Add more spacing to contract error messages

cli/tests/snapshot/inputs/errors/blame_custom_message_ansi_escaping.ncl

@@ -1,3 +1,3 @@
 # capture = 'stderr'
 # command = ['eval']
-null | std.FailWith "\x1B[9;33;47m strike through with background \"and\" \"quotes\""
+null | std.FailWith "\x1B[9;33;47mstrike through with background \"and\" \"quotes\""

cli/tests/snapshot/snapshots/snapshot__eval_stderr_array_at_empty_array.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by the caller of `at`: invalid array indexing
+error: contract broken by the caller of `at`
+       invalid array indexing
     ┌─ <stdlib/std.ncl>:162:9
     │
 162 │       | std.contract.unstable.IndexedArrayFun

cli/tests/snapshot/snapshots/snapshot__eval_stderr_array_at_out_of_bound.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by the caller of `at`: invalid array indexing
+error: contract broken by the caller of `at`
+       invalid array indexing
     ┌─ <stdlib/std.ncl>:162:9
     │
 162 │       | std.contract.unstable.IndexedArrayFun

cli/tests/snapshot/snapshots/snapshot__eval_stderr_array_merge_fail.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by a value: cannot merge unequal arrays
+error: contract broken by a value
+       cannot merge unequal arrays
   ┌─ <unknown> (generated by evaluation):1:1
   │
 1 │ contract.Equal some_array

cli/tests/snapshot/snapshots/snapshot__eval_stderr_array_range_reversed_indices.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by the caller of `range`: invalid range
+error: contract broken by the caller of `range`
+       invalid range
     ┌─ <stdlib/std.ncl>:649:9
     │
 649 │       | std.contract.unstable.RangeFun Dyn

cli/tests/snapshot/snapshots/snapshot__eval_stderr_array_range_step_negative_step.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by the caller of `range_step`: invalid range step
+error: contract broken by the caller of `range_step`
+       invalid range step
     ┌─ <stdlib/std.ncl>:624:9
     │
 624 │       | std.contract.unstable.RangeFun (std.contract.unstable.RangeStep -> Dyn)

cli/tests/snapshot/snapshots/snapshot__eval_stderr_blame_custom_message_ansi_escaping.ncl.snap

@@ -2,11 +2,12 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by a value:  strike through with background "and" "quotes"
+error: contract broken by a value
+       strike through with background "and" "quotes"
   ┌─ [INPUTS_PATH]/errors/blame_custom_message_ansi_escaping.ncl:3:1
   │
-3 │ null | std.FailWith "\x1B[9;33;47m strike through with background \"and\" \"quotes\""
-  │ ^^^^   ------------------------------------------------------------------------------ expected type
+3 │ null | std.FailWith "\x1B[9;33;47mstrike through with background \"and\" \"quotes\""
+  │ ^^^^   ----------------------------------------------------------------------------- expected type
   │ │       
   │ applied to this expression
 

cli/tests/snapshot/snapshots/snapshot__eval_stderr_contract_with_custom_diagnostic.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by a value: main error message
+error: contract broken by a value
+       main error message
    ┌─ [INPUTS_PATH]/errors/contract_with_custom_diagnostic.ncl:13:1
    │
 13 │ null | Contract

cli/tests/snapshot/snapshots/snapshot__eval_stderr_destructuring_closed_fail.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by a value: extra field `b`
+error: contract broken by a value
+       extra field `b`
   ┌─ [INPUTS_PATH]/errors/destructuring_closed_fail.ncl:3:11
   │
 3 │ let {a} = {a=1, b=2}

cli/tests/snapshot/snapshots/snapshot__eval_stderr_record_forall_constraints_contract.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by the caller: field not allowed in tail: `x`
+error: contract broken by the caller
+       field not allowed in tail: `x`
   ┌─ [INPUTS_PATH]/errors/record_forall_constraints_contract.ncl:3:19
   │
 3 │ let f | forall r. { ; r } -> { x: Number; r } = fun r => %record_insert% "x" r 1 in f { x = 0 }

cli/tests/snapshot/snapshots/snapshot__eval_stderr_subcontract_nested_custom_diagnostics.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by a value: child's message
+error: contract broken by a value
+       child's message
    ┌─ [INPUTS_PATH]/errors/subcontract_nested_custom_diagnostics.ncl:19:8
    │
 19 │ null | ParentContract

cli/tests/snapshot/snapshots/snapshot__eval_stderr_value_contract_violation.ncl.snap

