rustc_mir: adjust the type_length_limit diagnostic to be more useful.

Author: eddyb

src/librustc_mir/monomorphize/collector.rs

@@ -198,6 +198,8 @@ use monomorphize::item::{MonoItemExt, DefPathBasedNames, InstantiationMode};
 use rustc_data_structures::bit_set::GrowableBitSet;
 use rustc_data_structures::sync::{MTRef, MTLock, ParallelIterator, par_iter};
 
+use std::iter;
+
 #[derive(PartialEq, Eq, Hash, Clone, Copy, Debug)]
 pub enum MonoItemCollectionMode {
     Eager,
@@ -476,21 +478,33 @@ fn check_type_length_limit<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     // Bail out in these cases to avoid that bad user experience.
     let type_length_limit = *tcx.sess.type_length_limit.get();
     if type_length > type_length_limit {
-        // The instance name is already known to be too long for rustc. Use
-        // `{:.64}` to avoid blasting the user's terminal with thousands of
-        // lines of type-name.
-        let instance_name = instance.to_string();
-        let msg = format!("reached the type-length limit while instantiating `{:.64}...`",
-                          instance_name);
-        let mut diag = if let Some(node_id) = tcx.hir().as_local_node_id(instance.def_id()) {
-            tcx.sess.struct_span_fatal(tcx.hir().span(node_id), &msg)
-        } else {
-            tcx.sess.struct_fatal(&msg)
+        // The instance name is already known to be too long for rustc.
+        // Show only the first and last 32 characters to avoid blasting
+        // the user's terminal with thousands of lines of type-name.
+        let shrink = |s: String, before: usize, after: usize| {
+            // An iterator of all byte positions including the end of the string.
+            let positions = || s.char_indices().map(|(i, _)| i).chain(iter::once(s.len()));
+
+            let shrunk = format!(
+                "{before}...{after}",
+                before = &s[..positions().nth(before).unwrap_or(s.len())],
+                after = &s[positions().rev().nth(after).unwrap_or(0)..],
+            );
+
+            // Only use the shrunk version if it's really shorter.
+            // This also avoids the case where before and after slices overlap.
+            if shrunk.len() < s.len() {
+                shrunk
+            } else {
+                s
+            }
         };
-
+        let msg = format!("reached the type-length limit while instantiating `{}`",
+                          shrink(instance.to_string(), 32, 32));
+        let mut diag = tcx.sess.struct_span_fatal(tcx.def_span(instance.def_id()), &msg);
         diag.note(&format!(
             "consider adding a `#![type_length_limit=\"{}\"]` attribute to your crate",
-            type_length_limit*2));
+            type_length));
         diag.emit();
         tcx.sess.abort_if_errors();
     }

src/test/ui/issues/issue-22638.stderr

@@ -8,7 +8,7 @@ LL | |         a.matches(f)
 LL | |     }
    | |_____^
    |
-   = note: consider adding a `#![type_length_limit="40000000"]` attribute to your crate
+   = note: consider adding a `#![type_length_limit="26214380"]` attribute to your crate
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-37311-type-length-limit/issue-37311.stderr

@@ -1,12 +1,12 @@
-error: reached the type-length limit while instantiating `<(&(&(&(&(&(&(&(&(&(&(&(&(&(&(&(&(&(&(&(), &()), &(&(), &())), &...`
+error: reached the type-length limit while instantiating `<(&(&(&(&(&(&(&(&(&(&(&(&(&(&(&(...))))))))))))))) as Foo>::recurse`
   --> $DIR/issue-37311.rs:13:5
    |
 LL | /     fn recurse(&self) { //~ ERROR reached the type-length limit
 LL | |         (self, self).recurse();
 LL | |     }
    | |_____^
    |
-   = note: consider adding a `#![type_length_limit="2097152"]` attribute to your crate
+   = note: consider adding a `#![type_length_limit="2097149"]` attribute to your crate
 
 error: aborting due to previous error
 

src/test/ui/type_length_limit.stderr

@@ -1,6 +1,10 @@
-error: reached the type-length limit while instantiating `std::mem::drop::<std::option::Option<((((((G, G, G), (G, G, G), ...`
+error: reached the type-length limit while instantiating `std::mem::drop::<std::option::Op... G), (G, G, G), (G, G, G))))))>>`
+  --> $SRC_DIR/libcore/mem.rs:LL:COL
    |
-   = note: consider adding a `#![type_length_limit="512"]` attribute to your crate
+LL | pub fn drop<T>(_x: T) { }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: consider adding a `#![type_length_limit="1094"]` attribute to your crate
 
 error: aborting due to previous error