Strange matching behaviour in for loop - rust

Question

Rust noob here. I am puzzled as to why I get this strange behaviour when matching the variable over which I am iterating through a range, with the max of that range. Instead of only matching with the last item, it matches everything! Can someone please explain the output and compiler warning I get from this code:
fn main() {
let maxi = 2;
for i in 0..=maxi {
match i {
maxi => {
println!("i={} matched maxi={}",i,maxi);
}
_ => {
println!("i={} matched with _",i);
}
}
}
}
The compiler output is as follows
warning: unreachable pattern
--> se_match_loop.rs:7:15
|
5 | maxi => {
| ---- matches any value
6 | println!("i={} matched maxi={}",i,maxi);
7 | } _ => {
| ^ unreachable pattern
|
= note: `#[warn(unreachable_patterns)]` on by default
warning: 1 warning emitted
When I execute the compiled script, this is the output
i=0 matched maxi=0
i=1 matched maxi=1
i=2 matched maxi=2
But if I declare maxi as a constant (with const instead of let), I get the expected output
i=0 matched _
i=1 matched _
i=2 matched maxi=2

Answer 1

This is documented under Matching Named Variables.
match starts a new scope, variables declared as part of a pattern
inside the match expression will shadow those with the same name
outside the match construct, as is the case with all variables.
For example, the following code
fn main() {
let x = Some(5);
let y = 10;
match x {
Some(50) => println!("Got 50"),
Some(y) => println!("Matched, y = {:?}", y),
_ => println!("Default case, x = {:?}", x),
}
println!("at the end: x = {:?}, y = {:?}", x, y);
}
will print
Matched, y = 5
at the end: x = Some(5), y = 10
So let's consider your first snippet,
fn main() {
let maxi = 2;
for i in 0..=maxi {
match i {
maxi => {
println!("i={} matched maxi={}",i,maxi);
}
_ => {
println!("i={} matched with _",i);
}
}
}
}
The pattern in your match arm introduces a new variable(Remember match started with a new scope) named maxi that will match any value. That's why the rust compiler emits the following warning with your first snippet.
warning: unreachable pattern
--> se_match_loop.rs:7:15
|
5 | maxi => {
| ---- matches any value
6 | println!("i={} matched maxi={}",i,maxi);
7 | } _ => {
| ^ unreachable pattern
|
= note: `#[warn(unreachable_patterns)]` on by default
warning: 1 warning emitted
Now lets consider the second one(with const),
fn main() {
const maxi: i32 = 2;
for i in 0..=maxi {
match i {
maxi => {
println!("i={} matched maxi={}",i,maxi);
}
_ => {
println!("i={} matched with _",i);
}
}
}
}
This will work as expected because constants are inlined wherever they’re used, making using them identical to simply replacing the name of the const with its value