
+   |     ^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL ~     data.resize(100, 0);
+LL ~     f.read(&mut data).unwrap()
+   |
+
+error: aborting due to 15 previous errors