Fix non-constant value ICE (#90878)

This also fixes the same suggestion, which was kind of broken, because it just searched for the last occurence of `const` to replace with a `let`. This works great in some cases, but when there is no const and a leading space to the file, it doesn't work and panic with overflow because it thought that it had found a const.

I also changed the suggestion to only trigger if the `const` and the non-constant value are on the same line, because if they aren't, the suggestion is very likely to be wrong.

Also don't trigger the suggestion if the found `const` is on line 0, because that triggers the ICE.

compiler/rustc_resolve/src/diagnostics.rs

@@ -450,9 +450,23 @@ impl<'a> Resolver<'a> {
                 // let foo =...
                 //     ^^^ given this Span
                 // ------- get this Span to have an applicable suggestion
+
+                // edit:
+                // only do this if the const and usage of the non-constant value are on the same line
+                // the further the two are apart, the higher the chance of the suggestion being wrong
+                // also make sure that this line isn't the first one (ICE #90878)
+
                 let sp =
                     self.session.source_map().span_extend_to_prev_str(ident.span, current, true);
-                if sp.lo().0 == 0 {
+
+                let is_first_line = self
+                    .session
+                    .source_map()
+                    .lookup_line(sp.lo())
+                    .map(|file_and_line| file_and_line.line == 0)
+                    .unwrap_or(true);
+
+                if sp.lo().0 == 0 || self.session.source_map().is_multiline(sp) || is_first_line {
                     err.span_label(ident.span, &format!("this would need to be a `{}`", sugg));
                 } else {
                     let sp = sp.with_lo(BytePos(sp.lo().0 - current.len() as u32));

compiler/rustc_span/src/lib.rs

@@ -1935,6 +1935,7 @@ pub struct Loc {
 #[derive(Debug)]
 pub struct SourceFileAndLine {
     pub sf: Lrc<SourceFile>,
+    /// Index of line, starting from 0.
     pub line: usize,
 }
 #[derive(Debug)]

src/test/ui/consts/issue-90878-2.rs

@@ -0,0 +1,12 @@
+ #![l=|x|[b;x ]] //~ ERROR unexpected token: `|x| [b; x]`
+//~^ ERROR cannot find attribute `l` in this scope
+//~^^ ERROR attempt to use a non-constant value in a constant [E0435]
+//~^^^ ERROR cannot find value `b` in this scope [E0425]
+
+// notice the space at the start,
+// we can't attach any attributes to this file because it needs to be at the start
+
+// this example has been slightly modified (adding ]] at the end), so that it actually works here
+// it still produces the same issue though
+
+fn main() {}

src/test/ui/consts/issue-90878-2.stderr

@@ -0,0 +1,30 @@
+error: unexpected token: `|x| [b; x]`
+  --> $DIR/issue-90878-2.rs:1:7
+   |
+LL |  #![l=|x|[b;x ]]
+   |       ^^^^^^^^^
+
+error: cannot find attribute `l` in this scope
+  --> $DIR/issue-90878-2.rs:1:5
+   |
+LL |  #![l=|x|[b;x ]]
+   |     ^
+
+error[E0435]: attempt to use a non-constant value in a constant
+  --> $DIR/issue-90878-2.rs:1:13
+   |
+LL |  #![l=|x|[b;x ]]
+   |        -    ^
+   |        |
+   |        this would need to be a `const`
+
+error[E0425]: cannot find value `b` in this scope
+  --> $DIR/issue-90878-2.rs:1:11
+   |
+LL |  #![l=|x|[b;x ]]
+   |           ^ help: a local variable with a similar name exists: `x`
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0425, E0435.
+For more information about an error, try `rustc --explain E0425`.

src/test/ui/consts/issue-90878.rs

@@ -0,0 +1,4 @@
+ fn main() {
+    |x: usize| [0; x];  //~ ERROR attempt to use a non-constant value in a constant [E0435]
+    // (note the space before "fn")
+}

src/test/ui/consts/issue-90878.stderr

@@ -0,0 +1,11 @@
+error[E0435]: attempt to use a non-constant value in a constant
+  --> $DIR/issue-90878.rs:2:20
+   |
+LL |     |x: usize| [0; x];
+   |      -             ^
+   |      |
+   |      this would need to be a `const`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0435`.

src/test/ui/consts/non-const-value-in-const.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let x = 5;
+    const Y: i32 = x; //~ ERROR attempt to use a non-constant value in a constant [E0435]
+
+    let x = 5;
+    let _ = [0; x]; //~ ERROR attempt to use a non-constant value in a constant [E0435]
+}

src/test/ui/consts/non-const-value-in-const.stderr

@@ -0,0 +1,20 @@
+error[E0435]: attempt to use a non-constant value in a constant
+  --> $DIR/non-const-value-in-const.rs:3:20
+   |
+LL |     const Y: i32 = x;
+   |     -------        ^ non-constant value
+   |     |
+   |     help: consider using `let` instead of `const`: `let Y`
+
+error[E0435]: attempt to use a non-constant value in a constant
+  --> $DIR/non-const-value-in-const.rs:6:17
+   |
+LL |     let x = 5;
+   |     ----- help: consider using `const` instead of `let`: `const x`
+...
+LL |     let _ = [0; x];
+   |                 ^ non-constant value
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0435`.