Auto merge of rust-lang#88175 - camsteffen:let-desugar-span, r=Manishearth

Add expansion to while desugar spans

In the same vein as rust-lang#88163, this reverts a change in Clippy behavior as a result of rust-lang#80357 (and reverts some `#[allow]`s): This changes `clippy::blocks_in_if_conditions` to not fire on `while` loops. Though we might actually want Clippy to lint those cases, we should introduce the change purposefully, with tests, and possibly under a different lint name.

The actual change here is to add a desugaring expansion to the spans when lowering a `while` loop.

r? `@Manishearth`

Author: bors

tests/ui/crashes/issues_loop_mut_cond.rs

@@ -1,4 +1,3 @@
-#![allow(clippy::blocks_in_if_conditions)]
 #![allow(dead_code)]
 
 /// Issue: https://github.com/rust-lang/rust-clippy/issues/2596

tests/ui/infinite_loop.rs

@@ -1,5 +1,3 @@
-#![allow(clippy::blocks_in_if_conditions)]
-
 fn fn_val(i: i32) -> i32 {
     unimplemented!()
 }

tests/ui/infinite_loop.stderr

@@ -1,5 +1,5 @@
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:22:11
+  --> $DIR/infinite_loop.rs:20:11
    |
 LL |     while y < 10 {
    |           ^^^^^^
@@ -8,71 +8,71 @@ LL |     while y < 10 {
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:27:11
+  --> $DIR/infinite_loop.rs:25:11
    |
 LL |     while y < 10 && x < 3 {
    |           ^^^^^^^^^^^^^^^
    |
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:34:11
+  --> $DIR/infinite_loop.rs:32:11
    |
 LL |     while !cond {
    |           ^^^^^
    |
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:78:11
+  --> $DIR/infinite_loop.rs:76:11
    |
 LL |     while i < 3 {
    |           ^^^^^
    |
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:83:11
+  --> $DIR/infinite_loop.rs:81:11
    |
 LL |     while i < 3 && j > 0 {
    |           ^^^^^^^^^^^^^^
    |
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:87:11
+  --> $DIR/infinite_loop.rs:85:11
    |
 LL |     while i < 3 {
    |           ^^^^^
    |
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:102:11
+  --> $DIR/infinite_loop.rs:100:11
    |
 LL |     while i < 3 {
    |           ^^^^^
    |
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:107:11
+  --> $DIR/infinite_loop.rs:105:11
    |
 LL |     while i < 3 {
    |           ^^^^^
    |
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:173:15
+  --> $DIR/infinite_loop.rs:171:15
    |
 LL |         while self.count < n {
    |               ^^^^^^^^^^^^^^
    |
    = note: this may lead to an infinite or to a never running loop
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:181:11
+  --> $DIR/infinite_loop.rs:179:11
    |
 LL |     while y < 10 {
    |           ^^^^^^
@@ -82,7 +82,7 @@ LL |     while y < 10 {
    = help: rewrite it as `if cond { loop { } }`
 
 error: variables in the condition are not mutated in the loop body
-  --> $DIR/infinite_loop.rs:188:11
+  --> $DIR/infinite_loop.rs:186:11
    |
 LL |     while y < 10 {
    |           ^^^^^^