Missing help message when forgetting to use the let keyword

[shubham-93]

Code

fn main() {
    let n = 1;
    n1 = 2;
}

Current output

error[E0425]: cannot find value `n1` in this scope
 --> src/main.rs:3:5
  |
3 |     n1 = 2;
  |     ^^ help: a local variable with a similar name exists: `n`

For more information about this error, try `rustc --explain E0425`.

Desired output

error[E0425]: cannot find value `n1` in this scope
 --> src/main.rs:3:5
  |
3 |     n1 = 2;
  |     ^^ help: a local variable with a similar name exists: `n`
  |
help: you might have meant to introduce a new binding
  |
3 |     let n1 = 2;
  |     +++

For more information about this error, try `rustc --explain E0425`.

Rationale and extra context

Coming from a Python background, I sometimes forget to use the let keyword in Rust when binding a value to a new variable. In such cases, it would be useful to get a help message from the compiler that asks the programmer to do so (help: you might have meant to introduce a new binding). I noticed that sometimes the compiler does give such a message, but sometimes not. Suppose I want to define two variables and I forget to use the let statement for the second variable. After some experimentation, it seems like the desired help message appears after a pattern common to both variable names is matched and the non-matched part has a length that crosses a certain threshold. I will give some examples below. Assume that the first variable's name is never changed, and we tweak the second variable's name slightly to see when the message help: you might have meant to introduce a new binding is triggered. Note that the other help message help: a local variable with a similar name exists is always triggered, which is not always helpful/relevant, and is therefore one of the reasons why I raised this issue.

1. The matched pattern is 'n' and the differing characters between the two variable names are '11', and I do get the desired help message from the compiler. The difference here is in 2 characters, but this is not always the minimum number required to trigger the help message as shown in example 3. below.

let n = 1;
n11 = 2;

2. The desired help message in the following case is triggered when the number of differing characters is at least 2 (e.g. abc = 2 does not trigger the message).

let ab = 1;
abcd = 2;

3. The desired help message in the following case is triggered when the number of differing characters is at least 4 (e.g. abcdefghi = 2 does not trigger the message).

let abcdef = 1;
abcdefghij = 2;

4. Now let's flip the first 2 characters of the 2 variables. So suppose I have the following:

let abcdef = 1;
ba = 2;

where the first 2 characters of the 2 variables are flipped (ab versus ba). Then, I will start adding one new (and different) character (in alphabetical order) at a time to the second variable's name and compile to see when the help message appears. We see the message in the following cases: ba, bac, bacd, bacde. Then the message disappears in the following cases: bacdef, bacdefg. And then reappears in the case bacdefgh, etc.

5. As a last example, suppose a user is interested in doing some data analysis and names a variable data_attribute. Then they name another variable with a similar name but with an underscore '_' followed by a numerical suffix. They will not get the desired help message unless they append at least 7 digits. So this means that

let data_attribute = 1;
data_attribute_1234567 = 2;

is the first time the help message appears. The message does not appear when the second variable's name is one of the following: data_attribute_1, data_attribute_12, data_attribute_123, data_attribute_1234, data_attribute_12345, data_attribute_123456.

These are just some random cases I quickly thought of, but perhaps there are more rules to when a message is triggered. The variable names in the examples above (except the last one) are not the best, but I used them to show which events trigger the message. But from especially the last example, one would think that data_attribute is completely different from e.g. data_attribute_71 for many use cases, and so the desired help message would be quite useful.

Can we not have the message triggered every time a let statement is forgotten?

Other cases

No response

Rust Version

rustc 1.82.0 (f6e511e 2024-10-15)
binary: rustc
commit-hash: f6e511e
commit-date: 2024-10-15
host: aarch64-apple-darwin
release: 1.82.0
LLVM version: 19.1.1

Anything else?

No response

[uellenberg]

This is the relevant code:

rust/compiler/rustc_resolve/src/late/diagnostics.rs

Lines 897 to 899 in ef972a3

	if !self.r.add_typo_suggestion(err, suggestion, ident_span) {
	fallback = !self.let_binding_suggestion(err, ident_span);
	}

and

rust/compiler/rustc_resolve/src/late/diagnostics.rs

Line 879 in ef972a3

if !self.r.add_typo_suggestion(err, typo_sugg, ident_span) {

Making typo and let binding suggestions mutually exclusive seems to be an intentional decision, so I'm not sure whether it's something to be fixed. Reworking the logic yields the following error:

error[E0425]: cannot find value `n1` in this scope
 --> src/main.rs:3:5
  |
3 |     n1 = 2;
  |     ^^
  |
help: a local variable with a similar name exists
  |
3 |     n = 2;
  |     ~
help: you might have meant to introduce a new binding
  |
3 |     let n1 = 2;
  |     +++

For more information about this error, try `rustc --explain E0425`.
error: could not compile `rust-lang-test` (bin "rust-lang-test") due to 1 previous error

[uellenberg]

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