@@ -2,7 +2,8 @@
 source: cli/tests/snapshot/main.rs
 expression: err
 ---
-error: contract broken by the value of `foo`: no reason
+error: contract broken by the value of `foo`
+       no reason
   ┌─ [INPUTS_PATH]/errors/value_contract_violation.ncl:3:36
   │
 3 │ { foo | std.FailWith "no reason" = 1 }.foo

core/src/error/mod.rs

@@ -1589,13 +1589,24 @@ is None but last_arrow_elem is Some"
             .filter(|diag| !label::ContractDiagnostic::is_empty(diag));
         let head_contract_diagnostic = contract_diagnostics.next();
 
-        let mut msg = format!("{}{msg_addendum}", title(&label));
+        // The addendum and the custom contract diagnostic are important, so we want to display
+        // them as part of the main error message. However, they can make the message quite long.
+        // To avoid clutter, we display each component on a new line, indented with respect to the
+        // initial "error: "
+        let new_msg_block = "\n       ";
+        let mut msg = title(&label);
+
+        if !msg_addendum.is_empty() {
+            // unwrap(): write shouldn't fail on a String
+            write!(&mut msg, "{new_msg_block}{msg_addendum}").unwrap();
+        }
 
         if let Some(contract_msg) = head_contract_diagnostic
             .as_ref()
             .and_then(|diag| diag.message.as_ref())
         {
-            write!(&mut msg, ": {}", &super::escape(contract_msg)).unwrap();
+            // unwrap(): write shouldn't fail on a String
+            write!(&mut msg, "{new_msg_block}{}", &super::escape(contract_msg)).unwrap();
         }
 
         let contract_notes = head_contract_diagnostic

doc/manual/contracts.md

@@ -139,11 +139,13 @@ appropriate error messages. Let us try:
       std.contract.blame_with_message "not a string" label
 
 > 1 | IsFoo
-error: contract broken by a value: not a string
+error: contract broken by a value
+       not a string
 [...]
 
 > "a" | IsFoo
-error: contract broken by a value: not equal to "foo"
+error: contract broken by a value
+       not equal to "foo"
 [...]
 
 > "foo" | IsFoo
@@ -421,7 +423,8 @@ By default, record contracts are closed, meaning that additional fields are forb
 > let Contract = {foo | String}
 
 > {foo = "a", bar = 1} | Contract
-error: contract broken by a value: extra field `bar`
+error: contract broken by a value
+       extra field `bar`
 [...]
 ```
 
@@ -495,7 +498,8 @@ be what you want:
   }
 
 > {sub_field.foo = "a", sub_field.bar = "b"} | ContractPipe
-error: contract broken by the value of `sub_field`: extra field `bar`
+error: contract broken by the value of `sub_field`
+       extra field `bar`
 [...]
 
 > {sub_field.foo = "a", sub_field.bar = "b"} | ContractEq
@@ -653,7 +657,8 @@ always failing contract `std.FailWith` to observe where evaluation takes place:
 * documentation: Some information
 
 > config.fail
-error: contract broken by the value of `fail`: ooch
+error: contract broken by the value of `fail`
+       ooch
 [...]
 ```
 
@@ -831,15 +836,17 @@ it check anything, though?
   }
 
 > config."0"
-error: contract broken by a value: field name `not_a_number` is not a number
+error: contract broken by a value
+       field name `not_a_number` is not a number
 [...]
 
 > let config | NumberBoolDict = {
     "0" | doc "Some information" = "not a boolean",
   }
 
 > config."0"
-error: contract broken by a value: field `0` is not a boolean
+error: contract broken by a value
+       field `0` is not a boolean
 [...]
 ```