Handle multiple error fix suggestions carefuly

The existing code seems to assume that substitutions spans are disjoint,
which is not always the case.

In the example:

    pub trait AAAA {}
    pub trait B {}
    pub trait C {}
    pub type T<P: AAAA + B + C> = P;

, we get three substituions starting from ':' and ending respectively at
the end of each trait token.

With the former offset calculation, this would cause `underline_start` to
eventually become negative before being converted to `usize`...

The new version may report erroneous results for non perfectly overlapping
substitutions but I don't know if such examples exist. Alternatively, we
could detect these cases and trim out overlapping substitutions.

src/librustc_errors/emitter.rs

@@ -1530,7 +1530,7 @@ impl EmitterWriter {
 
             // This offset and the ones below need to be signed to account for replacement code
             // that is shorter than the original code.
-            let mut offset: isize = 0;
+            let mut offsets: Vec<(usize, isize)> = Vec::new();
             // Only show an underline in the suggestions if the suggestion is not the
             // entirety of the code being shown and the displayed code is not multiline.
             if show_underline {
@@ -1550,12 +1550,19 @@ impl EmitterWriter {
                         .map(|ch| unicode_width::UnicodeWidthChar::width(ch).unwrap_or(1))
                         .sum();
 
+                    let offset: isize = offsets
+                        .iter()
+                        .filter_map(
+                            |(start, v)| if span_start_pos <= *start { None } else { Some(v) },
+                        )
+                        .sum();
                     let underline_start = (span_start_pos + start) as isize + offset;
                     let underline_end = (span_start_pos + start + sub_len) as isize + offset;
+                    assert!(underline_start >= 0 && underline_end >= 0);
                     for p in underline_start..underline_end {
                         buffer.putc(
                             row_num,
-                            max_line_num_len + 3 + p as usize,
+                            ((max_line_num_len + 3) as isize + p) as usize,
                             '^',
                             Style::UnderlinePrimary,
                         );
@@ -1565,7 +1572,7 @@ impl EmitterWriter {
                         for p in underline_start - 1..underline_start + 1 {
                             buffer.putc(
                                 row_num,
-                                max_line_num_len + 3 + p as usize,
+                                ((max_line_num_len + 3) as isize + p) as usize,
                                 '-',
                                 Style::UnderlineSecondary,
                             );
@@ -1582,8 +1589,9 @@ impl EmitterWriter {
                     // length of the code to be substituted
                     let snippet_len = span_end_pos as isize - span_start_pos as isize;
                     // For multiple substitutions, use the position *after* the previous
-                    // substitutions have happened.
-                    offset += full_sub_len - snippet_len;
+                    // substitutions have happened, only when further substitutions are
+                    // located strictly after.
+                    offsets.push((span_end_pos, full_sub_len - snippet_len));
                 }
                 row_num += 1;
             }

src/test/ui/type/issue-67690-type-alias-bound-diagnostic-crash.rs

@@ -0,0 +1,8 @@
+// Regression test for issue #67690
+// Rustc endless loop out-of-memory and consequent SIGKILL in generic new type
+
+// check-pass
+pub type T<P: Send + Send + Send> = P;
+//~^ WARN bounds on generic parameters are not enforced in type aliases
+
+fn main() {}

src/test/ui/type/issue-67690-type-alias-bound-diagnostic-crash.stderr

@@ -0,0 +1,12 @@
+warning: bounds on generic parameters are not enforced in type aliases
+  --> $DIR/issue-67690-type-alias-bound-diagnostic-crash.rs:5:15
+   |
+LL | pub type T<P: Send + Send + Send> = P;
+   |               ^^^^   ^^^^   ^^^^
+   |
+   = note: `#[warn(type_alias_bounds)]` on by default
+help: the bound will not be checked when the type alias is used, and should be removed
+   |
+LL | pub type T<P> = P;
+   |            --
+

src/test/ui/type/type-alias-bounds.rs

@@ -1,6 +1,6 @@
 // Test `ignored_generic_bounds` lint warning about bounds in type aliases.
 
-// build-pass (FIXME(62277): could be check-pass?)
+// check-pass
 #![allow(dead_code)]
 
 use std::rc::Rc;

src/test/ui/type/type-alias-bounds.stderr

@@ -8,7 +8,7 @@ LL | type SVec<T: Send + Send> = Vec<T>;
 help: the bound will not be checked when the type alias is used, and should be removed
    |
 LL | type SVec<T> = Vec<T>;
-   |     --    --
+   |           --
 
 warning: where clauses are not enforced in type aliases
   --> $DIR/type-alias-bounds.rs:10:21
@@ -30,7 +30,7 @@ LL | type VVec<'b, 'a: 'b + 'b> = (&'b u32, Vec<&'a i32>);
 help: the bound will not be checked when the type alias is used, and should be removed
    |
 LL | type VVec<'b, 'a> = (&'b u32, Vec<&'a i32>);
-   |            --  --
+   |                --
 
 warning: bounds on generic parameters are not enforced in type aliases
   --> $DIR/type-alias-bounds.rs:14:18
@@ -41,7 +41,7 @@ LL | type WVec<'b, T: 'b + 'b> = (&'b u32, Vec<T>);
 help: the bound will not be checked when the type alias is used, and should be removed
    |
 LL | type WVec<'b, T> = (&'b u32, Vec<T>);
-   |           --  --
+   |               --
 
 warning: where clauses are not enforced in type aliases
   --> $DIR/type-alias-bounds.rs:16:25