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Add information about dependency scanning to the tour #10

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Feb 25, 2024
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Add information about dependency scanning to the tour #10

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Add information about dependency scanning to the tour.
bobg Feb 25, 2024
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Tweaks.
bobg Feb 25, 2024
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36 changes: 29 additions & 7 deletions Tour.md
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Expand Up @@ -21,12 +21,12 @@ about which, more below.

“Mingo” is a Go static-analysis tool that I created recently in order to answer the question,
“What is the oldest version of Go that can compile my code?”
The answer to that question is what belongs in [the go directive](https://go.dev/ref/mod#go-mod-file-go) in a Go program’s go.mod file,
The answer to that question is what belongs in [the go directive](https://go.dev/ref/mod#go-mod-file-go) in a Go program’s `go.mod` file,
but it has historically been challenging to know what to put there.

At a high level,
mingo works by parsing a Go program,
then walking its syntax tree looking for two things:
then walking its syntax tree looking for three things:

1. Language constructs introduced at a specific version of Go.
For example, if a program contains a `for … range` statement with no variable assignment,
Expand All @@ -38,6 +38,7 @@ then walking its syntax tree looking for two things:
it needs at least Go 1.6.
If it calls `bytes.Clone`,
it needs at least Go 1.20.
3. The minimum Go version declared by the dependencies in `go.mod`.

Each time it finds one of these,
it may bump up its idea of the minimum Go version needed.
Expand Down Expand Up @@ -129,11 +130,11 @@ that constitute a function body,
and then checks that there’s a final `return` statement.
(If there isn’t, then Go 1.1 or later is required.)

The type [ast.Stmt](https://pkg.go.dev/go/ast#Stmt),
together with `ast.Decl` and `ast.Expr`
The types [ast.Stmt](https://pkg.go.dev/go/ast#Stmt),
`ast.Decl`, and `ast.Expr`
(which we’ll see in a moment),
form the core of the `ast` package.
It’s an interface, and it’s implemented by
`Stmt` is an interface, and it’s implemented by
[a wide assortment](https://cs.opensource.google/go/go/+/master:src/go/ast/ast.go;l=849-871;drc=ef84d62cfc358ff62c60da9ceec754e7a389b5d5)
of concrete types,
representing variable assignments,
Expand Down Expand Up @@ -242,9 +243,30 @@ Back in `pkgScanner.ident`,
if we get a hit from this history lookup,
we [update the required Go version](https://github.com/bobg/mingo/blob/e25314c0cc521e743eb39543db37296d4239df46/expr.go#L98-L103) using the value from the lookup.

And that’s everything!
Of course there are a lot of other cases that mingo covers,
That takes care of detecting language changes and standard-library changes.
Of course there are a lot of other such cases that mingo covers,
but all of those are handled in one or another of the ways described here.
There’s one more way in which the minimum-required version of Go can increase, though,
and that’s if it depends on a module that declares a higher minimum Go version.
This is handled in [Scanner.scanDeps](https://github.com/bobg/mingo/blob/bddce647865471cc1b3214894582be9188b4af00/deps.go#L16).

That function parses the `go.mod` file
([here](https://github.com/bobg/mingo/blob/bddce647865471cc1b3214894582be9188b4af00/deps.go#L22))
to discover direct dependencies.
(Indirect ones are discarded [here](https://github.com/bobg/mingo/blob/bddce647865471cc1b3214894582be9188b4af00/deps.go#L28-L30).)
Each of those is then individually scanned by [Scanner.scanDep](https://github.com/bobg/mingo/blob/bddce647865471cc1b3214894582be9188b4af00/deps.go#L71).

The first thing that `Scanner.scanDep` does is to get a “download” of information about the dependency.
This is the result of running `go mod download -json`,
which happens [here](https://github.com/bobg/mingo/blob/bddce647865471cc1b3214894582be9188b4af00/deps.go#L52).
(The extra logic delegating that job to a `depScanner`
allows [a mock dependency scanner](https://github.com/bobg/mingo/blob/bddce647865471cc1b3214894582be9188b4af00/deps_test.go#L30) to be injected for testing.)
It is then possible to [parse _the dependency’s_ go.mod file](https://github.com/bobg/mingo/blob/bddce647865471cc1b3214894582be9188b4af00/deps.go#L86)
to discover the version in [its go directive](https://github.com/bobg/mingo/blob/bddce647865471cc1b3214894582be9188b4af00/deps.go#L94),
which mingo accepts on faith
(rather than scanning all the code in the dependency too).

That’s everything about how mingo does its job.
But there’s still one more topic to cover:
the `Analyzer` API.

Expand Down
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